Chilled Mirror Hygrometer
Author: Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser
back to Moisture measurement methods
The chilled mirror hygrometer[1][2] measures the dew point temperature directly, i.e. the absolute humidity (AH). A polished metal surface is chilled until vapor is deposited. Now the water vapor contained in the ambient air is at maximum saturation. For calculating the relative humidity, the temperature of the metal surface and the saturation vapor pressure of the water vapor can be combined (the value is taken from table: absolute humidity and saturation vapor pressure as a function of temperature). This method is mainly used for calibration.
The chilled mirror dew point meter measures the dew point directly.[3] However, when using a modern chilled mirror dew point hygrometer the small metal mirror in the sample gas stream is cooled by Peltier elements to such a degree, that the controlling photo sensors can only just detect dew or ice deposits. The mirror temperature, detected with thermo sensors, is the dewpoint temperature. The setting process takes only a few seconds and measuring takes place continuously. Automatic chilled mirror dewpoint hygrometers are relatively expensive. They can also be used for measuring corrosive gases and they detect other condensable vapors. The accuracy is ± 0.5 °C at dewpoints above 0°C.
Since a certain water or ice film is necessary in order to be detected by the photoelectrics, it is necessary to go slightly below dewpoint. In dew point temperatures below -20°C this may represent a few degrees Celsius. A difficulty is the deposition of dust on the mirror, which is always slightly damp.
Advantages:
- wide measuring range
- high accuracy, reliability and reproducibility
- does not depend on air pressure
Disadvantages
- complex and costly process
- needs mains electricity
- high weight
- temperature measurements must be very accurate
- slow adjustment time
- risk of soiling
Weblinks[edit]
- ↑ http://www.yesinc.com/products/data/cmh/index.html viewed on 15/10/2012
- ↑ http://www.sensorsmag.com/sensors/humidity-moisture/the-chilled-mirror-hygrometer-how-it-works-where-it-works-an-577?page_id=1 viewed on 15/10/2012
- ↑ http://www.mabatec.com.mx/Marcas/Download/Chilled_Mirror_intro.pdf viewed on 15/10/2012
Literature[edit]
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