Reviewed and Revised on 10/15/2013
Indoor air humidity is generally measured in terms of relative humidity by an instrument called hygrometer, or relative humidity meter. Relative humidity measures the percent amount of water vapor present in the air to the total amount of water vapor it could actually hold at the given temperature. The hygrometers can have either a dial or a digital readout. Digital hygrometers are not always more accurate. When purchasing a hygrometer, check the operating range, because electronic hygrometers may have a minimum relative humidity level they can read (e.g. 20%).
Calibrate for Accuracy
All hygrometers require calibration to increase their level of accuracy. To calibrate a hygrometer follow these steps:
1) Obtain an airtight container that is at least 3 times the size of the hygrometer. Examples of airtight containers include a plastic bag with a zip-type seal, a food storage container with a tight-fitting lid, or a coffee can with the original lid.
2) Place a water-filled cup in the sealed container along with the meter for 4-6 hours, or until water droplets are visible on the inner surface of the container (i.e., plastic bag). When the droplets begin to accumulate on the edge of the sealed container, this indicates a relative humidity level of 100%. The reading on the hygrometer should be at least 95%, but preferably 100%. Take note of the reading.
3) Next add table salt to the cup of water while stirring until the water cannot dissolve any more salt. Salt should be sitting on the bottom of the cup. Then place the cup back in the sealed container with the meter and let them sit again for 2-3 hours. The salt reduces the ability of the water to evaporate, and therefore, the humidity level. A salt solution should generate a humidity reading of 75%, but readings from 70-80% are acceptable.
4) Compare the 2 readings. If they are both different by the same amount (i.e, percentage), you can recalibrate your hygrometer by that amount. Check the owner's manual for specific instructions for calibrating your unit. If your unit does not have the ability to be calibrated, then you can adjust the readings mentally.