The Chilled Mirror hygrometer: How It Works, Where It Works-and Where It Doesn'tMay 1, 2005 By: David J. Beaubien Sensors
Chilled mirror hygrometers, with their simple principle of operation, wide measuring span, high level of precision, and inert construction, are widely used in industrial control systems.
The chilled mirror hygrometer (CMH) has been used as a NIST-referenced humidity transfer standard in metrology laboratories for the past four decades. Its principle of operation is simple and elegant. Its totally inert construction, wide measuring span, and high precision have led to thousands of industrial control installations. The proper application of any process monitoring device, and especially a humidity sensor, first requires a good understanding of how the sensor works and its advantages as well as its limitations. That is what this article is all about.
How They Work, and a Look Inside
CMHs make a direct measurement of the dew point temperature of a gas by allowing a sample of gas of unknown water vapor content to condense on an inert, chilled, mirror-polished metal surface. Thermoelectric modules are typically used to chill the surface. A beam of light, usually from an LED, is reflected from the surface into a photodetector (see Figure 1). There are a variety of other types of condensate detecting schemes, but light reflection from a mirrored surface is the classic method.
Figure 1. Chilled mirror hygrometers (CMHs) detect dew point by cooling a reflective condensation surface until water begins to condense. The condensed fine water droplets are detected optically by components such as shown here.
With a properly designed feedback system, the mirror is maintained at the temperature at which the rate of dew condensation exactly equals the rate of the dew layer's evaporation. In this state, the mass of the dew layer is neither increasing nor decreasing, and the deposit is in dynamic equilibrium with the water vapor pressure of the surrounding gas sample, thus defining the dew point temperature of the sample. Under such conditions, the surface temperature of the metallic condensation surface represents the saturation temperature for the water vapor in the gas under measurement. A second detector is sometimes used to monitor the polarization of the scattered light, and allows automatic determination of the phase of the condensate, i.e., dew point or frost point.
A typical CMH, in contrast to many other humidity sensors, can be made very inert, rendering it virtually indestructible and minimizing the need for recalibration. A full-range dew point sensor is capable of handling dew points from >100°C to as low as –70 °C The gas sample contacts only inert materials: a glass or quartz lens, a Teflon O-ring, and a stainless steel housing and metallic condensation surface. Among the inert mirror materials are gold, chromium-plated silver or copper, and titanium nitride. A common design for the mirror for use in harsh industrial applications is either a copper or silver mass for the mirror, covered by a thin, polished stainless steel sheath (see Figure 2).
Figure 2. In a typical CMH, the gas of interest contacts only inert materials: a glass or quartz lens, a Teflon O-ring, and a stainless steel housing and metallic condensation surface.
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