Relative humidity measurement using IC Sensors

Relative humidity measurement using IC Sensors Xie Changjiang youlihua Summary: Based on the comparison of various relative humidity measurement methods and sensors, this paper introduces the characteristics and usage of integrated humidity sensor based on examples, and points out the influencing factors and compensation methods of the conditions, the specific implementation line of relative humidity detection with temperature effect compensation is provided, and the expressions of the detection software calculation, the comparison experiment results of the segmented implementation, and the external conditions of the detection process are provided. Keywords:Relative humidity, sensor, integration, measurement, microprocessor Relative humidity detection by ic Sensor Abstract:Based on Comparation of relative-humidity detection manners and sensors, the IC relative-Humidity Sensor and Its applied conditions are beyond the factors which influence the detection accuracy and the compensation methods are studied. the actual circuit detection software and the environmental limits in detecting processes are also given. Key words:Relative humidity, sensor, IC, detect, microprocessor 1. Sensor Characteristics of relative humidity detection 1.1 comparison of common Humidity Sensors Humidity Measurement plays an increasingly important role in automation systems and many other technical fields. However, it is difficult to measure the humidity, temperature, and other physical parameters. The main reason is that the water vapor in the air is slightly smaller than that in the air, and the influence of water vapor on various substances is also complicated. At present, there are many sensors and detection methods used for humidity measurement. From the perspective of usage, it is generally required to have the following features: ① Accurately and quickly measure humidity; ② Wide temperature range; ③ Wide humidity detection range; ④ Small change in timeliness; ⑤ The peripheral circuit is simple. Currently, humidity sensors are mainly divided into Adsorption Type and non-adsorption type. Because of the price, use environment and other reasons, the widely used is the adsorption type sensor. The adsorption of water molecules on the surface or inside the object directly affects the electrical and physical properties of the object. With this feature, a variety of adsorption sensors are made. Several common humidity detection sensors and their characteristics are shown in table 1. Although it has been improved and improved for a long time, it is still difficult to fully meet the above requirements. With the development of integrated electronic technology, the combination of integrated circuits and sensor technology has greatly improved the original shortcomings of sensors. Working principle of 1.2 capacitive sensor Capacitive sensors are currently the mainstream in environmental humidity and process humidity detection. They are the only sensors capable of detecting relative humidity between 0 and 0 ~ 100% RH-range sensors for full detection [1]. Table 1 Comparison of common Humidity Sensors

Active Substances	Thermo-Solid Polymer	Thermoplastic Polymer	Thermoplastic Polymer	Thermoplastic	Aio3	Lithium Chloride Film	Evaporation saturation
Base Material	Ceramics or Silicon	Ceramics or glass	Polyester Film	Na	Na	Ceramics	Na
Measurement Parameters	% RH	% RH	% RH	% RH	% RH	% RH	Wet and dry ball humidity
Variable parameters	Capacitor	Capacitor	Capacitor	Resistance	Resistance	Conductivity	Temperature
RH range	0% ~ 100%	0% ~ 100%	0% ~ 100%	20% ~ 100%	2% ~ 90%	15% ~ <100%	20% ~ 100%
RH precision	± 1% ~ + 5%	± 3% ~ + 5%	± 3% ~ + 5%	± 3% ~ + 10%	± 1% ~ + 5%	+ 5%	+ 3%
Delay	1% ~ 3% RH	2% ~ 5% RH	2% ~ 5% RH	3% ~ 6% RH	& Lt; 2% RH	Poor	Difference
Response time	15 ~ 60 s	15 ~ 90 s	15 ~ 90 s	2 ~ 5 min	3 ~ 15 min	3 ~ 5 min	2 ~ 5 min
Linearity	+ 1% RH	+ 1% RH	+ 1% RH	Difference	Difference	Poor	Difference
Temperature range	-40 ~ 185 ℃	-30-30 ~ 190 ℃	-25 ~ 100 ℃	10 ~ 40 ℃	-10 ~ 75 ℃	-50 ~ 100 ℃	0 ~ 100 ℃
Temperature Effect	0.0022% Rh/℃	<0.3% Rh/℃	<0.3% Rh/℃	& Gt; 1% RH/℃	& Gt; 1% RH/℃	& Gt; 1% RH/℃	<0.5% Rh/℃
Timeliness	+ 1% RH/5yr	+ 1% RH/yr	+ 1% RH/yr	+ 3% Rh/yr	+ 3% Rh/yr	+ 1% RH/yr	± 0. 1% RH/yr
Corrosion Resistance	Excellent	Good	Average	Average	Average	Difference	Average
Anti-Condensation	Excellent	Excellent	Good	Average	Average	Average	Good

　

A capacitive humidity sensor consists of a protective layer, a screen layer, a capacitor, an electrode layer, and a base layer. When the sensor is working, the moisture of the active capacitor insulation layer is balanced with the surrounding gas. The shielding layer mainly isolates the sensor from the outside, reducing the influence of external non-humidity factors on the sensor. The protective layer is on the outermost layer of the sensor to protect the sensor from the erosion of dust and oil. The protective layer and shielding layer of capacitive sensors have high permeability requirements. When the relative saturation (water vapor equilibrium point) in the environment changes, the capacitance changes directly. This change is characterized by rapid response, good linearity, and long-term stability. After some peripheral circuits are added, the capacitance changes can be easily measured. This is a common form of humidity measurement by capacitive sensors. With the development of sensor technology and the increasing requirements of users, sensor manufacturers have already integrated signal conditioning circuits on multiple sensors, the sensor itself can output convertible or slightly sorted signals for conversion, which makes it easy for users to use. Honeywell released the ih3605 series for silicon Signal Integration at the end of 1997. This type of sensor (rhics for short) combines signal conditioning on the chip with sensitive capacitance to make the sensor output linearly. Its characteristics are laser corrected. When the power supply voltage is Vs = 5 V and the ambient temperature is 25 ℃ ~ Change the voltage output to 100% ~ within the relative humidity of 0.8 RH ~ 3.9 v. Due to the discretization in the production process, the specific sensor parameters may be different. When leaving the factory, the manufacturer should provide correction parameters for each sensor. For the power supply voltage vs 4 ~ When the range of 9 V changes, the output voltage (1) Among them, Vout is the sensor output when the relative humidity Hout, VS is the sensor supply voltage, v0 and VF are the output voltage calibrated by the sensor at a temperature of 25 ℃ and a relative humidity of 0 and HF (provided by the manufacturer's calibration data at factory time ). The output of all adsorption humidity sensors (capacitance, resistance, Conductive Film, etc.) is affected by the temperature. Due to the good temperature characteristics of rhics, during use, you do not need to make any compensation. When the temperature change range is large or away from 25 ℃, the temperature effect will be significantly reflected. Based on the relationship between sensor output and actual relative humidity at different temperatures (as shown in surface feature 1), the manufacturer establishes a temperature compensation formula: HR = HS/(1.0546-0.002 16 t); t is the temperature, unit: ℃ (2) In the formula, HR and HS are the actual relative humidity and sensor output values respectively. This formula has different compensation errors in different temperature ranges. When the temperature is high (T> 20 ℃, the formula is ± 1%). When the temperature is low, the formula is large and the maximum value is not greater than ± 5%, in a sense, you can compensate more than 95% of the temperature effect Error Based on the (2) formula. Figure 1 surface properties of Temperature Effect 2 measurement hardware implementation The sensing circuit has the weak signal processing function. You can use the ADC for direct conversion during measurement. Consider the following issues in conjunction with sensor features during implementation. 2.1 power supply voltage The power supply voltage range of the sensor is 4 ~ 9 V, because the manufacturer provides calibration data for 5 V and 25 ℃ power supply when the sensor leaves the factory, and in order to simplify the power supply design, a 5 v dc stable power supply can be used. When the humidity is high, the temperature in the active area of the sensor may fall near or below the dew point, and condensation may occur. In this case, high power supply voltage will make the sensor generate more heat, help make the active area temperature higher than the dew point temperature, avoid condensation. Therefore, the power supply voltage should be determined based on the usage. 2.2 Temperature Effect When making Temperature Compensation for rhics, a temperature sensor should be used, and the temperature measurement point should be as close as possible to the humidity active sensing area, that is, in the same microenvironment. Conventional Temperature Measurement components can be used in small products, of course, you can use rhics with additional temperature sensors, but the price is high. A convenient measurement circuit 2 is shown. The sensor is powered by a 5 V power supply and a small pt100 component is used for temperature measurement. The analog conversion uses the built-in ADC of INTEL80C196KC, considering the resolution of the ADC and ~ The conversion accuracy of 100 ℃ is ensured within the frequently used temperature range of 0.1 ℃, and the detection line in Figure 2 is adopted. To limit the influence of its own thermal effect, the constant current source output is 1 mA, which is-50 ~ after being adjusted by the common offset amplification circuit ~ The temperature range of 100 ℃ corresponds to the sensor output 0 ~ 5 V, the accuracy is completely guaranteed. Due to the fact that the output range of rhics is within the range, direct conversion without other processing is performed. Figure 2 measurement implementation hardware Diagram 3. Functional requirements and software implementation When using rhics for relative humidity measurement, the measurement circuit must have the following functions: ① Sensor parameter input, which determines the characteristics of the sensor based on the data independently calibrated by the manufacturer; ② Select the power supply voltage, and calculate the measurement HS based on this input and (1; ③ Calculate the actual relative humidity HR Based on HS and the measured thermometer. For ease of calculation, the formula (1) is processed to obtain: Where k = 5/Vs, set the constant (3) (4) Then the relative humidity measured is as follows: (5) It can be obtained from formula (2) and measured temperature T: The actual relative humidity value with compensation can be calculated based on the programming methods (3), (4), (5), and (2. The test is implemented in a non-condensation and relatively stable environment (micro-environment dynamic balance ). 4. Lab Verification The above methods and hardware are used to detect the humidity of the sensor. Considering the particularity of the humidity measurement, in order to test the results, A mixture of wet air and dry air is used to generate a gas with a certain humidity. Dry air is produced by self-developed gas source processing equipment. After being filtered by micro-vacuum and alumina zeolite, the relative humidity of dry air can reach below 0.5% at room temperature, after mixing with wet air, a gas with a relative humidity of more than 0.5% can be generated. The average humidity measurement instrument has poor accuracy in the low temperature range, and the measurement range is divided into 0 ~ 10% (Low Block) and 10% ~ 100% (High Block) second gear, the low-end range using measurement, and then look-up table calculation, high-end using polymer Humidity Measuring Instrument for measurement. For the low temperature range, the rhics measurement result is compared with the DWS-II micro moisture measuring instrument, because the DWS-II adopts the equilibrium analysis method of phosphorus oxide electrolysis, so only the moisture content ppm value of the gas can be output, relative humidity can be identified through the common gas moisture map by ppm value and temperature [4]. The experimental results show that, within the low temperature range, the error of rhics is less than 1% RH, and basically remains unchanged. In 10% ~ The humidity range of 95% is compared by using the polymer film humidity measuring instrument and rhics measuring device. The results show that the contrast deviation is small when the humidity is low, and the deviation is large when the humidity is high, the deviation range is 1% ~ In the range of 3%. For 95% ~ The humidity range of 100% is not verified because the lab conditions are limited and the non-condensation gas with stable humidity cannot be generated. Due to humidity measurement, you should pay attention to some restrictions when using rhics. Because the sensor requires a certain period of time, when the environment does not reach the dynamic balance, the error may be very large. Note that, on the other hand, air flows slowly (close to the static state) when the environment conditions are relatively stable ). When the air flow rate is fast (within a certain range), especially in low humidity conditions, the experiment proves that the error can be as large as unacceptable (related to the flow rate of the media ). The relationship between measurement results and flow velocity is difficult to carry out quantitative analysis due to the influence of many factors. For details, see references [6]. Due to the protective layer, rhics is highly resistant to chemical gases. If water vapor is exposed for a long time, the error is increased, but the performance is greatly improved compared with that of a common capacitive sensor. After Long-term experiments, I have proved that many sensors may become ineffective and many sensors cannot be recovered, while rhics has made significant improvements in this regard. Author unit:Xie Changjiang youlihua Department of Electronic Engineering, Wuxi Jiangnan University (214063) References 1 1993 ashare Handbook of fundaments. American Society of heating, refrigerating, and aircondition engineer, Inc. 2 RH sensor. Sensing & control. Honeywell, 1997.11 3 Jing Shangyu City. Air conditioning manual. Beijing: Construction Industry Press, 1986.9 4 Fu Xue. Sensor Application and Circuit Selection. Beijing: Electronics Industry Press, 1992.7 5. Measurement and adjustment of room humidity. Beijing: Building Industry Press, 1987.4 6 Zhao yuzhen et al. Correlation Between Wet Bulb Temperature and Its Geometric Size and wind speed. Journal of engineering thermal physics, 10 (3)