Input Devices and Sensors
Dew Point Measurements
Dew point is the temperature to which air must be cooled under constant pressure to cause condensation to occur. It can be an important parameter to consider in some HVAC applications were possible condensation is undesirable and therefore must be measured and controlled.
Methods for Measuring Dew Point
Dew point measurements for use in HVAC control systems are typically made by one of two methods. One method is by measuring temperature and relative humidity correctly and calculating the dew point using empirical mathematical formulas. The second is by direct measurement using a chilled mirror type sensor.
Calculation from Temperature and Relative Humidity
It is common practice when measuring relative humidity to combine a temperature sensor and transmitter into the same device as the humidity sensor. Using a microprocessor, it is then possible to calculate and transmit dew point. Accuracy is limited by the combined accuracy of the sensors and the electronics. Typical accuracy is 1.8 F. Typical repeatability is 0.7 F. Commonly, these devices can be configured to output calculated humidity ratio, wet bulb temperature, and absolute humidity as well as dew point.
Chilled Mirror Hygrometers
Chilled mirror sensing technology has been in use since the 1950's for determination of dew point temperature. Modern chilled mirror hygrometers use a thermoelectric heat pump (also called a Peltier device) to move heat away from a mirror. A light beam from an LED is directed to the mirror and back to a photocell. When condensation (above 0 C) or frost (below 0 C) forms on the mirrors surface, the light reaching the mirror is scattered and the intensity detected by the photocell is reduced. The mirror is maintained at the dew point temperature by controlling the output of the thermoelectric heat pump. A high accuracy, platinum resistance thermometer (RTD) senses the temperature of the mirrors surface and therefore reports the dew point temperature. Chilled mirror hygrometers require a vacuum pump to draw the sample through the sensor, and additional filtration elements in dirty environments.
Chilled mirror hygrometers are subject to inaccuracies resulting from soluble and insoluble contaminants depositing on the mirror. Insoluble contaminants affect the optical characteristics of the mirror. Soluble contaminants affect the vapor pressure of the condensed moisture on the mirror. Most sensors have insoluble contaminant compensation cycles that heat the mirror (to dry it) and then reset the optical parameters of the light sensor to the current mirror optical parameters. Unless the soluble contaminants are volatile, the insoluble contaminant compensation does not remove the soluble contaminants. Virtually all chilled mirror sensors require periodic inspection and cleaning.
Many chilled mirror hygrometers have microprocessor control and when combined with a dry bulb temperature sensor can calculate and output any humidity parameters desired in addition to or instead of dew point. Chilled mirror hygrometers are available for sensing dew/frost point temperatures from -100 to 185 F. Accuracy of better than 0.5 F is available.
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