① . Select measurement range
② . Select measurement accuracy
The measurement accuracy is the most important indicator of the humidity sensor. Every percentage point increase is a step up, or even a step up, for the humidity sensor. Because to achieve different precision, their manufacturing costs vary greatly, and their selling prices also vary greatly. Therefore, users must tailor their clothes accordingly, and should not blindly pursue "high, refined and sharp". If the humidity sensor is used at different temperatures, the influence of temperature drift shall also be considered for its indication. As we all know, the relative humidity is a function of temperature, and the temperature seriously affects the relative humidity in the designated space. Every 0.1 ℃ temperature change. A humidity change (error) of 0.5% RH will occur. If it is difficult to achieve constant temperature in the application, it is inappropriate to propose too high humidity measurement accuracy. In most cases, if there is no accurate temperature control means, or the measured space is unsealed, the accuracy of ± 5% RH is sufficient. For local space requiring accurate control of constant temperature and humidity, or for occasions requiring tracking and recording humidity changes at any time, humidity sensors with accuracy above ± 3% RH shall be selected. And the requirement of accuracy higher than ± 2% RH may be difficult even for the standard humidity generator to calibrate the sensor, let alone the sensor itself. It is very difficult to achieve the accuracy of 2% RH for the relative humidity measuring instrument even at 20-25 ℃. Generally, the characteristics given in the product data are measured at room temperature (20 ℃± 10 ℃) and in clean gas.
③ . Consider time drift and temperature drift
In actual use, due to the impact of dust, oil and harmful gases, the electronic humidity sensor will age and its accuracy will decline after a long time of use. The annual drift of the electronic humidity sensor is generally about ± 2%, or even higher. In general, the manufacturer will indicate that the effective service life of one calibration is one year or two years, and it needs to be recalibrated when it expires.
④ Other precautions
Humidity sensor is non sealing. In order to protect the accuracy and stability of measurement, it should be avoided to use it in acidic, alkaline and organic solvent atmosphere. Also avoid using in the environment with large dust. In order to correctly reflect the humidity of the space to be measured, it is also necessary to avoid placing the sensor too close to the wall or at a dead corner where the air is not circulating. If the measured room is too large, multiple sensors should be placed. Some humidity sensors require high power supply, otherwise the measurement accuracy will be affected. Or the sensors interfere with each other, or even fail to work. Suitable power supply meeting the accuracy requirements shall be provided according to the technical requirements during use. When the sensor needs to carry out long-distance signal transmission, attention should be paid to the attenuation of the signal. When the transmission distance exceeds 200m, it is recommended to select 4-20MA or 485 humidity sensor with output signal.