Analysis of the advantages and disadvantages of various humidity dew point measurement methods
2020-05-20 09:53:11Source：昶艾电子 Hits：
The humidity measuring instrument can be divided into cold mirror type, fully absorption electrolysis type, Al2O3 capacitance type, thin film capacitance type, resistance type, dry wet ball type and mechanical type. Among them, the fully absorption electrolytic micro-water meter and Al2O3 capacitive dew point meter are generally used for the measurement of low humidity range, while the resistance type, dry-wet ball, mechanical humidity meter can only be used for the measurement of relative humidity, the cold mirror type, thin film capacitance type (Vaisala's patent) humidity meter can not only be used for the measurement of low humidity, but also can be used for the measurement of medium and high humidity, that is, relative humidity. The above-mentioned instruments have their own advantages and disadvantages. Among them, the cold mirror dew point meter is the most accurate, the most reliable, the most basic measurement method, is widely used in the standard transmission, but its disadvantage is that the price is relatively expensive, and needs experienced operation and maintenance.
Cold mirror dew point meter
When the measured moisture enters into the dew point measuring chamber, the cold mirror surface is swept over; when the temperature of the mirror surface is higher than the dew point temperature of the moisture, the mirror surface is in a dry state; at this time, the light emitted by the light source in the photoelectric exposure device is almost reflected on the mirror surface, the photoelectric sensor senses and outputs the photoelectric signal, and the control circuit compares, amplifies and drives the thermoelectric pump to cool the mirror surface. When the temperature of the mirror surface drops to the dew point temperature of the moisture, the surface begins to dew (frost), the light appears diffuse reflection on the mirror surface, the reflection signal induced by the photoelectric sensor is weakened, the change is compared by the control loop, amplified, the thermoelectric pump is adjusted to make the refrigeration power properly reduced, finally, the temperature of the mirror surface is kept at the dew point temperature of the sample gas. The mirror temperature is induced by a platinum resistance temperature sensor which is close to the lower part of the cold mirror surface and is displayed on the display window.
At present, the world's companies that produce cold mirror dew-point meter, such as GE, Edgetech, Switzerland MBW and so on, all adopt this principle, Britain's MICHELL adopts double-optical path detection system, that is, the reflection light and scattering light are detected at the same time, Finland's Vaisala uses acoustic wave as detection system.
During the measurement process, the water vapor in the measured gas is close to the saturation state with the decrease of temperature. Due to the gravitational effect, the water molecules adsorb on the mirror surface to form a thin water film. This is the first stage of dew formation. When the mirror temperature continues to decrease, the thickness of the water film gradually increases, which is the second stage of the dew formation. In this phase, the force contrast between the gravitational force of water molecules and the surface tension of water film is changed, and the influence of the latter gradually dominates. At this time, any unstable factors on the cooling surface, such as the tiny scar on the mirror surface, will cause the water film to condense into droplets. With the further decrease of the mirror temperature, the dew drops begin to appear. Through the microscope, we can see the isolated growth and irregular distribution of the dew drops, and then the dew layer diffuse on the surface at a very fast speed. At this time, we can think that the liquid-vapor equilibrium begins, namely reaching the dew point.
The mirror should be hydrophobic, have good thermal conductivity, wear resistance, corrosion resistance and good optical performance. In the past, gold was used as a mirror, now rhodium is used as the mirror.
In the past, ethylene ether evaporation, mechanical refrigeration, liquefied gas or dry ice refrigeration, compressed air refrigeration have been used. The most commonly used is thermoelectric refrigeration or thermoelectric combined with mechanical refrigeration (dew point below -60°C). In this paper, thermoelectric refrigeration is emphasized.
Thermoelectric refrigeration, also known as semiconductor refrigeration, is Palput refrigeration (from its English name Peltier). The principle is that when a DC current passes through an NP element composed of two different metals, the heat will be transferred from one metal to another, which is just opposite to the thermocouple temperature measurement. Therefore, when the cold end of Palter is connected to the mirror and the other end is used as the heat dissipation end, the mirror can be cooled. In order to obtain different low temperature, a multi-level superposition method can be adopted. The data given by GE Company of the United States shows that, generally speaking, if the room temperature is 25°C, the temperature difference between the cold and cold end can reach 55°C, the temperature difference between the cold and cold end can reach 75°C, the temperature difference between the cold and cold end can reach 105°C, and the temperature difference between the cold and cold end can reach 120°C when the fifth refrigeration. The cooling capacity of different companies will vary slightly. The higher the temperature at the hot end, the higher the cooling efficiency, and the larger the temperature difference at the hot end. In order to reduce the temperature of the cold end, air cooling, water cooling and mechanical refrigeration are usually used to reduce the temperature of the hot end. But it is not possible to reduce without limits. It is important to note that its refrigeration capacity does not represent the measurement range of the dew point hygrometer. The definition of the measuring range of dew point hygrometer is that the temperature of the mirror surface can be obtained on the mirror surface, and the temperature of the mirror surface can be obtained when the dew or frost layer has a certain thickness. Therefore, under the general dew/frost point, the measuring range of dew point hygrometer is generally 5°C higher than its cooling ability, and under the low frost point, it is generally 10°C~12°C higher. For example, the DP19 dew point meter produced by MBW Company in Switzerland, when the room temperature is 10°C, its lowest measurement range is -60°C, when the room temperature is 20°C, its lowest measurement range is -55°C, when the room temperature is 35°C, its lowest measurement range is -45°C. Because of the high thermal conductivity of hydrogen and helium, the measurement range will be reduced by several degrees. When the pressure of the measured gas increases, the measurement range will also be reduced. For air and nitrogen, under the condition of higher than normal pressure, each additional atmospheric pressure, the measurement range will be reduced about 0.67°C.
Temperature measuring device
At present, most of the four-wire platinum resistance is used to measure temperature. The resistance value and temperature of the platinum resistance temperature sensing element are close to the linear relation in a fairly wide temperature range, the precision is high, the stability is good, the output signal is strong, the digital display is convenient.
At present, except for the cold mirror dew point meter developed recently by Vaisala Company of Finland, which adopts the sound wave principle to measure, the other are all used photoelectric detector to measure and control. The photoelectric detection technology has been used for several decades, and it is mature. But its disadvantage is that it can not distinguish the supercooled water from the frost.
Precautions for use
Supercooled water and frost
In the range of 0-20°C, supercooled water is easily formed on the mirror surface. Because the saturated vapor pressure on the ice surface and the water surface is different, if supercooled water is formed on the mirror surface, the measured value is lower than the frost point, and the temperature is different. For example, when the frost point is -10°C, the corresponding supercooled water temperature is -11.23°C. So be very careful at this temperature. If the instrument is equipped with an endoscope, it can be observed and distinguished by the endoscope. At present, most instruments have the function of test, that is, to test their minimum cooling capacity. At this time, we can use the function of test to make the temperature of the mirror be lower than -20°C, ensure the frost on the mirror, and then make a formal measurement.
The saturated water vapor pressure on the surface is different from that on the plane. When exposed to the metal surface, the equilibrium water vapor pressure, i.e., the saturated water vapor pressure at the curved water surface, is increased due to the effect of surface tension, which is known as the Kelvin effect. Because of the Kelvin effect, the dew point temperature obtained is lower than the dew point temperature of the real measured gas.
It means that the equilibrium water vapor pressure of the system is lower than the saturated water vapor pressure of the pure water when the water-soluble substance exists on the mirror. These water-soluble substances may be intrinsic to the mirror or contained in the measured gas. According to Raoul's law, the decrease of the equilibrium water vapor pressure is proportional to the solution concentration, which is the reason why there will be early condensation before reaching the dew point temperature of the measured gas
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 the water-soluble substances are inevitably more or less present in the mirror and measured gas, and the impurities in the gas may sometimes occur chemical reaction or photochemical reaction with the water-insoluble substances on the mirror to convert into soluble substances. This situation is more evident in the moisture measurement of industrial process gases. Therefore, it is necessary to remove the solid particles in the gas by adopting proper filter device, and further remove the soluble substances left on the mirror surface by repeated dew condensation and dedew operation, this method is widely used.
In practical work, we often find that the surface of the mirror is not uniform when the surface begins to expose, the layer always appears in a certain area of the mirror, the reason is often caused by scratches on the mirror, because in these defective areas, on the one hand, the residual material is not easy to remove, on the other hand, the defects of the corner play a role of "exposed core", accelerating the process of exposing. Therefore, in the use of the dew point meter, especially when cleaning the mirror, it is necessary to be careful to avoid mechanical damage to the mirror.
one is the raoul effect, the other is to change the level of specular background scattering. The raoul effect is mainly caused by water-soluble substances. If the substance in the measured gas (generally soluble salts), the mirror will dew out in advance, which will cause positive deviation of the measurement results. If the pollutants are insoluble in water particles, such as dust, etc., the scattering level of the background will be increased, so that the photoelectric dew-point meter zero drift.
Because the water content in the atmosphere is very high, and the water molecule is polar molecule, it is easy to absorb on the inner wall of the pipeline or through the pipeline. Therefore, the gas path system must be well sealed in the measurement, the thickness of the pipe wall is at least 1mm, so as to prevent the outside environment water intrusion into the leakage. If the temperature of the measuring environment changes greatly, the sealing of the pipeline should be checked again.
If the measured gas is directly discharged into the atmosphere, the problem of water diffusion into the measurement system should be considered. The most commonly used method is to connect a pipe with a proper length in the exhaust port. The length and diameter of the pipe are based on the principle of not affecting the pressure of the measuring chamber.
The sampling pipeline should be as short as possible, the number of joints should be reduced and the "dead space" should be avoided so as to reduce the interference of the background water.
The sampling pipeline and the measuring cavity wall are clean, the smoothness is good, and the hydrophobic material is selected. Figure 2-2 is the desorption-time curve of various materials when they are subjected to dry gas in the saturated adsorption state. From the experimental results we can get the following order of material selection: stainless steel, PTFE, copper, polyethylene, and, worst, nylon and rubber tubes, should not be used in low frost point measurements. In addition, the outer diameter of the tube is 6 mm or 1/4 inch, although the inner polished stainless steel tube is used in the low frost point measurement.
When measuring the high dew point, we must pay attention to that the dew point is lower than the ambient temperature of 3°C to avoid condensation of water vapor in the pipeline.
When the dew point hygrometer measures the humidity, the flow range is 0.25L/min~1L/min. In this range, the change of flow velocity does not affect the measurement results.
The sampling can be divided into two kinds, one is the pressure sampling, according to the different sampling methods, it can be divided into the pressure measurement and the atmospheric pressure measurement. See figures 2-3 and 2-4, respectively. The other is measured at atmospheric pressure, that is, the sample is taken by pump. In this case, it will often bring artificial positive pressure and negative pressure because of the different sampling methods. If sampling in the way shown in Figure 2-5, the dew point meter is measured under the condition of pressure, it will bring positive error to the measurement results. If the pump and the flow meter exchange position, the dew point meter is under the condition of negative pressure, it will bring negative error to the measurement. The correct sampling method is shown in figures 2-6.
The measurement range of the dew point hygrometer is wide. At present, the measurement range of a series of dew point hygrometers developed by MBW Company of Switzerland has reached -95°C~70°C, which can meet the majority of measurement requirements.
Pros and cons
Benefits: It is a basic measurement, accurate measurement, and the instrument is stable and drift-free. The instrument with the highest accuracy can reach ±0.1°C.
Disadvantage: High price, high requirements for operators, and need to maintain. Sensitive to pollutants. There is sometimes supercooled water in the -20°C~0°C range, so be especially careful to distinguish supercooled water from frost.
A micro water meter for complete absorption electrolysis
By means of continuous sampling, the gas sample flows through an electrolytic cell of a special structure, the moisture of which is absorbed by the phosphorus pentoxide layer as a hygroscopic agent and discharged by electrolysis into hydrogen and oxygen, and the phosphorus pentoxide is regenerated. The reaction process can be expressed as:
Combined (1), (2), will get:
When the absorption and electrolysis are balanced, the water entering into the electrolysis cell is absorbed by the phosphorus pentoxide film layer, and is electrolyzed. If the ambient temperature, ambient pressure and gas flow are known, the relationship between the electrolytic current of water and the water content of gas sample can be deduced according to Faraday's law of electrolysis and gas law:
Electrolytic current of water, μΑ;
Water content of gas sample, μL/L (i.e. volume ratio)
Gas flow, ml/min
Environmental pressure, Pa;
Absolute temperature of the environment, k;
As can be seen from the above formula, the magnitude of the electrolytic current is proportional to the water content in the gas sample, so the water content in the gas sample can be measured by measuring the electrolytic current of water. Under the condition of standard atmospheric pressure and 20°C, an ideal gas flows through the electrolytic cell at a flow rate of 100ml/min. When the water content of the gas sample is 1μL/L (ppmv), the electrolytic current is calculated from the formula as 13.4μΑ. This kind of instrument usually takes ppmv as unit, and can directly read the ppmv value of the moisture content in the gas sample.
Because of the catalytic effect of platinum electrode, the water electrolysis reaction is a reversible process, so when the measured gas sample is hydrogen, oxygen or contains enough hydrogen and oxygen, the balance moves to the left, the hydrogen and oxygen which have been produced by electrolysis are combined to generate water, and then the secondary electrolysis is carried out, so that the total electrolysis current value is higher, namely, the "hydrogen effect" and the "oxygen effect", or the "composite effect". The experiment shows that the reading of this kind of gas is higher by several to ten ppmv when the instrument is used to measure the water content of this kind of gas, but the deviation concentration reaction is on the background value, so it can be deducted.
The instrument is composed of two parts of gas path system and circuit, the gas path system mainly includes an electrolytic cell and a gas path control part.
In the glass tube, two platinum electrodes are wound into a double spiral shape, and phosphorus pentoxide film is uniformly coated between the electrodes as a hygroscopic agent. Under the specified measurement conditions, the inner winding structure can ensure the absorption and electrolysis of all water entering the pool. The glass pool wall is favorable for the uniform phosphorus pentoxide coating. Since platinum has the function of generating hydrogen and oxygen, especially hydrogen-rich gas, to react again to generate water, some companies have adopted rhodium instead of platinum.
For the dry phosphorus pentoxide coating, when a "absolutely dry" gas sample is introduced and an appropriate DC voltage is applied to the electrode, a small current-background value is generated in the circuit. The value of the background is only related to the structure of the cell, the condition of the coating, the temperature and the type of the sample, but not to the water content of the sample. Because the background value can always be added to the electrolytic current of the moisture contained in the gas sample, the real moisture content of the medium should be deducted from the instrument reading when measuring.
Pneumatic control system
The pneumatic system is composed of control valve, electrolytic cell, flow regulating valve, flow meter and dryer. The control of the air flow path is done by the control valve.
Precautions for use
From the formula, we can know that the measurement results, namely the humidity of the gas μL/L (ppmv) is calculated according to the gas flow and electrolytic current, so the gas flow must be accurately controlled and measured. This kind of instrument generally uses the floater flowmeter, under 20°C, 1atm, uses the air to calibrate. If the condition in use is not a standard condition, for example at a different temperature and pressure, or the measured gas is not air, the measured gas needs to be calibrated again or corrected according to a correction factor.
The measurement range is from several μL/L(ppmV) to 2000 μL/L(ppmV), and the accuracy is 5% of the reading or 1% of the full range. The invention can be used for a plurality of inert gases, some organic and inorganic gases which 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. It can not be used for certain corrosive gases and gases which can react with P2O5, such as ethanol, certain acidic gases, unsaturated hydrocarbon gases.
Pros and cons
Benefits: Absolute measurement, stable, no drift.
Disadvantage: The life of the battery is limited and needs to be regenerated. Both high and low humidity (<1ppmv) shorten their life. Slow response in low humidity. The demand for gas flow rate is high. It cannot be used in some corrosive gases and gases that react with P2O5. There's a background.
Aluminum oxide capacitance humidity meter
Measuring principle, structure and application range
The instrument has a variety of forms, such as portable battery-operated, data processing with a microprocessor, multi-parameter display, and so on. But its essence is a capacitor, by depositing a thin layer of porous alumina on a conductive substrate and then applying a thin layer of gold to the thin layer of alumina. The conductive substrate and the thin layer of gold form an electrode of the capacitor. The water vapor is absorbed by the porous alumina through the gold thin layer, and the impedance of the capacitor is proportional to the number of water molecules, namely the water vapor pressure. Moisture partial pressure can be obtained by measuring the impedance or capacitance of the capacitor, and dew point value can be obtained by conversion.
The thin layer of aluminum oxide located between the aluminum and gold electrodes is responsive throughout the range of saturated vapor pressure from 10-3Pa (approximately -110°C dew point) to water. Due to its strong affinity for water, coupled with the larger dielectric constant of water, such instruments are highly selective for water, but not responsive to other common gases and organic gases and liquids.
The accuracy is ±1~±2°C in the middle and high humidity range, and ±2~±3°C in the low humidity range, such as -100°C. The sensor does not react with hydrocarbon gas, CO, CO2 and HCFC-containing gas, but the drift of different gas is different. For certain corrosive gases, such as ammonia, SO3, and chlorine, will damage the sensor and should be avoided as far as possible.
Precautions for use
The usual measurement range of this kind of instrument is -110°C~+20°C. When the dew point is higher, the instrument will produce a larger drift. Attention should also be paid to the temperature coefficient. Because of its response to the partial pressure of water vapor, we should pay attention to the change of the total pressure of gas in the measurement.
It can avoid dust and oil pollution, and the gas flow rate is larger, which is 3~5(L/min) or even larger.
Pros and cons
Benefits: The invention has wide response range, from 1μL/L (ppmv) to 80%RH, can be installed remotely, can be used in the field, has relatively stable, fast response, small temperature coefficient, has no relation with flow rate change, has high selectivity to water, can be used in a wide range of temperature and pressure, has small daily maintenance amount and small volume.
Disadvantage: The method is an indirect measurement, operating at higher temperatures or some gases causing drift, affected by corrosive gases, which must be periodically calibrated to overcome aging, hysteresis and contamination. Since the response value is non-linear, each sensor needs to be calibrated and cannot be used universally.
Thin-film capacitive humidity meter
Measuring principle, structure and application range
A polyamine salt or cellulose acetate polymer film deposited on two conductive electrodes is used. The dielectric constant between the two electrodes can be changed when the film absorbs water or loses water. There is also a technique to use thermoset polymers that are resistant to high temperatures, which allows such sensors to be continuously measured at temperatures higher than 100°C. Now I'm using high-molecular films like Visala.
1.The main function is to support other parts of the sensor.
2.One of the electrodes is made of conductive material
3.Thin film layer. It is the heart of the sensor, the amount of water absorption of the film is related to the relative humidity of the surrounding environment. The thickness of the film is 1-10 (μm).
4.The upper electrode also plays an important role in the performance of the sensor. In order to get a quick response, it is necessary to have a higher water permeability. It is also a conductive material.
5.A contact pad for an upper electrode. Since there are many restrictions on the design of the upper electrode, a separate metal is needed to make good contact.
The measurement range is wide, from -50°C~100°C dew point. It can be used in a wide range of temperature, sometimes without temperature compensation. High temperature resistant thermoset resins allow for continuous measurements of such capacitive humidity sensors at temperatures of 185°C, with the highest temperature used depending on the sensor's packaging material. Another advantage for thermoset resin sensors is that the temperature coefficient is small in the -50°C~100°C temperature range, so it can be easily measured in a wide range.
All relative humidity sensors are temperature sensitive and, if calibrated at one temperature, will cause errors when used at another temperature. One advantage of polymer sensors is that they have less temperature dependence, i.e., smaller temperature coefficients. Therefore, when the use temperature is different from the calibration temperature, the error is small. Electronic temperature compensation is required if used at the limit temperature or if accuracy is high. When the temperature span is less than 50°C, it is easy to compensate the temperature. When the temperature range is wider, it is difficult to compensate the temperature. However, the accuracy of modern polymer sensor can reach ±1%RH in a narrow range, and ±3%RH in a wide range of temperature and humidity. After a period of use, or after contamination, a recalibration is required.
Pros and cons
Benefits: The system has the advantages of fast response, wide temperature and humidity measurement range, good linearity, little hysteresis, good stability and repeatability, low temperature coefficient and low cost.
Disadvantage: Almost nothing.
Resistance type humidity meter
Measuring principle and structure
The sensitive material takes the polymer solution of quaternary ammonium salt as the matrix, and the functional group is reacted with the resin polymer to produce a thermosetting resin with three dimensional and three dimensional, and has good stability. The change of relative humidity can lead to the change of resistance between the cathode and the anode.
Pros and cons
It has no hysteresis and aging, low temperature coefficient, low cost and low energy consumption. The temperature range is -10°C~80°C, the repeatability is better than 0.5%RH, the accuracy is higher, generally ±2%RH, in a very narrow range can reach ±1%RH.
Disadvantage: It is an indirect measurement instrument, which needs to be calibrated periodically, and is not suitable for some pollutants. If it is used in a wide range of temperature, it needs temperature compensation. It is more sensitive to pollutants than the capacitive sensor. It is not suitable for low humidity, loss of sensitivity when relative humidity is less than 15%RH, but it still has good performance when relative humidity is close to 100%RH, but condensation sometimes damages the sensor.
Some pollutants have great influence on resistance sensor, while others have great influence on capacitance sensor. Therefore, when selecting sensor, it mainly depends on the nature of pollutants.
Mechanical humidity meter
Measuring principle and structure
The length of organic polymer materials such as hair, intestinal membrane, nylon and polyimide will change with the relative humidity. The mechanical humidity meter utilizes this characteristic to make the above material make linear, strip-shaped humidity-sensing element or coat on the elastic material to roll into a loose-wire-shaped humidity-sensing element, and then through the mechanical amplifying device, the change of the geometric amount caused by the change of the humidity is indicated by a pointer or recorded by a recording pen, thereby directly indicating the relative humidity. The invention is suitable for measuring the temperature and humidity of indoor environment such as laboratory, computer room, warehouse and factory building.
Pros and cons
Benefits: Cheap, not sensitive to most pollutants, no power, and permanent recording
Disadvantage: drift, if the use of a certain humidity for a long time will lose its sensitivity, can not be used below 0°C, slow response, transport or vibration swing will damage its performance.
Dry-wet ball humidity meter
The dry-wet ball hygrometer is composed of two thermometers with identical specifications, one is called the dry-ball thermometer, and the temperature bubble is exposed in the measured gas to measure the ambient temperature, and the indication value is expressed by Ta (ta). The other is a wet-ball thermometer, which is wrapped with a specially made gauze cover to keep it wet. When the air around the wet ball is in the unsaturated state, the moisture on the wet ball gauze cover will evaporate continuously, because the moisture evaporates needs to absorb heat, so the temperature of the wet ball will decrease, its indication value is expressed by Tw(tw). The velocity of moisture evaporation of the wet sphere is related to the moisture content of the surrounding gas. When the humidity of the gas is lower, the moisture evaporation is faster, the temperature of the wet sphere is lower, and vice versa. After obtaining the accurate temperature of dry and wet balls, the humidity value is calculated by means of the equation of wet balls.
Due to its simplicity and low cost, dry-wet ball hygrometers have been the most used type for a considerable period of time in the past.
A humidity meter with good design and maintenance, in the temperature range of 5°C~80°C, if the temperature accuracy is ±0.2°C, the relative humidity accuracy is about ±3%RH. The accuracy of this principle is dependent on the accuracy of the thermometer. Platinum resistance thermometers are often used for some precise measurements. In general, the dry-wet ball hygrometer is a basic measurement method. If the calibrated thermometer is used and the operation is correct, such as the Assmann hygrometer, accurate, reliable and repeatable measurement results can be obtained. So in the past, this hygrometer was often used as a standard. However, many operators, especially in the industrial field, do not have enough energy and time, so the results are not accurate and unreliable. At present, the wet and dry spherical hygrometer is gradually replaced by modern instruments.
Pros and cons
Benefits: When the relative humidity is close to 100%RH, a higher accuracy can be obtained. Although there will be errors if the wet ball thermometer is polluted or used improperly, the maintenance cost is very low because of the simple device. The invention can be used in the situation that the room temperature is higher than 100°C, which is the basic measurement, the stability is good, the stability is simple, the cost is low.
Disadvantage: Some techniques are needed to obtain accurate measurements and calculations are needed to obtain the final results. A large number of gas samples are required, and the gas samples may be humidified by a wet gauze. When the relative humidity of the measured gas is lower than 15%RH, it is very difficult to reduce the temperature of the wet ball. When the temperature of wet ball is lower than 0°C, it is difficult to obtain reliable results. The volume cannot be too small because water is constantly supplied to the wet ball thermometer. Because dust, oil or other pollutants can pollute the gauze, or the water flow is insufficient, the temperature of the wet ball is higher, and the result of relative humidity is higher. In addition, the factors that affect the results are temperature measurement error, wind speed, radiation error and so on. When the temperature difference of dry and wet ball is 0.1°C at 20°C, the relative humidity error is 1%RH.