Humidity calibration ensures accurate measurement of moisture content in the air, which is important in laboratory and manufacturing processes. Hygrometers measure humidity using various methods, but calibration can be difficult. Saturated salt solutions are a widely used method for calibration, while unsaturated salt solutions and mixing dry and moist air can also be used. Professional calibration is necessary for tightly regulated environments.
Humidity calibration involves checking the accuracy and correct adjustment of the sensors that measure the moisture content in the air. Such measurements are of paramount importance in many laboratory and manufacturing processes. Especially difficult is accurate calibration for relative humidity (RH), the amount of moisture in the air relative to the maximum it can hold at a given temperature. A small change in ambient temperature can skew the reading even when the moisture content remains the same. Proper humidity calibration of monitoring devices helps ensure that ambient conditions will remain within required specifications for a reliable period of time.
Hygrometers are devices used to measure humidity. Most of these measure a quantity such as temperature, barometric pressure, or changes in electrical properties that are sensitive to change in humidity. Typically, these measurements indicate a moisture value relative to a standard reference. A difficulty in calibration is the generation of an accurate and stable relative humidity outside a special laboratory.
A convenient and widely used method of humidity calibration involves saturated salt solutions. Data was collected on the properties of 28 different salt solutions covering the full range of RH values. In this process, the bottom of an airtight box is coated with a saturated salt solution which generates a relative humidity in the space above with reasonable accuracy. The RH value depends on the salt used and is largely independent of the temperature. However, uniformity of temperature inside the box is required.
Unsaturated 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 like in the method above, the measuring device is supported in the space above the moisture calibration solution. Once the desired RH value is generated, the device reading can be noted and adjustments made.
In another method, a stream of dry air at 0% relative humidity is directed into two streams. 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 gases. While this method allows the humidity to be calibrated at a range of levels, it is itself dependent on the MFC being properly calibrated.
Implementations of the saturated salt method are often sold in kit form to companies without the resources to set up an in-house laboratory or contract with an instrument calibration company. However, pharmaceutical companies, microelectronics manufacturers and research laboratories depend on tightly regulated environments. At these sites, professional humidity calibration is standard procedure and regularly implemented.
Protect your devices with Threat Protection by NordVPN
