How to Calibrate a Hygrometer

How to Calibrate a Hygrometer

A hygrometer is an instrument used to measure relative humidity. Relative humidity is a measure of how much water the air is holding, relative to the maximum amount of water the air could possibly hold at a given temperature. 

Because of the delicate design of hygrometers, it is easy for them to become inaccurate (e.g. with shipping or just over time). A simple method of calibrating your hygrometer will help ensure you are measuring the relative humidity as accurately as possible. Hygrometers should be calibrated yearly.

Hygrometers are available in both digital and dial models, and both should be checked for calibration. Some dial types will have a small screw or adjustment on the back so you can adjust them after calibration. If there is no way to adjust them, make a note of the calibration result, and then you can simply add or subtract from the actual reading to get a more accurate humidity level (this is discussed in greater detail after the calibration process is explained). 

Tip: give a hygrometer about 2 hours to stabilize before taking a reading, as changes in the relative humidity may take a while to register accurately on a hygrometer.

This series of diagrams will show you how to calibrate a hygrometer. The last page in the series provides a summary of the steps that you can print out for reference.

To calibrate a hygrometer you will need:

• 1/2 cup table salt
• approximately 1/4 cup water
• coffee cup
• hygrometer
• large re-sealable freezer bag

1. Place 1/2 cup of salt in the coffee cup, and add the water. Stir for a bit to totally saturate the salt (the salt won't dissolve, it will be more like really wet sand).

2. Place the salt/water mix in a re-sealable plastic bag, along with the hygrometer, and seal the bag. Note: make sure none of the salt/water mix comes in direct contact with the hygrometer.

3. Let this bag aside at room temperature for 8-12 hours, in a location where the temperature is fairly constant.

4. After 8-12 hours, check the reading of the hygrometer. It is best to read it while still in the bag.

The relative humidity in the sealed bag with the salt/water mix should be 75 percent (mine read about 72 percent).

5. For adjustable hygrometers, adjust to read 75 percent. You will have to do this very quickly, or remember how much you need to adjust the setting (e.g. mine read 72 percent rather than 75 percent, so I would need to adjust the dial up 3 percentage points).

If yours is not adjustable (like mine), simply make a note of how "off" your hygrometer reads. If it reads below 75 percent, you will need to add the difference to your actual readings. If your hygrometer read above 75 percent on the calibration, you will need to subtract the difference from your actual reading. 

In my example: after sitting in the bag, my hygrometer read 72 percent, when it should have read 75 percent -- a difference of 3 percent. I now add 3 percent to the readings I take on the hygrometer (e.g. in a tank) to get the actual relative humidity.

Remember: always give a hygrometer about 2 hours to stabilize before taking a reading, as changes in the relative humidity may take a while to register accurately on a hygrometer.

Reference: http://exoticpets.about.com/od/herpresources/ss/hygrometer.htm