Selecting the Right Humidity Sensor for Your Application

Introduction	There are a large variety of humidity sensors on the market. The task of selecting the most suitable humidity sensor from a cost and performance standpoint is not always an easy one. The range of humidity sensors available extends from the simple hair hygrometer, to a large variety of polymer sensors, to chilled mirror dew point hygrometers, to the exotic Lyman-alpha or infrared hygrometers. All have their place, but selecting the correct sensor for a particular measurement is often confusing. Yankee offers two of the most popular and widely used types of humidity sensors; the chilled mirror dew point hygrometer and the polymer-based relative humidity sensor. The chilled mirror hygrometer offers one of the widest range and highest precision humidity measurements. In addition, the sensor is virtually indestructible. The polymer relative humidity sensor is much less expensive, covers a narrower range and is less precise, but is adequate for a large category of day-to-day environmental measurements.
Why and when to use a Chilled Mirror Hygrometer	The chilled mirror hygrometer (CMH) dew point has several distinct advantages over other water vapor sensing technologies: A CMH provides one of the few truly direct physical measurements of humidity. It is recognized as the most precise method of determining the water vapor content of a gas above 5% RH. Use it when you need maximum precision of measurement. The CMH's optical sensor is a totally inert device. The sample gas contacts glass and non-reactive metals. Thus, it can be easily cleaned and can last indefinitely. Use it when the gas sample contains contaminants and particulates, which would damage other types of humidity sensors. Unlike polymer RH sensors, lithium chloride dew cells, and other chemically-based sensors, a CM sensor does not lose its calibration. Use it when you need long-term calibration stability. The dew/frost point temperature defines the saturation point for the water vapor in the gas. From this unique equilibrium temperature, all other reporting formats of gas humidity can be derived. With measurements of gas temperature and pressure, other reporting forms for humidity can be derived.
How a Chilled Mirror Hygrometer works	The measurement of the water vapor content of a gas by the dew-point technique involves chilling a surface, usually a metallic mirror, to the temperature at which water on the mirror surface is in equilibrium with the water vapor pressure in the gas sample above the surface. At this temperature, the mass of water on the surface is neither increasing (too cold a surface) nor decreasing (too warm a surface). In the chilled-mirror technique, a mirror is constructed from a material with good thermal conductivity such as silver or copper, and properly plated with an inert metal such as iridium, nickel, or gold to prevent tarnishing and oxidation. The mirror is chilled using a thermoelectric cooler until dew just begins to form. A beam of light, typically from a solid-state LED, is aimed at the mirror surface and a photodetector monitors reflected light. As the gas sample flows over the chilled mirror, dew droplets form on the mirror surface, and the reflected light is scattered. As the amount of reflected light decreases, the photodetector output also decreases. This in turn controls the thermoelectric heat pump via an analog or digital control system that maintains the mirror temperature at the dew point. A precision miniature platinum resistance thermometer (PRT) properly embedded in the mirror monitors the mirror temperature at the established dew point. If the mirror is controlled to an equilibrium condition above the ice point, i.e., 0° C, the sensor is measuring the dew point. Below 0° C, the deposit cannot long persist as liquid water, and it is assumed that the deposit is frost, and that the sensor is measuring the frost point. However, if the mirror is kept extremely clean, it is possible for dew to exist below 0° C, and the only true way to verify that the sensor is controlling on the frost point is to visually inspect the mirror via a microscope. However, especially outdoors, it is typically impractical to maintain a perfectly clean mirror, as contaminants such as spores and other particulates serve as motes on which frost deposits can nucleate. Consequently, errors due to dew/frost point confusion at 0° C are seldom encountered.
Range and precision of the Chilled Mirror Hygrometer	A chilled mirror hygrometer works by detecting the scattered light from a condensation layer that forms on a cooled mirror from a gas sample. To function properly, a chilled mirror hygrometer must be able to cool its mirror to the dew or frost point of the sample. The range of dew point temperatures that a chilled mirror hygrometer can measure is dictated primarily by the heat pumping capability of the solid state thermoelectric mirror cooler. The graph below shows the typical useful working span of hygrometers with single- and two-stage coolers. In a single-stage hygrometer such as the YES 2010 series, if the sensor is installed in an ambient temperature environment of +20° C, dew points (frost point below 0° C) can be measured down to about -25° C. Thus, the mirror can "depress" 45° C. Systems with two stage coolers, such as the YES 2020 series, offer greater dew point depression. A two-stage unit running at +20° C can measure frost points down to -45° C. Thus, the mirror can "depress" 65° C. Lower frost points can be measured by reducing the temperature of the hot side of the cooler, or by using 3, 4, or 5 stage coolers or via a customer-supplied mechanical chiller.
What degree of precision can I expect?	Over the years a variety of sensing technologies have evolved to measure the water vapor content of a gas including: Gravimetric train Aspirated (or sling) psychrometer Condensation type (chilled mirror) Capacitance-type electret Hair-type mechanical hygrometer Lyman Alpha hygrometer Surface Acoustic Wave type hygrometer LiCl saturated salt hygrometer The absolute accuracy of the condensation-type (chilled mirror) hygrometer is exceeded only by the gravimetric train, an elaborate absorption system maintained as the primary standard for humidity measurements. Over the mid-range of dew point temperatures (± 40° C), the uncertainties of measurement for the chilled mirror hygrometer are in the order of one part in 100, thus making it the most precise means of measuring water vapor available. This graph was originally developed by the Mechanical/Humidity Section at the National Bureau of Standards in the 1960's and shows typical uncertainties for several popular humidity sensor technologies.
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