Characteristics of humidity sensors
From ASHRAE 1997, with permission
Type of Sensor |
Sensor Category |
Method of Operation |
Approximate Range |
Some Uses |
Approximate Accuracy |
Psychrometer |
Evaporative cooling |
Temperature measurement of wet bulb |
32 to 180°F |
Measurement, standard |
±3 to ±7% rh |
Adiabatic saturation psychrometer |
Evaporative cooling |
Temperature measurement of thermodynamic wet bulb |
40 to 85°F |
Measurement, standard |
±0.2 to ±2% rh |
Chilled mirror |
Dew point |
Optical determination of moisture formation |
-110 to 200°F dp |
Measurement, control, meteorology |
±0.4 to ±4°F |
Heated saturated salt solution |
Water vapor pressure |
Vapor pressure depression in salt solution |
-20 to 160°F dp |
Measurement, control, meteorology |
±3°F |
Hair |
Mechanical |
Dimensional change |
5 to 100% rh |
Measurement, control |
±5% rh |
Nylon |
Mechanical |
Dimensional change |
5 to 100% rh |
Measurement, control |
±5% rh |
Dacron thread |
Mechanical |
Dimensional change |
5 to 100% rh |
Measurement, control |
±7% rh |
Goldbeaters skin |
Mechanical |
Dimensional change |
5 to 100% rh |
Measurement, control |
±7% rh |
Cellulosic materials |
Mechanical |
Dimensional change |
5 to 100% rh |
Measurement, control |
±5% rh |
Carbon |
Mechanical |
Dimensional change |
5 to 100% rh |
Measurement, control |
±5% rh |
Dunmore type |
Electrical |
Impedance |
7 to 98% rh |
Measurement, control |
±1.5% rh |
Type of Sensor |
Sensor Category |
Method of Operation |
Approximate Range |
Some Uses |
Approximate Accuracy |
Ion exchange resin |
Electrical |
Impedance or capacitance |
10 to 100% rh |
Measurement, control |
±1.5% rh |
Porous ceramic |
Electrical |
Impedance or capacitance |
Up to 400°F |
Measurement, control |
±1 to ±1.5% rh |
Aluminum oxide |
Electrical |
Capacitance |
5 to 100% rh |
Measurement, control |
±3% rh |
Aluminum oxide |
Electrical |
Capacitance |
-110 to 140°F dp |
Trace moisture measurement, control |
±2°F dp |
Electrolytic hygrometer |
Electrical |
Capacitance |
|
|
|
Coulometric |
Electrolytic cell |
Electrolyzes due to adsorbed moisture |
1 to 1000 ppm |
Measurement |
|
Infrared laser diode |
Electrical |
Optical diodes |
0.1 to 100 ppm |
Trace moisture measurement |
±0.1 ppm |
Surface acoustic wave |
Electrical |
SAW attenuation |
85 to 98% rh |
Measurement, control |
±1% rh |
Piezoelectric |
Mass sensitive |
Mass changes due to adsorbed moisture |
-100 to 0°F |
Trace moisture measurement, control |
±2 to ±10°F dp ±1 to ±5 K dp |
Radiation absorption |
Moisture adsorption |
Moisture absorption of UV or IR radiation |
0 to 180°F dp |
Measurement, control, meteorology |
±4°F (±2°C) dp, |
Gravimetric |
Direct measurement of mixing ratio |
Comparison of sample gas with dry airstream |
120 to 20 000 ppm |
Primary standard, research and laboratory |
±0.13% of reading |
Color change |
Physical |
Color changes |
10 to 80% rh |
Warning device |
±10% rh |
Notes: 1. This table does not encompass all of the available technology for the measurement of humidity 2. The approximate range for the device types listed is based on surveys of device manufacturers 3. The approximate accuracy is based on manufacturers data. 4. Presently, the National Institute of Standards and Technology (NIST) will only certify instruments whose operating range is within -75 to 100°C dew point