How to Use Humidity Sensors With the development of the times, in the industrial sectors such as scientific research, agriculture, HVAC, textiles, computer rooms, aerospace, and electricity, there is a growing need for humidity sensors. The higher the quality of products, the higher the demand for product quality. The control of humidity and the monitoring and analysis of the moisture content of industrial materials have become one of the more common technical conditions. Humidity sensor products and humidity measurement belong to the industry that rose in the 1990s. How to use a good humidity sensor, how to judge the performance of the humidity sensor, which is still a relatively complicated technical problem for the average user. The following article is for your reference.
First, the classification of humidity sensors and moisture characteristics Humidity sensor, divided into two kinds of resistive and capacitive, the basic form of the product is to form a moisture sensing material in the substrate wet film. After the water vapor in the air is adsorbed on the moisture-sensitive material, the impedance and the dielectric constant of the element are greatly changed, thereby making a moisture sensor. Domestic and foreign manufacturers have different levels of humidity sensor products, and the quality and price are quite different. It is indeed difficult for users to choose the ideal product with the best performance and price ratio. It is necessary to make in-depth understanding of this aspect.
Humidity sensor has the following characteristics: 1. Accuracy and long-term stability The accuracy of humidity sensor should reach ± ​​2% ~ ± 5% RH, it is difficult to achieve this level as a measuring instrument, humidity sensor to reach ± ​​2% ~ ± 3 The accuracy of %RH is difficult, and the characteristics given in the product literature are usually measured at room temperature (20°C ± 10°C) and clean gas. In actual use, due to the influence of dust, oil, and harmful gases, the use of a long time, will produce aging, accuracy decline, the accuracy of the humidity sensor to determine its long-term stability, in general, long-term stability and use Lifetime is the first problem that affects the quality of the humidity sensor. Products with annual drift control at 1% RH level are few, generally around ±2%, or even higher.
2. Temperature coefficient of humidity sensor Humidity sensor is not only sensitive to environmental humidity, but also sensitive to temperature. Its temperature coefficient is generally in the range of 0.2-0.8%RH/°C, and some humidity sensors are under different relative humidity. The temperature coefficient is different. Temperature drift nonlinear, which requires temperature compensation on the circuit. The use of single-chip software compensation, or humidity sensor without temperature compensation can not guarantee the accuracy of the full temperature range, the linearization of humidity sensor temperature drift curve directly affects the effect of compensation, nonlinear temperature drift often does not compensate for better results Only the use of hardware temperature follow-up compensation will obtain a true compensation effect. The temperature range in which the humidity sensor operates is also an important parameter. Most humidity sensors are difficult to work normally above 40°C.
3, the humidity sensor power supply metal oxide ceramics, high polymer and lithium chloride and other moisture-sensitive materials, when applied DC voltage, will lead to performance changes, or even failure, so this type of humidity sensor can not use DC voltage or DC components AC voltage. Must be AC ​​powered.
4, interchangeability At present, the humidity sensor generally has the phenomenon of poor interchangeability, the same model of the sensor can not be interchangeable, seriously affecting the use of the effect, to the maintenance, debugging increased difficulties, some manufacturers have made all efforts in this regard, (But the interchangeability is still very poor) and achieved better results.
5, humidity correction correction humidity is much more difficult than the correction temperature. Temperature calibration often uses a standard thermometer as a standard, but the humidity calibration standard is more difficult to achieve, wet and dry bulb thermometers and some common pointer hygrometers can not be used for calibration, the accuracy can not be guaranteed, because of the requirements of the environment The conditions are very rigorous. In general, (preferably under conditions suitable for a humid environment), in the absence of sophisticated verification equipment, a simple saturated salt solution test is usually used and the temperature is measured.
Second, several methods for the preliminary judgment of the performance of the humidity sensor In the case of the actual calibration of the humidity sensor is difficult, the humidity sensor performance can be judged and checked by some simple methods.
1. Determination of Consistency. The same type, the same manufacturer of humidity sensor products is best to purchase more than two at a time, the more the more the problem is explained, put together the power to compare the detection output value, in a relatively stable condition, observe the consistency of the test . If it is further tested, it can be recorded at intervals of 24 hours. Generally, there are three kinds of humidity and temperature conditions in a day, which are high, medium and low, which can comprehensively observe the consistency and stability of the product, including temperature compensation characteristics.
2. Moisten the sensor by mouth or using other means of humidification to observe its sensitivity, reproducibility, humidity and dehumidification performance, resolution, and the maximum range of the product.
3. Test the product in both open and closed boxes. Whether the comparison is consistent or not, observe the thermal effect.
4. Test the product at high temperature and low temperature (according to the specification standard), and return it to the normal state for comparison with the record before the test, examine the temperature adaptability of the product, and observe the consistency of the product. The performance of the product must ultimately be based on the formal and complete testing methods of the quality inspection department. The use of saturated salt solution for calibration, can also use brand-name products for comparison testing, the product should also be long-term use of long-term calibration in order to more fully determine the quality of the humidity sensor.
Third, some analysis of the humidity sensor products on the market Many domestic and foreign humidity sensor products have appeared in the domestic market. Capacitive humidity sensors are more common. The types of moisture sensing materials are mainly polymer polymers, lithium chloride and metal. Oxide. Capacitive humidity sensors have the advantages of fast response, small size, good linearity, and stability. Some foreign products also have high temperature performance. However, the products that achieve the above performance are mostly foreign brands, and the prices are relatively expensive. Some of the low cost products of capacitive moisture sensors sold in the market often fail to reach the above-mentioned levels, and their linearity, consistency, and repeatability are not satisfactory. Below 30% RH, 80% RH or more of the wet segment is seriously deformed. Some products use SCM compensation correction, so that the humidity "step" jump, so that the accuracy is reduced, there is a lack of consistency, poor linearity. No matter the high-grade or low-grade capacitive humidity sensor, the long-term stability is not ideal. Most of the long-term use of drift is serious. The capacitance of the humidity sensor changes to pF level, the change of 1% RH is less than 0.5pF, and the drift of capacitance changes. It often causes error of several tens of RH%. Most capacitive humidity sensors do not have the performance of working at temperatures above 40°C, and often fail and damage.
Capacitive humidity sensors are also less resistant to corrosion and often require higher cleanliness of the environment. Some products also suffer from light failure, electrostatic failure, etc. Metal oxides are ceramic moisture sensitive resistors with the same humidity sensitive capacitance. Advantages, but in the dust environment, ceramic pores will be blocked components will fail, often using the method of power dust removal, but the effect is not ideal, and can not be used in flammable and explosive environment, alumina wet sensing material can not be overcome The weakness of the surface structure is "natural aging", the impedance is unstable, and the humidity sensitive resistance of the metal oxide ceramic also has the disadvantage of poor long-term stability. Lithium chloride humidity sensitive resistors have the most outstanding advantage of long-term stability, so through the strict process production, the manufactured instrument and sensor products can achieve high accuracy, and the stability is strong. The product has good linearity. , precision and consistency are reliable guarantees for long-term service life. The long-term stability of lithium chloride humidity sensors does not replace other moisture sensitive materials.
First, the classification of humidity sensors and moisture characteristics Humidity sensor, divided into two kinds of resistive and capacitive, the basic form of the product is to form a moisture sensing material in the substrate wet film. After the water vapor in the air is adsorbed on the moisture-sensitive material, the impedance and the dielectric constant of the element are greatly changed, thereby making a moisture sensor. Domestic and foreign manufacturers have different levels of humidity sensor products, and the quality and price are quite different. It is indeed difficult for users to choose the ideal product with the best performance and price ratio. It is necessary to make in-depth understanding of this aspect.
Humidity sensor has the following characteristics: 1. Accuracy and long-term stability The accuracy of humidity sensor should reach ± ​​2% ~ ± 5% RH, it is difficult to achieve this level as a measuring instrument, humidity sensor to reach ± ​​2% ~ ± 3 The accuracy of %RH is difficult, and the characteristics given in the product literature are usually measured at room temperature (20°C ± 10°C) and clean gas. In actual use, due to the influence of dust, oil, and harmful gases, the use of a long time, will produce aging, accuracy decline, the accuracy of the humidity sensor to determine its long-term stability, in general, long-term stability and use Lifetime is the first problem that affects the quality of the humidity sensor. Products with annual drift control at 1% RH level are few, generally around ±2%, or even higher.
2. Temperature coefficient of humidity sensor Humidity sensor is not only sensitive to environmental humidity, but also sensitive to temperature. Its temperature coefficient is generally in the range of 0.2-0.8%RH/°C, and some humidity sensors are under different relative humidity. The temperature coefficient is different. Temperature drift nonlinear, which requires temperature compensation on the circuit. The use of single-chip software compensation, or humidity sensor without temperature compensation can not guarantee the accuracy of the full temperature range, the linearization of humidity sensor temperature drift curve directly affects the effect of compensation, nonlinear temperature drift often does not compensate for better results Only the use of hardware temperature follow-up compensation will obtain a true compensation effect. The temperature range in which the humidity sensor operates is also an important parameter. Most humidity sensors are difficult to work normally above 40°C.
3, the humidity sensor power supply metal oxide ceramics, high polymer and lithium chloride and other moisture-sensitive materials, when applied DC voltage, will lead to performance changes, or even failure, so this type of humidity sensor can not use DC voltage or DC components AC voltage. Must be AC ​​powered.
4, interchangeability At present, the humidity sensor generally has the phenomenon of poor interchangeability, the same model of the sensor can not be interchangeable, seriously affecting the use of the effect, to the maintenance, debugging increased difficulties, some manufacturers have made all efforts in this regard, (But the interchangeability is still very poor) and achieved better results.
5, humidity correction correction humidity is much more difficult than the correction temperature. Temperature calibration often uses a standard thermometer as a standard, but the humidity calibration standard is more difficult to achieve, wet and dry bulb thermometers and some common pointer hygrometers can not be used for calibration, the accuracy can not be guaranteed, because of the requirements of the environment The conditions are very rigorous. In general, (preferably under conditions suitable for a humid environment), in the absence of sophisticated verification equipment, a simple saturated salt solution test is usually used and the temperature is measured.
Second, several methods for the preliminary judgment of the performance of the humidity sensor In the case of the actual calibration of the humidity sensor is difficult, the humidity sensor performance can be judged and checked by some simple methods.
1. Determination of Consistency. The same type, the same manufacturer of humidity sensor products is best to purchase more than two at a time, the more the more the problem is explained, put together the power to compare the detection output value, in a relatively stable condition, observe the consistency of the test . If it is further tested, it can be recorded at intervals of 24 hours. Generally, there are three kinds of humidity and temperature conditions in a day, which are high, medium and low, which can comprehensively observe the consistency and stability of the product, including temperature compensation characteristics.
2. Moisten the sensor by mouth or using other means of humidification to observe its sensitivity, reproducibility, humidity and dehumidification performance, resolution, and the maximum range of the product.
3. Test the product in both open and closed boxes. Whether the comparison is consistent or not, observe the thermal effect.
4. Test the product at high temperature and low temperature (according to the specification standard), and return it to the normal state for comparison with the record before the test, examine the temperature adaptability of the product, and observe the consistency of the product. The performance of the product must ultimately be based on the formal and complete testing methods of the quality inspection department. The use of saturated salt solution for calibration, can also use brand-name products for comparison testing, the product should also be long-term use of long-term calibration in order to more fully determine the quality of the humidity sensor.
Third, some analysis of the humidity sensor products on the market Many domestic and foreign humidity sensor products have appeared in the domestic market. Capacitive humidity sensors are more common. The types of moisture sensing materials are mainly polymer polymers, lithium chloride and metal. Oxide. Capacitive humidity sensors have the advantages of fast response, small size, good linearity, and stability. Some foreign products also have high temperature performance. However, the products that achieve the above performance are mostly foreign brands, and the prices are relatively expensive. Some of the low cost products of capacitive moisture sensors sold in the market often fail to reach the above-mentioned levels, and their linearity, consistency, and repeatability are not satisfactory. Below 30% RH, 80% RH or more of the wet segment is seriously deformed. Some products use SCM compensation correction, so that the humidity "step" jump, so that the accuracy is reduced, there is a lack of consistency, poor linearity. No matter the high-grade or low-grade capacitive humidity sensor, the long-term stability is not ideal. Most of the long-term use of drift is serious. The capacitance of the humidity sensor changes to pF level, the change of 1% RH is less than 0.5pF, and the drift of capacitance changes. It often causes error of several tens of RH%. Most capacitive humidity sensors do not have the performance of working at temperatures above 40°C, and often fail and damage.
Capacitive humidity sensors are also less resistant to corrosion and often require higher cleanliness of the environment. Some products also suffer from light failure, electrostatic failure, etc. Metal oxides are ceramic moisture sensitive resistors with the same humidity sensitive capacitance. Advantages, but in the dust environment, ceramic pores will be blocked components will fail, often using the method of power dust removal, but the effect is not ideal, and can not be used in flammable and explosive environment, alumina wet sensing material can not be overcome The weakness of the surface structure is "natural aging", the impedance is unstable, and the humidity sensitive resistance of the metal oxide ceramic also has the disadvantage of poor long-term stability. Lithium chloride humidity sensitive resistors have the most outstanding advantage of long-term stability, so through the strict process production, the manufactured instrument and sensor products can achieve high accuracy, and the stability is strong. The product has good linearity. , precision and consistency are reliable guarantees for long-term service life. The long-term stability of lithium chloride humidity sensors does not replace other moisture sensitive materials.
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