What is Humidity Calibration?

Humidity affects many properties of air and the materials that are exposed to it. Monitoring and measuring humidity is important wherever there is a need to prevent condensation, corrosion, mold, warping or spoilage of products. For example, it’s important to measure humidity in industries that manufacture and distribute foods, pharmaceuticals, chemicals, fuels, wood, textiles, and paper. Museums, art galleries, data centers, hospitals, research laboratories, and semiconductor manufacturers are other types of organizations where measuring humidity is vital.

The sensors that measure humidity need to be calibrated regularly to ensure they continue to operate within their specifications. Humidity sensor calibration is typically done using an instrument called a humidity generator.

Most humidity generators are based on one of two designs. A mixed-flow generator controls humidity by using the split-stream method. In this method, dry gas is drawn into the generator and divided into two parts. One part is partially or completely saturated with water vapor; the other part is mixed in until the targeted humidity output is reached. The humidity depends on the wet air’s humidity and the mixing ratio. A high-volume fan in the test chamber keeps the temperature and humidity uniform.

A two-pressure generator has two chambers. The first, called the saturator, contains air saturated with water vapor at a high pressure. The air passes from the saturator to the second chamber, called the test chamber. The test chamber is at a lower pressure. As the air passing into the test chamber reduces in pressure, its relative humidity also drops.

A mixed-flow generator is faster than a typical two-pressure generator, and is also more portable, which makes this type of calibrator a good solution for work in the lab or on-site in the field. 

Selecting a Solution

For measuring humidity, you’ll want a dependable, accurate solution that’s convenient and easy to use. The ability to monitor temperature as well as humidity, or barometric pressure and humidity, allows some instruments to provide a single solution. Do you need to monitor humidity in multiple locations? If so, the ability to network the measurement instruments together and analyze data on a computer can be helpful.

Other nice-to-have features include the ability to view data in real time in both statistical and graphical views; a front panel that can be viewed from a distance; the ability to export data to a spreadsheet for analysis.

For calibrating humidity, look for a dependable solution that maintains stability and uniformity over time– two key specifications that impact the quality of a humidity probe calibration. Ask how fast a humidity calibrator stabilizes, because that affects how long you have to wait before beginning the calibration.

Performing a thorough calibration is important. You can perform a spot-check or one-point probe calibration in the field with a handheld humidity meter, but its value is limited. Calibration with a handheld meter needs to be carefully managed. Temperature differences between the probe and its environment, technician body heat, and even moisture from breath can all cause RH measurement errors. Furthermore, one-point tests can cause out-of-tolerance readings when ambient conditions change.

Consider your workload and where you will calibrate it. Will you work primarily in a controlled laboratory environment? On-site in a field environment? Or both? If your work takes you to a variety of environments, consider a solution that works in all of them to save equipment cost and, so your technicians don’t have to learn different instruments for each environment.

The amount of workload you calibrate is important too. Some units only calibrate one humidity sensor at a time, while others can calibrate multiple probes at once.

Any time you make a capital investment you need to consider post-sale service and support. It’s best to work with a company that stands behind its products, as well as one that you feel confident will remain in business for many years to come.