How do you calibrate a hygrometer?

First, you should check that the hygrometer has a calibration function, since some analogue and even some digital hygrometers do not have this function. Analogue hygrometers that can be calibrated normally have a screw on the back which is used for calibration. For digital hygrometers, consult the manufacturer's instructions. As described in Which is the best type of hygrometer?, analogue hygrometers are particularly sensitive to vibration and must be calibrated regularly.

The following common calibration methods can be used for both analogue and digital hygrometers:

• Salt Method: Fill a container with water so that the bottom is covered by about 5 millimetres. Then make a saturated saline solution i.e. approximately 40 grams of salt per 100 mL of water. Place a glass with the opening facing down into the container and then place the hygrometer on it. Seal the container and leave it for about four hours. It is of paramount importance that the temperature remains constant during this process. The temperature should be between 19 and 21°C. This results in a 75% RH level inside the container. For the next step you will need to be quick: Place the hygrometer in your hand and immediately adjust the hygrometer to read 75%. If you take too long in setting it to 75% it will not be exact. The longer you take and the more sensitive your hygrometer is to fluctuations, the less accurate will be the result of the calibration.
  


• Wet Cloth Method: The following method is not quite as accurate, but it is faster and easier than the salt method. The reference value here is 100% relative humidity. This is simulated with a wet cloth which is wrapped around the hygrometer. Leave the hygrometer wrapped in the cloth for an hour and then adjust the humidity to 100%.