Understanding Relative Humidity and the Hygrometer

Humidity is the amount of moisture or water vapor present in the air. "Relative Humidity" is the ratio of water vapor present in a given volume of air at a given temperature to the most water vapor the air can hold, expressed as a percent. So 40% relative humidity means 40% saturation of the air.

How does humidity relate to your comfort?

Whether we feel warm or cold is determined (among other factors) by the rate at which moisture is evaporated from our bodies. On a humid summer day, we feel "uncomfortable" because there is so much moisture in the air that evaporation from our skin takes place very slowly. When the air is "dry", evaporation is more rapid and we feel cooler.

In winter, the air in our homes tends to be very dry which means rapid body evaporation and so we "feel cold". Example, 72 degrees is a comfortable temperature only if we have appropriate relative humidity. if humidity is extremely low, we may not feel warm at 80 degrees.

The table below equates the percent relative humidity to a positive temperature comfort level.

Keep in mind that high indoor humidity with very low outdoor temperatures can cause condensation harmful to the home. Icing on windows and sills indicates excessive indoor humidity.

This chart shows recommended maximum humidity to maintain at lower outdoor temperatures.



Indoor Humidity

30° ... 40%
20° ... 35%
10° ... 30%
... 25%
-10° ... 20%
-20° ... 15%

A reminder that your hygrometer indicates the humidity of room air where located and has no relation to outdoor humidity announced on weather reports.

The "hair hygrometer" offers high sensitivity to humidity by relying on the minute expansions and contractions of a hair. The "spiral hygrometer" has an internal, spiral metal strip which reacts to humidity.

Calibrating your hygrometer.

Calibrating your hygrometer depends on your model. In some, a small screwdriver is used on the back panel's set screw to change the indicator reading.

Hygrometer back showing movement setting
In other models, the entire hygrometer movement is rotated until the dial reads appropriately. Manufacturer's instructions for other configurations would apply.

Testing hygrometer accuracy.

You can test an hygrometer's accuracy in two common ways:

1. Wrap a damp cloth on the back of the hygrometer for 6 hours. The indicator should then read approx. 95%. If it reads lower or is off scale completely, merely adjust the indicator to read 95% using one of the methods described above. 

2. Another way to check calibration uses table salt, a see-through container such as a Ziplock baggie, and a small shallow open container, like a bottle cap.
Place a teaspoon or so of salt in the shallow container and add a few drops of water - just enough to get it wet. Remember, you don't want a salt water solution, just damp salt. Place the shallow container in the baggie along with the hygrometer. (Make sure the salt does not get on the hygrometer, this may damage it).
Seal the baggie with some air trapped inside and let it sit for 6 to 8 hours. Check the hygrometer reading without opening up the baggie. It should read 75%. If not, remove the instrument from the baggie and adjust the reading to 75% with a small screwdriver.
To maintain accuracy of the instrument, it is advisable to recalibrate every six months.