The Masy Blog

Humidity Calibration and The Importance of Chamber Stabilization

Estimated reading time: 3 minutes

Measuring, monitoring, and calibrating humidity is critical in many industries and processes. Pharmaceutical manufacturers, research laboratories, and museums are just a few of the industries that employ humidity sensors as an integral part of their daily operations. These vital sensors must be calibrated regularly to ensure their accuracy.

RH calibration requires specific and stable air conditions for accurate readings. Temperature and humidity conditions typically change slowly in air and are easily affected by outside factors. This slow rate of change means that a calibration chamber, even a chamber with the most current technology, takes time to stabilize and reach equilibrium before a technician can take any reading. It may take a humidity chamber anywhere from ½ hour to several hours to reach this equilibrium, depending on the conditions required.

At Masy BioServices we use Thunder Scientific Humidity Generators to perform relative humidity calibrations. The Thunder 2500 Humidity Generator is a primary standard and operates as a two-pressure generator. This system works by producing known humidity levels by saturating air at a given pressure and then isothermally bringing the saturated air to chamber pressure. The measurement of the water vapor is solely dependent on the measurements of pressure and temperature.

The two-pressure system of humidity calibration was originally developed by NIST and is the primary standard for humidity calibrations. Depending on the number of calibration points requested, a calibration performed using this two-pressure system will take anywhere from 2 hours to 4 days to complete.

This system makes it much easier to complete high accuracy, traceable, consistent temperature and humidity calibrations, compared to the more traditional salt bath method, which involves unique techniques with inconsistent and inaccurate results, and no uncertainty data.

Data collection for ‘As Found’ and ‘As Left’ data points will typically be taken once the device under test has achieved stability at the defined setpoint. An instrument often requires calibration at several different set points. The chamber must be allowed to re-stabilize at each new test point before taking each reading.

Is there a way to speed up humidity calibration and still get an accurate calibration? No. When it comes to Humidity calibration, there is no substitute for taking the time to let the chamber come to equilibrium. By allowing the appropriate amount of time for proper chamber stabilization, the accuracy and reliability of the calibration is increased. Stabilization is a critical step in humidity calibration.

At Masy we confidently complete SI traceable primary RH calibrations (within 10 to 95%RH @ 0oC to 70oC) with uncertainties as low as 0.5% RH. You can see our scope here.  We have 7 Thunder Scientific Humidity chambers available in our NVLAP accredited laboratory, operated by our experienced calibration technicians. Masy’s expertise, experience and state-of-the art equipment allows us to provide efficient, high-accuracy humidity calibrations.  If you have questions about the humidity calibration process, or a question about your specific calibration requirements, please contact our calibration team. We are always happy to answer your questions and help you determine the right process for your calibration needs.

For more information about Masy’s calibration services, you can visit us at or read this article about how our primary standards laboratory supported a local calibration lab with their critical equipment calibration.