How to Calibrate a Hygrometer

We want to assume that when we take medication, fly in an airplane, eat food, use chemical products or perform other normal tasks that the products and processes we use are safe. But it’s a dangerous world out there and that is why industries like pharmaceutical, semiconductors, chemicals, aerospace, food processing and the like have regulations in place to ensure the quality of their products and processes. These regulations typically require regular measurements of many parameters including temperature and humidity.

Why is it important to measure humidity?

Maintaining the correct temperature and humidity helps to keep products from spoiling. Products can spoil for a number of reasons, including:

• Condensation
• Bacteria and germ growth
• Mold
• Electrostatic discharge
• Corrosion
• Warping

To prevent these problems, industries with products at risk of spoilage install instruments to measure humidity and temperature continuously through the manufacturing process and also while products are stored.

What types of instruments measure humidity?

Instruments that measure humidity include but are not limited to:

• RH probes, meters and transmitters
• RH monitors and wall-mounted transmitters
• Humidity data loggers
• Digital thermometer hygrometer
• Psychrometers (web/dry bulb)
• Metal-paper coil hygrometers
• Hair tension hygrometers

Why calibrate humidity?

Instruments that measure humidity must be calibrated regularly to make sure they are performing within their manufacturers’ specifications. Calibration lets you know that measurements made with these instruments are accurate so that quality can be maintained.

How to calibrate a hygrometer or other humidity sensor

To calibrate humidity, the humidity generator generates and measures a variety of temperatures and relative humidity levels. These levels are compared with the measurements of the device under test (DUT). The difference between the two measurements lets you calculate the uncertainty of the DUTs measurements.

The calibration process described here uses a mixed-flow humidity calibrator to generate and measure the humidity. This type of instrument has a chamber in which air is circulated through both a drier system and a humidifier system. A controller determines how much dry air and moist air to mix in the chamber. A calibrated chamber sensor determines the chamber's humidity and temperature. This type of generator performs the calibration quickly and accurately.

1. Fill the humidity generator with distilled water according to the manufacturer’s directions.
2. Insert the device(s) under test (DUTs) into the chamber of the humidity generator. Depending on the type of sensor you are calibrating, you might be able to place it entirely inside the chamber or insert just the sensor portion through a port hole that keeps it in place.
3. Remove any existing condensation from the chamber.
4. Set the chamber to the desired temperature and humidity set point.
   1. Set the lower relative humidity points first and ramp up to the higher levels on each subsequent calibration step.
   2. Set the lower temperature calibration points first to minimize the possibility of water vapor condensing inside the chamber.
   3. In general, best practice is to start at the lowest dew point and then move up.
5. Wait for the chamber and DUT to stabilize.
6. Record the actual temperature and humidity displayed on the calibrator.
7. Record the reading on the DUT(s).
8. The difference between the calibrator’s readings and the DUT(s) readings tell you how much your DUT is “off.” If the difference it too great your sensor may need to be adjusted or repaired.

Using a chilled-mirror probe or hygrometer as a reference

If your humidity calibration workload requires a high level of accuracy, consider using an external reference to get the best measurement by comparing the reading on the DUT with the reference instead of with the generator’s display (we recommend 0.5 °C dew point uncertainty). It’s common to use a chilled mirror probe (also called a chilled mirror hygrometer) as the reference. A chilled mirror probe is a humidity transfer standard in which a polished metal surface is chilled until water begins to condense on it. At this point the dew point is defined; an optical detection system measures the reflected light from the mirror’s surface. During the calibration the DUT’s reading is compared to the highly accurate dew point humidity on the chilled mirror probe.

Here are three ways to configure a calibration with a chilled-mirror hygrometer.

Remote head. In this method the chilled mirror sensor head is inserted into the humidity generator’s chamber. This method reduces the risk of condensation and an external pump is normally not required.

Remote head with pumped air. Draw a gas sample from the humidity chamber to the chilled mirror sensor by inserting a tube inside the generator. Note: when drawing gas out of the chamber, the dew point of the gas must be lower than ambient temperature; otherwise a heated tube must be used to prevent condensation from forming in the tube.

Pump style. Pump air from the humidity generator’s chamber to the chilled mirror and back again. Again, watch for condensation in the tubing by keeping the dew point of the gas lower than ambient temperature.

Tips for ensuring a successful humidity measurement

• Separate the measurement area from ambient.
• Don’t allow air leaks between the inside and outside of the calibrator’s chamber.
• Ensure that you immerse your sensors properly.
• Don’t allow condensation to form.
• Wait long enough for the humidity generator to become stable and uniform.
• Correctly handle materials such as plastics, wood and foam that absorb and emit water vapor.
• Use materials like Teflon™ nonstick coating and stainless steel for making humidity measurements.
• Avoid exposing the DUT or calibrator to volatile chemicals.
• Keep the calibrator’s humidity chamber clean.
• Use only distilled water in the calibrator.
• Keep the door on the calibrator’s humidity chamber at all times.
• Monitor for drift using an external check standard. A chilled mirror hygrometer is often used for this purpose.

Learn more

• If you’d like more information about humidity and humidity calibration, I’ve recorded a webinar called “How to Calibrate Humidity Sensors.” You can download and watch it any time.

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