Understanding a few common terms will make your hygrometer readings more useful when interpreting the moisture information. Hygrometers use mechanical and electronic technologies to determine moisture content. Psychrometers or wet bulb/dry bulb hygrometers are often used to measure humidity in HVAC environments in homes and businesses, and sling psychrometers are attached to ropes and whirled around to take humidity measurements in outdoor and open indoor spaces. Others terms of interest include:
- Relative humidity
Most hygrometers measure relative humidity or the percentage moisture that air can hold at certain temperatures and pressure. Increasing pressure allows air to hold more than 100 percent, but normal atmospheric pressure limits the amount of water vapor a given volume of air can hold.
- Absolute humidity
Mathematical formulas can convert water vapor in a given volume of air at a certain temperature and pressure into an absolute quantity of water in grams per cubic meter or kilogram of air.
- Dew point
The dew point is when air at a certain temperature becomes saturated, which results in water condensing on objects. These objects include plants, mirrors, automobiles and glass. Some hygrometers use mirror surfaces to measure condensation to determine relative humidity.
- Wind chill index
Wind makes moisture evaporate more quickly, which increases the chill factor. In winter, cold winds cause temperatures to feel significantly colder than the actual temperature.
- Heat index
Moisture that approaches maximum saturation makes heat feel hotter because moisture can’t evaporate as efficiently
Rain or snow occur for various reasons, but just because the air has 100 percent humidity doesn’t necessarily mean that rain is inevitable. The reading merely means that the air can’t absorb any more moisture at its current pressure and temperature. Meteorologists study air humidity at various levels within the atmosphere to make their weather forecasts. Winds, clouds and intersections of weather fronts influence how water will precipitate from the air.
- Capacitive and resistive sensors
Electronic sensors in some hygrometers use capacitive or resistive sensors that measure humidity by studying the electrical effects of moisture. Capacitive sensors use polymer constants to measure humidity changes in electric current by running electrical charges between two plates to test each plate’s ability to hold a change. Resistive hygrometers absorb water from moisture-laden air and measure sensor resistance to determine humidity.