requires a high-frequency excitation in alternating current (Campbell 1997) which
can be provided by modern data acquisition devices.
The time constant of temperature and moisture sensors is given in Table A1.3
(Foken 2017).
A1.6 Air Speed Measurement
Air velocity profiles are, mentioned in Chaps. 2 and 3, crucial for the processes of
heat transfer by convection and for the development of heat and mass flows in
environmental systems. Anemometers are the instruments that are used to measure
air velocity, with different types of anemometers, such as cup anemometers
(Fig. A1.11), helical anemometers, hot wire anemometers, anemometers of
dynamic pressure and, for the application of the turbulent covariance method, the
Fig. A1.10 Capacitive sensor
for measurement of air relative humidity and with a
radiation shield in installation
Table A1.3 Time constant of temperature and humidity measurement systems
Sensor
Time constant (seconds)
Sonic thermometer
<0.01
Optical humidity measurement system
<0.01
Thermocouples
<0.01
Thermistors
0.1–1
Resistance thermometers
10–30
Liquid in glass-thermometers
80–150
Annex A1: Instrumentation in Environmental Physics
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