Selecting the Right Chilled Mirror Hygrometer for Your Application

1. What range of dew/frost temperatures of the sample do you intend to measure? This determines the amount of mirror cooling, i.e., "depression" required to reach the dew point. It defines whether you need single or multiple stage thermoelectric cooling.

2. What is the span of ambient temperatures of the gas sample? Keep in mind that the temperature of the sampling lines, if used, must be higher than the highest dew point anticipated. High dew point temperatures must be handled in heat-traced sample lines in order to prevent dew from condensing on the sample line walls, thus giving a lower and incorrect measurement. The highest temperature at which a standard thermoelectrically cooled dew point sensor can operate is limited by the characteristics of the solid state optical dew detectors, usually 85^oC.

3. What will be the ambient temperature of the environment where you will install the dew/frost point sensor? This determines the temperature to which the heat from the hot side of the thermoelectric heat pump will be dumped. This can be the prevailing room temperature or water coolant to obtain additional depression.

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.

Consult the Range Chart below 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.

• Is the gas corrosive? For example, does it contain chlorine or other active gasses? If so, the active component will often react with the dew deposit, causing erroneous "acid" dew point readings. We recommend you avoid chilled mirror dew point sensors for this application (regardless of the manufacturer’s claims!).

• Is the gas a mixture of gasses such as natural gas, containing methane as well as water vapor? Keep in mind that the chilled mirror will tend to control on the first dew point it encounters as it cools. In such instances, the methane dew/frost point temperature can be above the water dew point, causing an erroneous reading.

• Is the gas radioactive, such as UF₆? (Users handling radioactive gasses know how to handle contaminated instrumentation…….)

• How dirty is the gas sample and how often can you clean the mirror? Most importantly, how difficult is it for you to clean the mirror? Do you have to remove screws and plumbing? How long will the threads last?

Before making a decision, take a good look at the guts of what you’re buying. The chilled mirror sensor design and construction defines, more that any other factor, how well the chilled mirror technique will solve your problem.

Single-stage sensors

Many of the more routine dew point measurements can be made with a single stage sensor; for a typical ambient temperature span of +10^oC to +70^oC, a dew point span of –45^oC to a room temperature of +25^oC can be made with an absolute dew point accuracy of ± 0.2^oC. To reach lower dew points, a two stage thermoelectric heat pump will attain frost points down to –65^oC. 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).