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
0 to 80°C

Measurement, standard

±3 to ±7% rh

Adiabatic saturation psychrometer

Evaporative cooling

Temperature measurement of thermodynamic wet bulb

40 to 85°F
5 to 30°C

Measurement, standard

±0.2 to ±2% rh

Chilled mirror

Dew point

Optical determination of moisture formation

-110 to 200°F dp
-75 to 95°C dp

Measurement, control, meteorology

±0.4 to ±4°F
±0.2 to ±2°C

Heated saturated salt solution

Water vapor pressure

Vapor pressure depression in salt solution

-20 to 160°F dp
-30 to 70°C dp

Measurement, control, meteorology

±3°F
±1.5°C

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

Goldbeater’s 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
at 40 to 140°F
(5 to 60°C)

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
at -40 to 190°F
(-40 to 90°C)

Measurement, control

±1.5% rh

Porous ceramic

Electrical

Impedance or capacitance

Up to 400°F
Up to 200°C

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
-80 to 60°C dp

Trace moisture measurement, control

±2°F dp
±1°C 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
-75 to -20°C

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
-20 to 80°C dp

Measurement, control, meteorology

±4°F (±2°C) dp,
±5% rh

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