A DEW POINT HYGROMETER USING A SYSTEM OF PRIMARY MEASUREMENT
Franklin W. Kirk and Nicholas R. Rimboi, Instrumentation, Third Edition, American Technical Society, 1975
The typical instrument for measuring dew point is shown [at right]. It uses a gold-plated mirror surface which is bonded to a copper themistor holder. This assembly is chilled by a Peltier effect thermoelectric cooler. (The Peltier effect is discussed in Chapter 10.) The air or other gas being measured for dew point is passed by the mirror. A neon lamp is beamed on the mirror which reflects the beam toward a photoelectric resistor. As dew forms on the mirror and clouds it, there is a change in the amount of light reflected. This change is dected by an optical sensing bridge.