Chilled Mirror Hygrometer Temperature Depression

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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 effected 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 sensors to determine that dew or frost has formed, and stabilizes on the temperature at which the dew/frost 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.....more.....


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Edgetech Instruments
Edgetech Instruments, Inc. is a global leader in the design and manufacture of chilled mirror dew/frost point hygrometers and other moisture and oxygen measurement devices. The company manufactures absolute and relative humidity hygrometers as well as oxygen concentration analyzers. In 2015 Edgetech Instruments achieved ISO 9001 registration certification and in 2016 the company became ISO/IEC 17025 accredited. Edgetech hygrometers are made and supported in the USA. All calibrations and certifications are traceable to NIST.

Edgetech Instrument’s products include trace moisture analyzers and instrumentation, wireless and hand held portable hygrometers, absolute and relative humidity transmitters, aircraft hygrometers, humidity and temperature test chambers and instrumentation for engine test cells. Equipment can be purchased or rented.