Calibration and testing of humidity in temperature and humidity sensors - News - Global IC Trade Starts Here Free

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There are two main approaches to calibrating and testing humidity in temperature and humidity sensors: using a humidity reference or comparing it with a known accurate instrument. A humidity reference can be created using solutions like thionyl chloride or potassium hydroxide, or by using a saturated salt solution. When setting up such references, it's crucial to ensure the container is well-sealed to prevent any changes in temperature or moisture content. The system must be allowed enough time to reach equilibrium, which can sometimes take a long time. However, using an air pump to circulate air through the solution can significantly speed up the process.

Regardless of the method, it’s always beneficial to have a separate way of measuring the reference humidity. Even if this method isn’t extremely precise, it helps determine when equilibrium has been reached and reduces the risk of significant errors. One major limitation of using these types of humidity references is that they may not be suitable for all sensors without causing inaccuracies. Some sensors, like dew point meters or wet and dry thermometers, can alter their local environment by absorbing or releasing moisture. Others, such as chloride buttons or dew point meters, may introduce temperature gradients. For accurate readings, a proper airflow is necessary during the calibration process.

The most reliable way to calibrate any sensor is to compare it with a known good instrument under real-world humidity conditions. In summer, relative humidity typically ranges from 50% to 90% between noon and midnight. Another method involves using a water spray in a bathroom to create high humidity levels, while a central heating system can produce lower humidity levels indoors. For example, at 20°C, the relative humidity in a room is usually around 60%, but raising the temperature to 32°C can reduce it to about 30%. The best instruments for use as humidity references include wet and dry thermometers and dew point meters. Dew point meters, equipped with accurately calibrated thermometers, provide highly reliable humidity values, though they tend to be more expensive.

The Asman ventilated wet and dry thermometer is considered the ideal choice, while the rotary wet and dry thermometer is also sufficiently accurate for most applications. Many professionals rely on a simple rotary wet and dry thermometer as a standard for testing and calibrating more specialized equipment. This approach ensures consistency and accuracy across different devices and environments.