How to Adjust a Hygrometer

It takes two thermometers to adjust a hygrometer.

Hygrometers are used to measure relative humidity (RH). Over time, hygrometers can become inaccurate and need to be adjusted or calibrated. The most accurate way to adjust or calibrate a hygrometer is to use a psychrometer to get an accurate RH reading. Accurate results can be calculated using a simple, sling psychrometer. Digital aspirated psychrometers can also be used. A psychrometer is a device that consists of two thermometers. One records room temperature, and the other records the dew point for that temperature.

Things You'll Need

• Sling psychrometer

Instructions

Using a Sling Psychrometer

1. 

   Dew forms when relative humidity is at or near 100 percent.

   Saturate the wick on the wet bulb of the sling psychrometer, preferably with distilled water (since some minerals, like salts, can alter evaporation). Psychrometers have two thermometers, a dry bulb and a wet bulb. The wet bulb is wrapped in gauze (the wick) or has some other mechanism to wet the thermometer. Relative humidity (RH) is shown by the difference between the temperatures of the dry and wet bulb thermometers (difference between air temperature and dew point).

2. Whirl the psychrometer using the sling handle until the temperatures quit falling. Alternately, you can use a fan to blow on the thermometers.

3. Record immediately the temperature readings of the two thermometers and subtract the wet bulb reading from the dry bulb reading.

4. Take the dry bulb temperature, plus the difference between it and the wet bulb temperature, and use a Relative Humidity chart (such as the one provided by the Iowa Department of Transportation) to find the RH.

5. Increase accuracy by taking two or more readings with the psychrometer, then compute the average of the readings.

6. Read the RH on your hygrometer in the same location and at the same time and compare the RH from the psychrometer. Adjust the hygrometer as needed. Many hygrometers (especially inexpensive ones) will not have a function to adjust or calibrate the RH. To get accurate readings with hygrometers that cannot be adjusted, simply note the difference and add or subtract it from the hygrometer reading.

Other Methods

1. Use a digital, aspirated psychrometer if your budget allows for purchasing one. This type of psychrometer is the easiest to use, but accurate ones are expensive. The digital psychrometer will automatically calculate RH, but it may also need to be calibrated from time to time. It will produce inaccurate results if the battery is weak.

2. Send the hygrometer to the manufacturer for calibration. Certain complex types of hygrometers, such as dataloggers, can be calibrated using computer software, but many need to be sent to the manufacturer for proper calibration.

3. Take your hygrometer to a location where the RH is known. For example, a museum storage room in which hygrometers are routinely calibrated and constantly monitored will have a known RH. A hygrometer can be placed inside such a room for a few minutes, and then calibrated based on the difference between the room's RH and the hygrometer reading.

Tips & Warnings

• Adjust and calibrate hygrometers at least once a year.

• A sling psychrometer can be made with easy-to-acquire supplies (two thermometers, tape, gauze, rubber band and a fan).

• Inaccuracies can occur with a sling psychrometer as a result of stopping the “slinging” too early before the true wet bulb temperature can be established, or waiting too long after slinging before recording the temperature readings.

References

• Northeast Document Conservation Center: Preservation 101 Monitoring the Environment
• Iowa Department of Transportation: Determining Relative Humidity with a Sling Psychrometer
• “The Museum Environment;” Garry Thomson; 1986
• Society for Historical Archaeology: How do I monitor temperature and relative humidity?
• Miami Museum of Science: Make a Sling Psychrometer