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Humidity calibration involves checking the accuracy of and making the proper adjustment to sensors that measure moisture content in the air. Such measurements are critically important in many laboratory and manufacturing processes. Accurate calibration for relative humidity (RH), the amount of moisture in the air compared to the maximum it can hold at a particular temperature, is particularly difficult. A small change in ambient temperature can alter the value even when the moisture content remains the same. Proper humidity calibration of monitoring devices helps ensure that environmental conditions will remain within required specifications for a reliable length of time.
Hygrometers are devices used to measure humidity. Most of these measure a quantity such as temperature, atmospheric pressure or changes in electrical properties that are sensitive to change in humidity. Typically, these measurements indicate a humidity value by comparison with a standard reference. One difficulty in calibration is the generation of an accurate and stable relative humidity outside a special lab.
A convenient and widely used method of humidity calibration involves saturated salt solutions. Data has been compiled on the properties of 28 different salt solutions that cover the entire range of RH values. In this process, the bottom of an airtight box is covered with a saturated salt solution that generates a relative humidity in the space above it with reasonable accuracy. The RH value depends on the salt used and is largely independent of temperature. Uniformity of temperature within the box is necessary, however.
Non-saturated salt solutions can also be used to generate a certain relative humidity. In this case, the RH values produced depend on careful management of the salt concentration and ambient temperature. Just as in the above method, the measuring device is supported in the space above the solution for humidity calibration. Once the desired RH value has been generated, the device's reading can be noted and adjustments made.
In another method, a stream of dry air at 0% RH is directed into two flows. One is saturated with water vapor up to 100% RH. The relative humidity in the measuring chamber can then be adjusted by mixing the two flows with a mass flow controller (MFC), an instrument used to control the flow of gasses. While this method allows for humidity calibration at a range of levels, it is itself dependent on the proper calibration of the MFC.
Implementations of the saturated salt method are often sold in kit form to companies without the resources to set up an in-house lab or contract with an instrument calibration firm. Pharmaceutical companies, microelectronic manufacturers and research labs depend on closely regulated environments, however. In these sites, professional humidity calibration is a standard and regularly implemented procedure.