Instrument Measurement Principle The humidity measurement instrument can be divided into a cold mirror type, a completely absorbing electrolytic type, an Al2O3 capacitive type, a thin film capacitive type, a resistive type, a wet and dry ball, and a mechanical type. The fully-absorbing electrolytic water meter and the Al2O3 capacitive dew-point meter are generally used for measurement of the low-humidity range, while the resistive, wet-and-dry ball, and mechanical hygrometer can only be used for the measurement of relative humidity, cold mirror type, and thin film type ( Vaisala's patented hygrometer can be used not only for low-humidity measurements but also for medium-high humidity, which is the measurement of relative humidity. Each of the above principles of the instrument has its own advantages and disadvantages. Among them, the cold mirror type dew-point meter is the most accurate, most reliable, and most basic measurement method. It is widely used for standard transmission, but its disadvantage is that the price is relatively expensive, and it requires an experienced person to operate and maintain.
1.1 Cold mirror dew point meter
1.1.1 Measurement Principle When the measured moisture enters the dew point measurement chamber, it passes through the cold mirror. When the mirror temperature is higher than the dew point temperature of the moisture, the mirror surface is in a dry state. At this time, the light from the light source in the photoelectric detection device shines on the mirror surface. , almost completely reflected by the photoelectric sensor and photoelectric signal output, the control loop comparison, amplification, driving the thermoelectric pump, mirror cooling. When the mirror temperature drops to the moisture dew point temperature, dew condensation begins on the mirror surface, diffuse reflection occurs on the mirror surface, and the reflected signal sensed by the photoelectric sensor weakens. This change is compared with the control loop, and the thermoelectricity is adjusted after amplification. The pump is energized so that its cooling power is appropriately reduced. Finally, the mirror temperature is maintained at the sample gas dew point temperature. The mirror temperature is sensed by a platinum resistance temperature sensor that clings beneath the cold mirror and is displayed on the display window.
At present, companies that produce cold mirror dew point meters in the world, such as GE, Edgetech in the United States, and MBW in Switzerland, adopt this principle. British MICHELL uses a dual-optical path detection system that reflects both reflected light and scattered light. For testing, Vaisala in Finland uses sound waves as a detection system.
During the measurement process, as the temperature decreases, the water vapor in the measured gas approaches saturation, and due to the gravitational effect, water molecules adsorb on the mirror surface to form a thin film of water. This is the first stage of forming dew. As the mirror temperature continues to decrease, the thickness of the water film gradually increases, which is the second stage of formation of dew. In this phase, the force contrast between the free surface's gravity of the water molecules and the surface tension of the water film begins to change, and the influence of the latter gradually dominates. At this point, any unstable factors on the cooling surface, such as tiny flaws on the mirror surface, will cause the water film to be condensed into droplets. As the mirror temperature drops further, the dew drops begin to appear, and the isolated and irregularly distributed dew drops can be seen through the microscope. The dewdrop then diffuses on the surface at a very fast rate. At this point the liquid-vapor equilibrium can be assumed to begin. , that is, dew point.
1.1.2 Structure
1.1.2.1 The mirror surface shall be water-repellent, have good thermal conductivity, and be wear-resistant, corrosion-resistant, and have good optical properties. In the past, gold was used as a mirror. At present, it is mainly used as a mirror.
1.1.2.2 Mirror cooling has been used in the past in ether evaporation, mechanical refrigeration, liquefied gas or dry ice cooling, and compressed air refrigeration. The most commonly used is currently the combination of thermoelectric cooling or thermoelectricity and mechanical refrigeration (low dew point At -60 °C). This article focuses on thermoelectric cooling.
Thermoelectric cooling, also known as semiconductor cooling, Peltier cooling (from its English name Peltier). The principle is that when there is a direct current through an NP element made up of two different metals, heat is transferred from one metal to another, just opposite the thermocouple temperature measurement. Therefore, when the cold end of Peltier is connected with the mirror surface and the other end is used as the heat sink end, the mirror surface can be cooled. In order to obtain different degrees of low temperature multi-level stacking can be used. According to the data given by the United States GE company, in general, if the room temperature is 25 °C, when the first-class cooling, the temperature difference between the hot and cold side can reach 55 °C, the temperature difference between the hot and cold side can reach 75 °C, In the four-stage cooling, the temperature difference between the hot and cold ends can reach 105°C, and the temperature difference between the hot and cold ends in the five-stage cooling can reach 120°C. Different companies' products may have slightly different cooling capacity. The higher the hot end temperature, the higher the cooling efficiency and the greater the temperature difference between the hot and cold ends. In order to reduce the temperature of the cold end, air-cooling, water-cooling, and mechanical cooling are usually used to reduce the temperature of the hot end. However, it cannot be reduced without limit. It should be noted that its cooling capacity does not represent the measurement range of the dewpoint meter. The dew point meter's measuring range is defined as the mirror surface temperature at which a stable, certain thickness of dew or frost layer is obtained on the mirror surface. Therefore, in general dew/frost points, the measurement range is generally 5°C higher than its cooling capacity. In the case of low frost point, it is usually 10°C~12°C. For example, the DP19 dew point meter manufactured by MBW, Switzerland, has a minimum measurement range of -60°C when the room temperature is 10°C, a minimum measurement range of -55°C when the room temperature is 20°C, and a room temperature of 35°C. The lowest measurement range is -45°C. Due to the high thermal conductivity of hydrogen and helium, the measurement range can be reduced by a few degrees. When the pressure of the gas to be measured increases, the measurement range will also be reduced. For air and nitrogen, the measurement range is reduced by about 0.67°C for every additional atmospheric pressure above atmospheric pressure.
1.1.2.3 Most temperature measuring devices currently use four-wire platinum resistance thermometers. The platinum resistance temperature sensor has a nearly linear relationship between resistance and temperature over a wide temperature range. The accuracy is high, the stability is good, and the output signal is strong, making it easy to display digitally.
1.1.2.4 Detection System Currently, the cold mirror dew point instrument developed and produced by Vaisala of Finland is measured by the principle of sound wave. Others use the photoelectric detector to measure and control. Photoelectric detection technology has a history of several decades and is relatively mature, but its disadvantage is that it cannot distinguish between cold water and frost.
1.1.3 Precautions for use 1.1.3.1 Supercooled water and frost In the range of 0 to -20°C, supercooled water is easily formed on the mirror surface. Since the saturated water vapor pressures on the ice surface and the water surface are different, they are formed on the mirror surface. With cold water, the measured value is lower than the frost point, and the difference in temperature is different. For example, when the frost point is -10°C, the temperature of the corresponding supercooled water is -11.23°C. Therefore, be very careful in this temperature range. If the instrument is equipped with an endoscope, it can be distinguished by observation through the endoscope. At present, most instruments have the function of test, that is, testing their minimum cooling capacity. At this time, the test function can be used first to make the mirror temperature lower than -20°C to ensure that frost is formed on the mirror surface, and then the formal measurement is performed.
1.1.3.2 Kelvin Effect
The saturated vapor pressure on the surface is different from that on the plane. When exposed on a metal surface, due to the effect of surface tension, the equilibrium vapor pressure, that is, the saturated vapor pressure of the curved water surface, is increased. This effect is called the Kelvin effect. Due to the Kelvin effect, the dew point temperature reached is lower than the dew point temperature of the actual measured gas.
1.1.3.2 Raoul Effect
Refers to the existence of water-soluble substances on the mirror, the equilibrium vapor pressure of the system is lower than the saturated vapor pressure of pure water. These water-soluble substances may be inherent on the mirror or contained in the gas being measured. According to Raoul's law, the reduction of the equilibrium vapor pressure of a solution is proportional to the concentration of the solution, which is why early condensation occurs before the dew point temperature of the measured gas is reached.
The Kelvin effect is just the opposite of the Raoul effect, so it will offset some. However, in the dew point measurement, the Raoul effect is more significant than the Kelvin effect because water-soluble substances inevitably exist more or less in the mirror and the gas to be measured, and the impurities in the gas may sometimes be present on the mirror surface. The water-insoluble substances undergo chemical or photochemical reactions and are converted into soluble substances. This situation is more pronounced in the measurement of moisture in industrial process gases. Therefore, a suitable filtering device is used to remove the solid particles in the gas, and the residual soluble substances on the mirror surface are further removed by repeatedly carrying out condensation and deodorization operations. This method is widely used by people.
In practical work, we often find that the dew condensation on the mirror surface is not uniform, and the dew layer always appears first on a certain area of ​​the mirror surface. The reason is often caused by scratches on the mirror surface, because in these Where there is a defect, on the one hand, the remaining material is not easy to remove, and on the other hand, the edges of the defect play the role of “nucleation†and accelerate the condensation process. Therefore, in the use of the dew point meter, especially when cleaning the mirror, be careful to avoid mechanical damage to the mirror.
1.1.3.3 Mirror pollution
One is the Raoul effect, and the second is to change the specular background scattering level. The Raoul effect is mainly caused by water-soluble substances. If this substance (usually soluble salts) is detected in the gas, dew condensation will occur ahead of time in the mirror, causing positive deviations in the measurement results. If the contaminants are particles that are insoluble in water, such as dust, it will increase the scattering level of the background, thus causing zero drift of the photoelectric dew point meter.
1.1.3.4 sampling line
Because the water content in the atmosphere is very high, and the water molecule is a polar molecule, it is easy to suck on the inner wall of the pipeline or through the pipeline. Therefore, the airway system must be sealed during measurement, and the wall thickness should be at least 1mm to prevent external environment moisture intrusion. If the temperature of the measuring environment changes greatly, the pipeline seal should be checked again.
If the measured gas is directly discharged into the atmosphere, the problem of the diffusion of atmospheric moisture into the measurement system should be considered. The most common method is to connect a suitable length of pipe to the exhaust port. Its length and pipe diameter are based on the principle of not affecting the pressure in the measuring chamber.
Sampling piping should be as short as possible to minimize the number of joints and avoid "dead space" to reduce the background moisture interference.
Sampling pipeline and the inner wall of the measuring chamber strive to be clean, and the finish is better. The material with strong hydrophobicity is selected. Figure 2-2 shows the desorption-time curves of various materials when dry gas is applied under saturated adsorption conditions. From the experimental results we can obtain the following order of materials: stainless steel, PTFE, copper, polyethylene, and the worst are nylon and rubber tubes, which should not be used in low frost point measurements. In addition, when measuring at low frost points, the outside diameter of the tube is typically 6 mm or 1/4 inch despite the use of internally polished stainless steel tubes.
When making high dew point measurements, be sure to note that the dew point to be measured is less than 3°C below the ambient temperature in order to prevent condensation of water vapor in the piping.
When the dew point meter measures humidity, the flow rate is generally 0.25L/min~1L/min. Within this range, changes in flow rate do not affect the measurement results.
Sampling can generally be divided into two cases, one is a pressure sampling, according to the different sampling methods, can be divided into pressure measurement and atmospheric pressure measurement. See Figure 2-3 and Figure 2-4 respectively. The other is to measure at atmospheric pressure, that is to pump the sample. In this case, the positive pressure and negative pressure are often caused by different sampling methods. If sampling is performed as shown in Figure 2-5, the dew point meter is measured under pressure and will give The measurement result brings positive errors. If the pump and flowmeter are switched, the dewpoint meter is under negative pressure, which will bring negative error to the measurement. The correct sampling method is shown in Figure 2-6.
1.1.4 Application The dew point meter has a wide measurement range. Currently, a series of dew point meters developed by British MICHELL and Swiss MBW have reached the measurement range of -95°C~70°C, which can satisfy most of the measurement requirements.
1.1.5 advantages and disadvantages
Advantages: It is a basic measurement, accurate measurement, and the instrument is stable without drift. The current highest accuracy instrument can reach ±0.1°C.
Disadvantages: Higher prices, higher demands on operators, and maintenance. Sensitive to pollutants. In the range of -20°C to 0°C, there is sometimes supercooled water, so special care must be taken to distinguish between supercooled water and frost.
1.2 Absolutely absorbs the electrolysis micro moisture meter
1.2.1 Measuring principle Continuous sampling is used to make the gas sample flow through a specially constructed electrolytic cell. Its moisture is absorbed by the phosphorus pentoxide layer as a moisture absorbent and is electrolyzed to discharge hydrogen gas and oxygen. Phosphorus is regenerated. The reaction process can be expressed as:
P2O5+H2O=2HPO3
2HPO3=H2+1/2O2+P2O5
Merge (1), (2)
2H2O=2H2+O2
When the balance between absorption and electrolysis is reached, the water entering the electrolysis cell is completely absorbed by the phosphorus pentoxide membrane and is totally electrolyzed. If the ambient temperature, the ambient pressure and the gas sample flow are known, the relationship between the electrolysis current of the water and the water content of the sample can be derived from Faraday's law of electrolysis and gas law:
(Equation 14)
In the formula: - electrolysis current of water, μA;
- Water sample moisture content, μL / L (ie volume ratio);
-Gas sample flow, ml/min;
- Environmental pressure, Pa;
- The absolute temperature of the environment, k;
.
From the above equation, the size of the electrolytic current is proportional to the moisture content in the gas sample, so the water content in the gas sample can be measured by measuring the electrolytic current of the water. At the standard atmospheric pressure and 20°C conditions, an ideal gas flows through the electrolytic cell at a flow rate of 100 ml/min. When the moisture content of the sample is 1 μl/L (ppmv), the electrolytic current calculated from the above formula is 13.4 μA. This type of instrument is generally measured in ppmv and can directly read the ppmv of moisture content in the sample.
Due to the catalytic action of the platinum electrode, the electrolysis of water is a reversible process, so when the sample to be tested is hydrogen, oxygen, or contains a sufficient amount of hydrogen and oxygen, the equilibrium shifts to the left, and part of the hydrogen and oxygen that have been generated by electrolysis Compound water is generated, followed by secondary electrolysis, so that the total electrolytic current value is high, which is the "hydrogen effect" and "oxygen effect", or collectively referred to as "composite effect." Experiments show that when using this instrument to determine the moisture content of this type of gas sample, the readings will be higher by a few to a dozen ppmv, but this deviation is concentrated on the background value, so it can be deducted.
1.2.2 Structure The instrument consists of an air circuit system and a circuit. The air circuit system mainly includes an electrolytic cell and a pneumatic control section.
1.2.2.1 Electrolytic cell Inside the glass tube, two platinum electrodes are wound into a double spiral, and phosphorus pentoxide film is evenly coated between the electrodes as a moisture absorbent. Under the specified measuring conditions, this inner winding structure can ensure that all the water entering the tank is absorbed and electrolyzed. The wall of the glass pool facilitates uniform coating of phosphorus pentoxide. Since platinum has the effect of reacting generated hydrogen and oxygen, especially hydrogen-rich gas, to generate water again, some companies use helium instead of platinum.
For a dry phosphorus pentoxide coating, when an "absolutely dry" gas sample is introduced and a suitable DC voltage is applied to the electrode, a small current-background value will be generated in the circuit. The size of the background value is only related to the structure of the electrolytic cell, the condition of the coating, the temperature and the type of the gas sample, and has nothing to do with the moisture content of the gas sample. Since the background value can always be added to the electrolytic current contained in the moisture content of the sample, the true water content of the medium should be subtracted from the instrument reading after the measurement.
1.2.2.2 Air Control System The air system consists of control valves, electrolytic cells, flow control valves, flow meters, and dryers. The air flow path is controlled by a control valve.
1.2.3 Precautions for Use From Equation 12, it can be known that the measurement result, that is, the gas's humidity μL/L (ppmv) is calculated based on the gas flow rate and the electrolytic current. Therefore, the gas flow must be accurately controlled and measured. This type of instrument generally uses a float meter and is calibrated with air at 20°C and 1 atm. If the conditions used are not standard conditions, such as at a different temperature and pressure, or if the measured gas is not air, then the measured gas must be recalibrated or corrected based on the correction factor.
1.2.4 Application Range The measurement range is typically from a few μL/L (ppmv) to 2000 μL/L (ppmv). Accuracy is typically 5% of reading or 1% of full scale. It can be used for a variety of inert gases, some organic and inorganic gases that do not react with P2O5. Examples include air, nitrogen, hydrogen, oxygen, argon, helium, neon, carbon monoxide, carbon dioxide, sulfur hexafluoride, methane, ethane, propane, butane, natural gas, and certain Freon gases. Can not be used for certain corrosive gases and gases that can react with P2O5, such as ethanol, certain acid gases, and unsaturated hydrocarbon gases.
1.2.5 Advantages and Disadvantages Advantages: Absolute measurement method, stable, no drift.
Disadvantages: Electrolytic cell life is limited and requires regeneration. High humidity or low humidity (<1ppmv) will shorten its life. Slow response at low humidity. High gas flow requirements. It cannot be used for certain corrosive gases and gases that react with P2O5. There is a background.
1.3 Alumina Capacitive Hygrometer
1.3.1 Measurement principle, structure and application range
The instruments come in many forms, such as portable battery-operated, microprocessor-enabled data processing, multi-parameter display, and the like. But its essence is a capacitor, by depositing a thin layer of porous alumina on a conductive substrate, and then coating the thin layer of alumina with a layer of thin gold. The conductive substrate and the thin gold layer form the electrodes of the capacitor. Water vapor is absorbed through the pore-like alumina through the thin layer of gold. The impedance of this capacitor is proportional to the number of water molecules, that is, the partial pressure of water vapor. The moisture vapor partial pressure can be obtained by measuring the impedance or capacitance of the capacitor, and the dew point value can be obtained by conversion. The structure is shown in Figure 2-7.
The thin layer of alumina between the aluminum and gold electrodes responds to the full range of saturation vapor pressures of water at 10-3 Pa (approximately -110°C dew point). Due to its strong affinity for water, coupled with the large dielectric constant of water, this type of instrument has a high degree of selectivity for water and no response to other common gases and organic gases and liquids.
In the high-humidity range, the accuracy is generally ±1~±2°C, and in the low-humidity range, such as -100°C, the accuracy is generally ±2~±3°C. Such sensors do not react with hydrocarbon gases, CO, CO2, and chlorofluorocarbon gases, but they drift differently for different gases. For some corrosive gases, such as ammonia, SO3, and chlorine, the sensor can be damaged, so it should be avoided.
1.3.2 Precautions for Use This type of instrument usually has a measurement range of -110°C to +20°C. When the measured dew point is high, the instrument will cause a large drift. At the same time pay attention to the temperature coefficient. Because of its response to moisture partial pressure, attention should be paid to the change in the total gas pressure at the time of measurement.
Avoid dust, oil, gas flow during measurement, generally 3 ~ 5 (L/min), or even greater.
1.3.3 advantages and disadvantages
Advantages: Wide response range, from 1μL/L (ppmv) to 80%RH, can be remotely installed, can be used in the field, relatively stable, fast response, small temperature coefficient, has nothing to do with flow changes, has a higher moisture content The selectivity can be used within a wide range of temperature and pressure, with less routine maintenance and small size.
Disadvantages: This method is an indirect measurement. Operation at higher temperatures or certain gases can cause drift. Due to the influence of corrosive gases, it must be regularly calibrated to overcome aging, hysteresis, and contamination. Because the response value is non-linear, each sensor needs to be calibrated and cannot be used universally.
1.4 Film Capacitive Hygrometer
1.4.1 Measurement principle, structure and application range
The use of polyamine salt or acetate fiber polymer films deposited on two conductive electrodes. When the film absorbs or loses water, it changes the dielectric constant between the two electrodes. There is also a technology that uses a thermosetting polymer that can withstand high temperatures, allowing such sensors to perform continuous measurements at temperatures above 100°C. At present, Vaisala is a polymer film.
1. The substrate, usually glass, is used to support the rest of the sensor.
2. One of the electrodes, made of a conductive material.
3. Thin film layer. It is the heart of the sensor. The amount of water absorbed by the membrane is related to the relative humidity of the surrounding environment. The thickness of this film is generally 1 to 10 (μm).
4. The upper electrode also plays an important role in the performance of the sensor. In order to get a fast response, there must be a high water permeability. It is also a conductive material.
5. Contact pads of the upper electrode. Since there are many restrictions on the design of the upper electrode, a separate metal is required for good contact.
Its wide measurement range, from -50 °C ~ 100 °C dew point. Can be used over a wide temperature range, sometimes without temperature compensation. High-temperature thermosetting resins allow this type of capacitive humidity sensor to perform continuous measurements at a temperature of 185°C, depending on the sensor's packaging material. Another advantage of sensors for thermosetting resins is that the temperature coefficient is small in the temperature range of -50°C to 100°C, so accurate measurements can be easily achieved over a wide range.
All relative humidity sensors are sensitive to temperature. If calibrated at one temperature, it will introduce errors when used at another temperature. One of the advantages of polymer sensors is that they are less dependent on temperature, ie the temperature coefficient is smaller. Therefore, when the use temperature is different from the calibration temperature, the error is small. Electronic temperature compensation is required if used at extreme temperatures or where accuracy is critical. When the temperature span is less than 50°C, temperature compensation is easier. When the temperature range is wider, temperature compensation is somewhat difficult. However, modern polymer sensors can achieve ±1% RH accuracy over a narrow range and ±3% RH over a wide temperature and humidity range. After a period of use, or after being contaminated, recalibration is required.
1.4.2 Advantages and disadvantages
Advantages: Fast response, wide temperature and humidity measurement range, good linearity, almost no lag, good stability and repeatability, low temperature coefficient, low cost.
Disadvantages: Basic None.
1.5 resistance hygrometer
1.5.1 Measurement principle and structure The sensitive material is based on a polymer solution of a quaternary ammonium salt. By reacting this functional group with a resin polymer, a three-dimensional, three-dimensional thermosetting resin can be produced with good stability. Changes in relative humidity can cause changes in resistance between the cathode and the anode.
1.5.2 Advantages and disadvantages
Advantages: There is basically no hysteresis and aging, low temperature coefficient, low cost, and low energy consumption. Temperature range -10 °C ~ 80 °C, repeatability better than 0.5% RH, high accuracy, generally ± 2% RH, up to ± 1% RH in a very narrow range.
Disadvantages: It is an indirect measuring instrument that requires regular calibration and is not suitable for certain contaminants. If it is used within a wide temperature range, temperature compensation is required, which is slower than capacitive sensors and sensitive to contaminants. Not suitable for low humidity, loses sensitivity when relative humidity is below 15% RH, but still has better performance when relative humidity is close to 100% RH, but condensation can sometimes damage the sensor.
Some contaminants have a greater impact on resistive sensors and others have a greater impact on capacitive sensors. Therefore, when selecting a sensor, it is mainly based on the nature of the contaminant.
1.6 Mechanical Hygrometer
1.6.1 Measurement principle and structure The length of organic polymer materials such as hair, gut, nylon, and polyimide will change with relative humidity. Mechanical hygrometer is the use of this feature, the above materials made of linear, ribbon-like moisture-sensitive elements or coated on the elastic material wound into a hair-like moisture-sensitive element, and then by the mechanical amplification device will be caused by the change in humidity geometry Changes are indicated with a pointer or recorded with a stylus pen to directly indicate relative humidity. It is suitable for the measurement of temperature and humidity in indoor environments such as laboratories, computer rooms, warehouses, and factories.
1.6.2 Advantages and Disadvantages Advantages: Cheap, insensitive to most pollutants, no need for power supply, permanent records.
Disadvantages: Drift, if used under a certain humidity for a long time will lose its sensitivity, can not be used below 0 °C, the response is slow, transport or vibration swing will destroy its performance.
1.7 Wet and hygrometer hygrometer
1.7.1 Principle Wet and hygroscopic hygrometers consist of two thermometers of exactly the same specifications. One is called a dry-bulb thermometer. A warm bubble is exposed to the gas to be measured to measure the ambient temperature. The indication is expressed in Ta(ta). . The other one is a wet-bulb thermometer whose warm bubbles are wrapped with special gauze and the gauze cover is kept moist. When the air around the wet bulb is in an unsaturated state, the moisture on the wet bulb gauze will evaporate continuously. Since the moisture needs to absorb heat, the temperature of the wet bulb will drop, and the indication is expressed by Tw(tw). The rate of moisture evaporation from the wet bulb is related to the moisture content of the surrounding gas. When the humidity of the gas is lower, the water evaporates faster and the wet bulb temperature is lower, and vice versa. After obtaining the correct dry and wet bulb temperature, the humidity value is converted by means of the wet-ball equation.
Because of its simplicity and low cost, wet and dry ball hygrometers have been the most used type for a long time in the past.
A well-designed and well-maintained hygrometer has a temperature range of 5°C to 80°C. If the temperature accuracy is ±0.2°C, the relative humidity accuracy is about ±3%RH. The accuracy of the hygrometer based on this principle depends on the accuracy of the thermometer. For some precise measurements, platinum resistance thermometers are often used. In summary, a wet and dry ball hygrometer is a basic measurement method. If a calibrated thermometer is used and the correct operation, such as the Assmann hygrometer, can provide accurate, reliable, repeatable measurements. Therefore in the past such hygrometers were often used as standards. However, many operators, especially in the industrial field, do not have enough energy and time. Therefore, the results obtained are inaccurate and unreliable. Currently, hygrometers are being replaced by modern instruments.
1.7.2 Advantages and disadvantages
Advantages: Higher accuracy can be achieved when the relative humidity is close to 100% RH. Although the wet bulb thermometer may cause errors if it is contaminated or improperly used, the maintenance cost is very low because the device is relatively simple. Can be used at room temperature higher than 100 °C occasions, is the basic measurement, good stability, simple, cheap, low cost.
Disadvantages: Some tricks are needed to get accurate measurements and calculations are needed to get the final result. A large amount of gas sample is required, and the gas sample may be humidified by wet gauze. When the measured relative humidity of the gas is less than 15% RH, it is difficult to sufficiently reduce the wet bulb temperature. When the wet bulb temperature is lower than 0°C, it is difficult to obtain reliable results. Due to the constant water supply to the wet bulb thermometer, the volume cannot be too small. Since dust, oily substances, or other contaminants can contaminate the gauze or the water flow is insufficient, the wet bulb temperature will be high and the resulting relative humidity will be high. In addition, the factors that affect the results include temperature measurement errors, wind speed, and radiation errors. At 20°C, when the difference between the wet bulb temperature difference is 0.1°C, the relative humidity error is 1% RH.
CAT5E of twisted pair also use 4 winding pairs and 1 anti-tension wire, the color of the pair is exactly the same as the CAT6 of twisted pair, namely white orange, orange, white green, green, white blue, blue, white brown and brown respectively.
Although the CAT5E of unshielded twisted pairs can provide up to 1000Mb/s of transmission bandwidth, they often need to be supported by expensive special equipment.Therefore, it is usually only used for 100Mb/s fast Ethernet to connect desktop switches to computers.If you are not prepared to upgrade your network to gigabit Ethernet in the future, then you might as well use CAT5E of unshielded twisted pair in horizontal wiring.
CAT6 of unshielded twisted pair: all parameters of the six classes of unshielded twisted pair have been greatly improved and the bandwidth has been extended to 250MHz.The CAT6 of twisted pair are different from the CAT5 or CAT5E types of twisted pair in shape and structure, which not only increases the insulated cross skeleton, but also places the four pairs of twisted pair wires in the four grooves of the cross skeleton, and the diameter of the cable is also thicker.
In addition to the traditional voice system still use three types of twisted pair, network cabling is basically using CAT5E or CAT6 of unshielded twisted pair Ethernet cable.
1.1 Cold mirror dew point meter
1.1.1 Measurement Principle When the measured moisture enters the dew point measurement chamber, it passes through the cold mirror. When the mirror temperature is higher than the dew point temperature of the moisture, the mirror surface is in a dry state. At this time, the light from the light source in the photoelectric detection device shines on the mirror surface. , almost completely reflected by the photoelectric sensor and photoelectric signal output, the control loop comparison, amplification, driving the thermoelectric pump, mirror cooling. When the mirror temperature drops to the moisture dew point temperature, dew condensation begins on the mirror surface, diffuse reflection occurs on the mirror surface, and the reflected signal sensed by the photoelectric sensor weakens. This change is compared with the control loop, and the thermoelectricity is adjusted after amplification. The pump is energized so that its cooling power is appropriately reduced. Finally, the mirror temperature is maintained at the sample gas dew point temperature. The mirror temperature is sensed by a platinum resistance temperature sensor that clings beneath the cold mirror and is displayed on the display window.
At present, companies that produce cold mirror dew point meters in the world, such as GE, Edgetech in the United States, and MBW in Switzerland, adopt this principle. British MICHELL uses a dual-optical path detection system that reflects both reflected light and scattered light. For testing, Vaisala in Finland uses sound waves as a detection system.
During the measurement process, as the temperature decreases, the water vapor in the measured gas approaches saturation, and due to the gravitational effect, water molecules adsorb on the mirror surface to form a thin film of water. This is the first stage of forming dew. As the mirror temperature continues to decrease, the thickness of the water film gradually increases, which is the second stage of formation of dew. In this phase, the force contrast between the free surface's gravity of the water molecules and the surface tension of the water film begins to change, and the influence of the latter gradually dominates. At this point, any unstable factors on the cooling surface, such as tiny flaws on the mirror surface, will cause the water film to be condensed into droplets. As the mirror temperature drops further, the dew drops begin to appear, and the isolated and irregularly distributed dew drops can be seen through the microscope. The dewdrop then diffuses on the surface at a very fast rate. At this point the liquid-vapor equilibrium can be assumed to begin. , that is, dew point.
1.1.2 Structure
1.1.2.1 The mirror surface shall be water-repellent, have good thermal conductivity, and be wear-resistant, corrosion-resistant, and have good optical properties. In the past, gold was used as a mirror. At present, it is mainly used as a mirror.
1.1.2.2 Mirror cooling has been used in the past in ether evaporation, mechanical refrigeration, liquefied gas or dry ice cooling, and compressed air refrigeration. The most commonly used is currently the combination of thermoelectric cooling or thermoelectricity and mechanical refrigeration (low dew point At -60 °C). This article focuses on thermoelectric cooling.
Thermoelectric cooling, also known as semiconductor cooling, Peltier cooling (from its English name Peltier). The principle is that when there is a direct current through an NP element made up of two different metals, heat is transferred from one metal to another, just opposite the thermocouple temperature measurement. Therefore, when the cold end of Peltier is connected with the mirror surface and the other end is used as the heat sink end, the mirror surface can be cooled. In order to obtain different degrees of low temperature multi-level stacking can be used. According to the data given by the United States GE company, in general, if the room temperature is 25 °C, when the first-class cooling, the temperature difference between the hot and cold side can reach 55 °C, the temperature difference between the hot and cold side can reach 75 °C, In the four-stage cooling, the temperature difference between the hot and cold ends can reach 105°C, and the temperature difference between the hot and cold ends in the five-stage cooling can reach 120°C. Different companies' products may have slightly different cooling capacity. The higher the hot end temperature, the higher the cooling efficiency and the greater the temperature difference between the hot and cold ends. In order to reduce the temperature of the cold end, air-cooling, water-cooling, and mechanical cooling are usually used to reduce the temperature of the hot end. However, it cannot be reduced without limit. It should be noted that its cooling capacity does not represent the measurement range of the dewpoint meter. The dew point meter's measuring range is defined as the mirror surface temperature at which a stable, certain thickness of dew or frost layer is obtained on the mirror surface. Therefore, in general dew/frost points, the measurement range is generally 5°C higher than its cooling capacity. In the case of low frost point, it is usually 10°C~12°C. For example, the DP19 dew point meter manufactured by MBW, Switzerland, has a minimum measurement range of -60°C when the room temperature is 10°C, a minimum measurement range of -55°C when the room temperature is 20°C, and a room temperature of 35°C. The lowest measurement range is -45°C. Due to the high thermal conductivity of hydrogen and helium, the measurement range can be reduced by a few degrees. When the pressure of the gas to be measured increases, the measurement range will also be reduced. For air and nitrogen, the measurement range is reduced by about 0.67°C for every additional atmospheric pressure above atmospheric pressure.
1.1.2.3 Most temperature measuring devices currently use four-wire platinum resistance thermometers. The platinum resistance temperature sensor has a nearly linear relationship between resistance and temperature over a wide temperature range. The accuracy is high, the stability is good, and the output signal is strong, making it easy to display digitally.
1.1.2.4 Detection System Currently, the cold mirror dew point instrument developed and produced by Vaisala of Finland is measured by the principle of sound wave. Others use the photoelectric detector to measure and control. Photoelectric detection technology has a history of several decades and is relatively mature, but its disadvantage is that it cannot distinguish between cold water and frost.
1.1.3 Precautions for use 1.1.3.1 Supercooled water and frost In the range of 0 to -20°C, supercooled water is easily formed on the mirror surface. Since the saturated water vapor pressures on the ice surface and the water surface are different, they are formed on the mirror surface. With cold water, the measured value is lower than the frost point, and the difference in temperature is different. For example, when the frost point is -10°C, the temperature of the corresponding supercooled water is -11.23°C. Therefore, be very careful in this temperature range. If the instrument is equipped with an endoscope, it can be distinguished by observation through the endoscope. At present, most instruments have the function of test, that is, testing their minimum cooling capacity. At this time, the test function can be used first to make the mirror temperature lower than -20°C to ensure that frost is formed on the mirror surface, and then the formal measurement is performed.
1.1.3.2 Kelvin Effect
The saturated vapor pressure on the surface is different from that on the plane. When exposed on a metal surface, due to the effect of surface tension, the equilibrium vapor pressure, that is, the saturated vapor pressure of the curved water surface, is increased. This effect is called the Kelvin effect. Due to the Kelvin effect, the dew point temperature reached is lower than the dew point temperature of the actual measured gas.
1.1.3.2 Raoul Effect
Refers to the existence of water-soluble substances on the mirror, the equilibrium vapor pressure of the system is lower than the saturated vapor pressure of pure water. These water-soluble substances may be inherent on the mirror or contained in the gas being measured. According to Raoul's law, the reduction of the equilibrium vapor pressure of a solution is proportional to the concentration of the solution, which is why early condensation occurs before the dew point temperature of the measured gas is reached.
The Kelvin effect is just the opposite of the Raoul effect, so it will offset some. However, in the dew point measurement, the Raoul effect is more significant than the Kelvin effect because water-soluble substances inevitably exist more or less in the mirror and the gas to be measured, and the impurities in the gas may sometimes be present on the mirror surface. The water-insoluble substances undergo chemical or photochemical reactions and are converted into soluble substances. This situation is more pronounced in the measurement of moisture in industrial process gases. Therefore, a suitable filtering device is used to remove the solid particles in the gas, and the residual soluble substances on the mirror surface are further removed by repeatedly carrying out condensation and deodorization operations. This method is widely used by people.
In practical work, we often find that the dew condensation on the mirror surface is not uniform, and the dew layer always appears first on a certain area of ​​the mirror surface. The reason is often caused by scratches on the mirror surface, because in these Where there is a defect, on the one hand, the remaining material is not easy to remove, and on the other hand, the edges of the defect play the role of “nucleation†and accelerate the condensation process. Therefore, in the use of the dew point meter, especially when cleaning the mirror, be careful to avoid mechanical damage to the mirror.
1.1.3.3 Mirror pollution
One is the Raoul effect, and the second is to change the specular background scattering level. The Raoul effect is mainly caused by water-soluble substances. If this substance (usually soluble salts) is detected in the gas, dew condensation will occur ahead of time in the mirror, causing positive deviations in the measurement results. If the contaminants are particles that are insoluble in water, such as dust, it will increase the scattering level of the background, thus causing zero drift of the photoelectric dew point meter.
1.1.3.4 sampling line
Because the water content in the atmosphere is very high, and the water molecule is a polar molecule, it is easy to suck on the inner wall of the pipeline or through the pipeline. Therefore, the airway system must be sealed during measurement, and the wall thickness should be at least 1mm to prevent external environment moisture intrusion. If the temperature of the measuring environment changes greatly, the pipeline seal should be checked again.
If the measured gas is directly discharged into the atmosphere, the problem of the diffusion of atmospheric moisture into the measurement system should be considered. The most common method is to connect a suitable length of pipe to the exhaust port. Its length and pipe diameter are based on the principle of not affecting the pressure in the measuring chamber.
Sampling piping should be as short as possible to minimize the number of joints and avoid "dead space" to reduce the background moisture interference.
Sampling pipeline and the inner wall of the measuring chamber strive to be clean, and the finish is better. The material with strong hydrophobicity is selected. Figure 2-2 shows the desorption-time curves of various materials when dry gas is applied under saturated adsorption conditions. From the experimental results we can obtain the following order of materials: stainless steel, PTFE, copper, polyethylene, and the worst are nylon and rubber tubes, which should not be used in low frost point measurements. In addition, when measuring at low frost points, the outside diameter of the tube is typically 6 mm or 1/4 inch despite the use of internally polished stainless steel tubes.
When making high dew point measurements, be sure to note that the dew point to be measured is less than 3°C below the ambient temperature in order to prevent condensation of water vapor in the piping.
When the dew point meter measures humidity, the flow rate is generally 0.25L/min~1L/min. Within this range, changes in flow rate do not affect the measurement results.
Sampling can generally be divided into two cases, one is a pressure sampling, according to the different sampling methods, can be divided into pressure measurement and atmospheric pressure measurement. See Figure 2-3 and Figure 2-4 respectively. The other is to measure at atmospheric pressure, that is to pump the sample. In this case, the positive pressure and negative pressure are often caused by different sampling methods. If sampling is performed as shown in Figure 2-5, the dew point meter is measured under pressure and will give The measurement result brings positive errors. If the pump and flowmeter are switched, the dewpoint meter is under negative pressure, which will bring negative error to the measurement. The correct sampling method is shown in Figure 2-6.
1.1.4 Application The dew point meter has a wide measurement range. Currently, a series of dew point meters developed by British MICHELL and Swiss MBW have reached the measurement range of -95°C~70°C, which can satisfy most of the measurement requirements.
1.1.5 advantages and disadvantages
Advantages: It is a basic measurement, accurate measurement, and the instrument is stable without drift. The current highest accuracy instrument can reach ±0.1°C.
Disadvantages: Higher prices, higher demands on operators, and maintenance. Sensitive to pollutants. In the range of -20°C to 0°C, there is sometimes supercooled water, so special care must be taken to distinguish between supercooled water and frost.
1.2 Absolutely absorbs the electrolysis micro moisture meter
1.2.1 Measuring principle Continuous sampling is used to make the gas sample flow through a specially constructed electrolytic cell. Its moisture is absorbed by the phosphorus pentoxide layer as a moisture absorbent and is electrolyzed to discharge hydrogen gas and oxygen. Phosphorus is regenerated. The reaction process can be expressed as:
P2O5+H2O=2HPO3
2HPO3=H2+1/2O2+P2O5
Merge (1), (2)
2H2O=2H2+O2
When the balance between absorption and electrolysis is reached, the water entering the electrolysis cell is completely absorbed by the phosphorus pentoxide membrane and is totally electrolyzed. If the ambient temperature, the ambient pressure and the gas sample flow are known, the relationship between the electrolysis current of the water and the water content of the sample can be derived from Faraday's law of electrolysis and gas law:
(Equation 14)
In the formula: - electrolysis current of water, μA;
- Water sample moisture content, μL / L (ie volume ratio);
-Gas sample flow, ml/min;
- Environmental pressure, Pa;
- The absolute temperature of the environment, k;
.
From the above equation, the size of the electrolytic current is proportional to the moisture content in the gas sample, so the water content in the gas sample can be measured by measuring the electrolytic current of the water. At the standard atmospheric pressure and 20°C conditions, an ideal gas flows through the electrolytic cell at a flow rate of 100 ml/min. When the moisture content of the sample is 1 μl/L (ppmv), the electrolytic current calculated from the above formula is 13.4 μA. This type of instrument is generally measured in ppmv and can directly read the ppmv of moisture content in the sample.
Due to the catalytic action of the platinum electrode, the electrolysis of water is a reversible process, so when the sample to be tested is hydrogen, oxygen, or contains a sufficient amount of hydrogen and oxygen, the equilibrium shifts to the left, and part of the hydrogen and oxygen that have been generated by electrolysis Compound water is generated, followed by secondary electrolysis, so that the total electrolytic current value is high, which is the "hydrogen effect" and "oxygen effect", or collectively referred to as "composite effect." Experiments show that when using this instrument to determine the moisture content of this type of gas sample, the readings will be higher by a few to a dozen ppmv, but this deviation is concentrated on the background value, so it can be deducted.
1.2.2 Structure The instrument consists of an air circuit system and a circuit. The air circuit system mainly includes an electrolytic cell and a pneumatic control section.
1.2.2.1 Electrolytic cell Inside the glass tube, two platinum electrodes are wound into a double spiral, and phosphorus pentoxide film is evenly coated between the electrodes as a moisture absorbent. Under the specified measuring conditions, this inner winding structure can ensure that all the water entering the tank is absorbed and electrolyzed. The wall of the glass pool facilitates uniform coating of phosphorus pentoxide. Since platinum has the effect of reacting generated hydrogen and oxygen, especially hydrogen-rich gas, to generate water again, some companies use helium instead of platinum.
For a dry phosphorus pentoxide coating, when an "absolutely dry" gas sample is introduced and a suitable DC voltage is applied to the electrode, a small current-background value will be generated in the circuit. The size of the background value is only related to the structure of the electrolytic cell, the condition of the coating, the temperature and the type of the gas sample, and has nothing to do with the moisture content of the gas sample. Since the background value can always be added to the electrolytic current contained in the moisture content of the sample, the true water content of the medium should be subtracted from the instrument reading after the measurement.
1.2.2.2 Air Control System The air system consists of control valves, electrolytic cells, flow control valves, flow meters, and dryers. The air flow path is controlled by a control valve.
1.2.3 Precautions for Use From Equation 12, it can be known that the measurement result, that is, the gas's humidity μL/L (ppmv) is calculated based on the gas flow rate and the electrolytic current. Therefore, the gas flow must be accurately controlled and measured. This type of instrument generally uses a float meter and is calibrated with air at 20°C and 1 atm. If the conditions used are not standard conditions, such as at a different temperature and pressure, or if the measured gas is not air, then the measured gas must be recalibrated or corrected based on the correction factor.
1.2.4 Application Range The measurement range is typically from a few μL/L (ppmv) to 2000 μL/L (ppmv). Accuracy is typically 5% of reading or 1% of full scale. It can be used for a variety of inert gases, some organic and inorganic gases that do not react with P2O5. Examples include air, nitrogen, hydrogen, oxygen, argon, helium, neon, carbon monoxide, carbon dioxide, sulfur hexafluoride, methane, ethane, propane, butane, natural gas, and certain Freon gases. Can not be used for certain corrosive gases and gases that can react with P2O5, such as ethanol, certain acid gases, and unsaturated hydrocarbon gases.
1.2.5 Advantages and Disadvantages Advantages: Absolute measurement method, stable, no drift.
Disadvantages: Electrolytic cell life is limited and requires regeneration. High humidity or low humidity (<1ppmv) will shorten its life. Slow response at low humidity. High gas flow requirements. It cannot be used for certain corrosive gases and gases that react with P2O5. There is a background.
1.3 Alumina Capacitive Hygrometer
1.3.1 Measurement principle, structure and application range
The instruments come in many forms, such as portable battery-operated, microprocessor-enabled data processing, multi-parameter display, and the like. But its essence is a capacitor, by depositing a thin layer of porous alumina on a conductive substrate, and then coating the thin layer of alumina with a layer of thin gold. The conductive substrate and the thin gold layer form the electrodes of the capacitor. Water vapor is absorbed through the pore-like alumina through the thin layer of gold. The impedance of this capacitor is proportional to the number of water molecules, that is, the partial pressure of water vapor. The moisture vapor partial pressure can be obtained by measuring the impedance or capacitance of the capacitor, and the dew point value can be obtained by conversion. The structure is shown in Figure 2-7.
The thin layer of alumina between the aluminum and gold electrodes responds to the full range of saturation vapor pressures of water at 10-3 Pa (approximately -110°C dew point). Due to its strong affinity for water, coupled with the large dielectric constant of water, this type of instrument has a high degree of selectivity for water and no response to other common gases and organic gases and liquids.
In the high-humidity range, the accuracy is generally ±1~±2°C, and in the low-humidity range, such as -100°C, the accuracy is generally ±2~±3°C. Such sensors do not react with hydrocarbon gases, CO, CO2, and chlorofluorocarbon gases, but they drift differently for different gases. For some corrosive gases, such as ammonia, SO3, and chlorine, the sensor can be damaged, so it should be avoided.
1.3.2 Precautions for Use This type of instrument usually has a measurement range of -110°C to +20°C. When the measured dew point is high, the instrument will cause a large drift. At the same time pay attention to the temperature coefficient. Because of its response to moisture partial pressure, attention should be paid to the change in the total gas pressure at the time of measurement.
Avoid dust, oil, gas flow during measurement, generally 3 ~ 5 (L/min), or even greater.
1.3.3 advantages and disadvantages
Advantages: Wide response range, from 1μL/L (ppmv) to 80%RH, can be remotely installed, can be used in the field, relatively stable, fast response, small temperature coefficient, has nothing to do with flow changes, has a higher moisture content The selectivity can be used within a wide range of temperature and pressure, with less routine maintenance and small size.
Disadvantages: This method is an indirect measurement. Operation at higher temperatures or certain gases can cause drift. Due to the influence of corrosive gases, it must be regularly calibrated to overcome aging, hysteresis, and contamination. Because the response value is non-linear, each sensor needs to be calibrated and cannot be used universally.
1.4 Film Capacitive Hygrometer
1.4.1 Measurement principle, structure and application range
The use of polyamine salt or acetate fiber polymer films deposited on two conductive electrodes. When the film absorbs or loses water, it changes the dielectric constant between the two electrodes. There is also a technology that uses a thermosetting polymer that can withstand high temperatures, allowing such sensors to perform continuous measurements at temperatures above 100°C. At present, Vaisala is a polymer film.
1. The substrate, usually glass, is used to support the rest of the sensor.
2. One of the electrodes, made of a conductive material.
3. Thin film layer. It is the heart of the sensor. The amount of water absorbed by the membrane is related to the relative humidity of the surrounding environment. The thickness of this film is generally 1 to 10 (μm).
4. The upper electrode also plays an important role in the performance of the sensor. In order to get a fast response, there must be a high water permeability. It is also a conductive material.
5. Contact pads of the upper electrode. Since there are many restrictions on the design of the upper electrode, a separate metal is required for good contact.
Its wide measurement range, from -50 °C ~ 100 °C dew point. Can be used over a wide temperature range, sometimes without temperature compensation. High-temperature thermosetting resins allow this type of capacitive humidity sensor to perform continuous measurements at a temperature of 185°C, depending on the sensor's packaging material. Another advantage of sensors for thermosetting resins is that the temperature coefficient is small in the temperature range of -50°C to 100°C, so accurate measurements can be easily achieved over a wide range.
All relative humidity sensors are sensitive to temperature. If calibrated at one temperature, it will introduce errors when used at another temperature. One of the advantages of polymer sensors is that they are less dependent on temperature, ie the temperature coefficient is smaller. Therefore, when the use temperature is different from the calibration temperature, the error is small. Electronic temperature compensation is required if used at extreme temperatures or where accuracy is critical. When the temperature span is less than 50°C, temperature compensation is easier. When the temperature range is wider, temperature compensation is somewhat difficult. However, modern polymer sensors can achieve ±1% RH accuracy over a narrow range and ±3% RH over a wide temperature and humidity range. After a period of use, or after being contaminated, recalibration is required.
1.4.2 Advantages and disadvantages
Advantages: Fast response, wide temperature and humidity measurement range, good linearity, almost no lag, good stability and repeatability, low temperature coefficient, low cost.
Disadvantages: Basic None.
1.5 resistance hygrometer
1.5.1 Measurement principle and structure The sensitive material is based on a polymer solution of a quaternary ammonium salt. By reacting this functional group with a resin polymer, a three-dimensional, three-dimensional thermosetting resin can be produced with good stability. Changes in relative humidity can cause changes in resistance between the cathode and the anode.
1.5.2 Advantages and disadvantages
Advantages: There is basically no hysteresis and aging, low temperature coefficient, low cost, and low energy consumption. Temperature range -10 °C ~ 80 °C, repeatability better than 0.5% RH, high accuracy, generally ± 2% RH, up to ± 1% RH in a very narrow range.
Disadvantages: It is an indirect measuring instrument that requires regular calibration and is not suitable for certain contaminants. If it is used within a wide temperature range, temperature compensation is required, which is slower than capacitive sensors and sensitive to contaminants. Not suitable for low humidity, loses sensitivity when relative humidity is below 15% RH, but still has better performance when relative humidity is close to 100% RH, but condensation can sometimes damage the sensor.
Some contaminants have a greater impact on resistive sensors and others have a greater impact on capacitive sensors. Therefore, when selecting a sensor, it is mainly based on the nature of the contaminant.
1.6 Mechanical Hygrometer
1.6.1 Measurement principle and structure The length of organic polymer materials such as hair, gut, nylon, and polyimide will change with relative humidity. Mechanical hygrometer is the use of this feature, the above materials made of linear, ribbon-like moisture-sensitive elements or coated on the elastic material wound into a hair-like moisture-sensitive element, and then by the mechanical amplification device will be caused by the change in humidity geometry Changes are indicated with a pointer or recorded with a stylus pen to directly indicate relative humidity. It is suitable for the measurement of temperature and humidity in indoor environments such as laboratories, computer rooms, warehouses, and factories.
1.6.2 Advantages and Disadvantages Advantages: Cheap, insensitive to most pollutants, no need for power supply, permanent records.
Disadvantages: Drift, if used under a certain humidity for a long time will lose its sensitivity, can not be used below 0 °C, the response is slow, transport or vibration swing will destroy its performance.
1.7 Wet and hygrometer hygrometer
1.7.1 Principle Wet and hygroscopic hygrometers consist of two thermometers of exactly the same specifications. One is called a dry-bulb thermometer. A warm bubble is exposed to the gas to be measured to measure the ambient temperature. The indication is expressed in Ta(ta). . The other one is a wet-bulb thermometer whose warm bubbles are wrapped with special gauze and the gauze cover is kept moist. When the air around the wet bulb is in an unsaturated state, the moisture on the wet bulb gauze will evaporate continuously. Since the moisture needs to absorb heat, the temperature of the wet bulb will drop, and the indication is expressed by Tw(tw). The rate of moisture evaporation from the wet bulb is related to the moisture content of the surrounding gas. When the humidity of the gas is lower, the water evaporates faster and the wet bulb temperature is lower, and vice versa. After obtaining the correct dry and wet bulb temperature, the humidity value is converted by means of the wet-ball equation.
Because of its simplicity and low cost, wet and dry ball hygrometers have been the most used type for a long time in the past.
A well-designed and well-maintained hygrometer has a temperature range of 5°C to 80°C. If the temperature accuracy is ±0.2°C, the relative humidity accuracy is about ±3%RH. The accuracy of the hygrometer based on this principle depends on the accuracy of the thermometer. For some precise measurements, platinum resistance thermometers are often used. In summary, a wet and dry ball hygrometer is a basic measurement method. If a calibrated thermometer is used and the correct operation, such as the Assmann hygrometer, can provide accurate, reliable, repeatable measurements. Therefore in the past such hygrometers were often used as standards. However, many operators, especially in the industrial field, do not have enough energy and time. Therefore, the results obtained are inaccurate and unreliable. Currently, hygrometers are being replaced by modern instruments.
1.7.2 Advantages and disadvantages
Advantages: Higher accuracy can be achieved when the relative humidity is close to 100% RH. Although the wet bulb thermometer may cause errors if it is contaminated or improperly used, the maintenance cost is very low because the device is relatively simple. Can be used at room temperature higher than 100 °C occasions, is the basic measurement, good stability, simple, cheap, low cost.
Disadvantages: Some tricks are needed to get accurate measurements and calculations are needed to get the final result. A large amount of gas sample is required, and the gas sample may be humidified by wet gauze. When the measured relative humidity of the gas is less than 15% RH, it is difficult to sufficiently reduce the wet bulb temperature. When the wet bulb temperature is lower than 0°C, it is difficult to obtain reliable results. Due to the constant water supply to the wet bulb thermometer, the volume cannot be too small. Since dust, oily substances, or other contaminants can contaminate the gauze or the water flow is insufficient, the wet bulb temperature will be high and the resulting relative humidity will be high. In addition, the factors that affect the results include temperature measurement errors, wind speed, and radiation errors. At 20°C, when the difference between the wet bulb temperature difference is 0.1°C, the relative humidity error is 1% RH.
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