Chilled Mirror Hygrometer Temperature Depression Explained.
There is often confusion about the dewpoint measurement range of chilled mirror hygrometers and their temperature depression range. The hygrometer dewpoint measurement range and the temperature depression are interrelated, but they are not the same. The hygrometer’s dewpoint range is determined by the amount of temperature depression that the hygrometer’s sensor is physically capable of, plus the actual sensor temperature as it is affected by the ambient temperature, the sample gas temperature, and any ancillary cooling of the sensor (i.e. coolant flowing through the sensor’s cooling jacket).
Chilled mirror hygrometers determine the dewpoint temperature of a gas or gas mixture by cooling the sensor’s mirror until dew or frost forms on the mirror’s surface. The sensor then uses its optical components to determine that dew or frost has formed and stabilizes on the temperature at which it is just forming. That is by definition, the dewpoint temperature.
The dewpoint temperature is a measurement of the amount of water vapor in a gas. For any dewpoint temperature there is a singular aqueous vapor pressure. Dewpoint temperature is a primary measurement that can be used to calculate other moisture parameters including parts per million or relative humidity if pressure or temperature are known.
The sensor’s mirror in most chilled mirror hygrometers is cooled by a thermoelectric cooling device. Thermoelectric cooling is the cooling effect that occurs as a result of current flowing between two different conductors or semiconductors; heat is produced at one juncture and a cooling effect at another juncture, creating a temperature differential. This can be used to transfer heat from one place to another. A system that uses this effect is called a Peltier heat pump.1
Thermoelectric coolers come in various ranges of cooling capability. Depression is the potential difference in temperature between the temperature of the sensor base and the cooling capability of the thermoelectric cooler. The sensor temperature could be ambient, or the sample temperature, or the temperature of a coolant flowing through the sensor’s cooling jacket. Multiple thermoelectric coolers can be used together in stages to increase the available amount of temperature depression.
Example: An oxygen production plant purchased an Edgetech Instruments chilled mirror hygrometer having a three-stage thermoelectric cooler, for a dryer application with an expected dewpoint range of +20°C to -75°C. The challenges were a high sample temperature of +160°C and a low expected dewpoint temperature of -75°C. This represents a mirror cooling range requirement of 235°C. The three-stage thermoelectric cooler in the hygrometer had a depression capability of 115°C.
In this example it is clear that the depression capability of the three-stage thermoelectric cooler was not sufficient to determine the dewpoint from the highest sample temperature of +160°C to the lowest possible dewpoint temperature of -75°C. However, two installation adjustments solved the problem.