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RLC Engineering, LLC.

Determining Relative Humidity

Relative Humidity is a characteristic of air. When the relative humidity is high, the air feels clammy or muggy. We may sweat more and have a hard time staying cool. When the relative humidity is low, our lips get chapped, skin dries and cracks, and things contacting our skin feel a little rougher. Other living (or once-living) things like plants, insects, mold and wood also react to changes in relative humidity.

Relative humidity is a ratio or the "relative" amount of moisture in the air, as compared to the maximum amount of moisture the air can hold at that temperature. For example, air with a relative humidity of 50% contains one half the amount of moisture the air can hold at that temperature. Warmer air can hold more moisture than colder air, so when air is warmed, its relative humidity decreases. Air a 70 degrees and 50% RH will have a RH of only 25% when heated to 90 degrees. Conversely, the 70 degree, 50% RH air will have a 100% relative humidity when cooled to 50 degrees. Click here to see a sample of how relative humidity varies with temperature.

Controlling relative humidity in a building is important because of its affects on both the structure and living organisms within the building. People are most comfortable in a relative humidity of 30-60% while mold grows best at relative humidity over 80%. Dust mites like a relative humidity over 60%, and can be controlled by maintaining a RH below 50%. Wood expands at higher RH, and shrinks at low RH. Situations where hardwood floors cup, warp or gap are usually caused by RH problems. An ideal situation would be a stable indoor relative humidity between 30 and 50% all year long.

Determining relative humidity: Several methods are available for determining relative humidity. The easiest method is to purchase a digital hygrometer from some place like Radio Shack. These devices cost less than $30, and often provide temperature as well as RH readouts. Those these devices are not the most accurate, they can show trends and indications of potential problems. They can be calibrated for more accurate readings. To calibrate, fill a soup bowl 2/3's full with tap water. Add table salt until no more salt dissolves, and some salt crystals remain in the bottom of the bowl. Place the bowl of salt water and the hygrometer in a plastic bag, seal it shut, and read the RH after about 6 hours. The reading should be 75% RH. If not, add or subtract the difference as a calibration factor. For example, if your hygrometer reads 70%, you should add 5% to all readings. If the reading was 85%, you should subtract 10% from all readings.

A second method for determining relative humidity is by using a simple thermometer, glass of ice water and a psychometric chart. (Or you can use my RH calculator.) Step 1. Measure the temperature of the air with the thermometer. Step 2. Fill a drinking glass 2/3's full with tap water. Add a few ice cubes and stir with the thermometer. Step 3. Watch the outside of the glass as you stir. As soon as you see condensation (a haze) form on the outside of the glass, read the thermometer. Step 4. Enter the two temperatures in the calculator or use them on a psych chart. Click here to calculate the relative humidity from your readings.


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Last Update: 01/21/2002