Ask Jon Eakes

Last Updated: Wednesday, September 15th, 1999, Created: Wednesday, September 15th, 1999

Calibrating your Hygrometer

Yes you can check if your hygrometer or humidistat is giving you the right readings. Mix into a glass, one part salt to two parts water -- like 1/4 cup salt to 1/2 cup water. Mix it thoroughly. Now put this glass and the hygrometer into a sealed plastic bag at 22 degrees C, room temperature. Wait for 12 hours and the atmosphere inside the bag will be exactly 75% RH, Relative Humidity.

If your gauge doesn't say 75% and you can adjust it, put it to 75. If you can't adjust it, like an electronic humidistat, then tape a note right next to the read-out that tells you to add or subtract however much it is off. Now you will know what the humidity really is in the house.

But the big catch is that the Relative Humidity will change all over the house. It is not the same at the floor as it is at the ceiling, and it is not the same on the window sill when the sun is shining or when it is dark. RH is very dependent on temperature. If you are trying to spot problem areas, put your hygrometer right in the problem area, but not in the sun.

Keywords: Environmental, Humidity, Measuring

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