Chilled Mirror Hygrometer
The chilled mirror hygrometer measures the dew-point temperature directly. A polished metal surface is chilled until vapor starts condensing on it which indicates that 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 a 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. 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 by the thermo-sensors is the dew-point temperature. The setting process takes only a few seconds and measuring takes place continuously. Automatic chilled mirror dew-point hygrometers are relatively expensive. They can also be used for measuring corrosive gases and as well as detecting other condensable ones. The accuracy is ±0.5 °C at dew-points above 0°C.
Since a certain water or ice film is necessary for the detection by the photoelectric system, it is necessary to cool slightly below the dew-point. For dew-point temperatures below -20°C this may represent a few degrees Celsius. A problem encountered is the deposition of dust on the mirror as it is always slightly damp. A second detector is sometimes used to monitor the polarization of the scattered light, and allows automatic determination of the phase of the condensate, i.e., dew point or frost point.
- wide measuring range
- high accuracy, reliability and reproducibility
- does not depend on air pressure
- complex and costly process
- needs connecting to an electrical outlet
- high weight
- temperature measurements must be very accurate
- slow adjustment time
- risk of soiling
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