Chilled Mirror Hygrometer Information
Principle of Operation
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, rubidium, 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 broadband light emitting diode, 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.
Why select a Chilled Mirror Hygrometer?
The chilled mirror hygrometer (CMH) dew point has several distinct advantages over other water vapor sensing technologies:
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.
Working Range of Dew/Frost Point Chilled Mirror Hygrometers
A CMH 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 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.
Interpretation of Uncertainty
When measuring dew point or frost point temperatures that are below the freezing point, it is desirable to know the phase (liquid or solid) of the condensate. When the temperature of the mirror surface is slightly below zero degrees Celsius the condensate may be super cooled liquid water or it may be in the form of solid ice. Generally, if the dew point has been dropping, the condensate will initially be in the form of super cooled water. In time, it will usually change to the ice phase because that has a lower energy state. At very low temperatures, the change to ice is more rapid and at –40oC it generally exists only in the ice form. It is desirable to know the state of the condensate because for a given mirror temperature, the vapor pressure in equilibrium with the condensate is higher for water and lower for ice.
For a given vapor pressure over the mirror, if the condensate is ice then the mirror temperature at equilibrium is slightly higher than it would be if the condensate were liquid.
Measurement span of one-and two-stages
The above chart shows the difference in CMH mirror temperatures for a given vapor pressure in the gas sample. The ordinate is the equilibrium temperature over ice minus the equilibrium temperature over water as a function of the mirror temperature.
In some industrial and metrology applications, the hygrometer has a microscope providing direct observation so the user can accurately discern the phase state on the mirror surface. In lieu of a microscope, users must assume instruments operating for long periods of time without interruption have an ice layer at temperatures below 0° C and are measuring frost point. The relationship in the above chart is very nearly a straight line with a slope magnitude of 0.1127 DT/T mirror.
The YES Chilled Mirror Hygrometer Family
YES manufactures both one- and two-stage chilled mirror hygrometers in its Model 2000 series. Models ending in 10 are single-stage sensors while those ending in 20 are dual stage (3, 4, or 5 stage sensors are available on customer order). Although packaged for different applications, models share the same sensor technology to leverage proven designs.
Selecting the Right Chilled Mirror Hygrometer for Your Application
First, you must answer three questions:
The answers to these three questions define whether you require a single stage or multiple stage dew/frost point sensor, as well as the type of heat sink required for the sensor, thus determining the feasibility of the measurement and the cost of the solution.
Next, consult the Range Chart above for single and two stage sensors; all manufacturer’s range charts are essentially the same, as the "depression" (the amount of cooling the mirror can achieve) of a single or two stage sensor is determined solely by the physics of thermoelectric heat pumping mechanics.
Other factors to consider:
Finally, before making a decision, take a good look at the "guts" of what you’re buying. More that any other factor the chilled mirror sensor design and construction defines how well the chilled mirror technique will solve your problem.
Many of the more routine dew point measurements can be made with a single stage sensor; for a typical ambient temperature span of +10oC to +70oC, a dew point span of –45oC to a room temperature of +25oC can be made with an absolute dew point accuracy of ± 0.1oC. To reach lower dew points, a two stage thermoelectric heat pump will attain frost points down to –65oC. Let’s see what’s available! (Note: All chilled mirror hygrometers offer pretty much the same dew point depression as a function of the number of stages used in the thermoelectric heat pump; this is a matter of physics of the Peltier cooling function. But that’s where the similarities end).