Monitoring and/or controlling the moisture content of a gas is critical for many industrial processes. Industries where these processes are found include HVAC, pharmaceutical, semi-conductor, food storage, medical air as well as the compressed air market. The two most common measurements which provide an indication of air moisture content are humidity and dew point, or more precisely dew point temperature. There are a wide variety of instruments available for monitoring and controlling humidity and dew point, but they vary greatly in performance, reliability, application and price. In this article we will discuss two of the most common types of instruments; they are capacitance-based sensors and chilled mirror hygrometers. We will discuss the principles of operation of both types of instruments as well as their benefits and limitations in various applications. Let's begin by briefly reviewing humidity and dew point.
Humidity is a measurement of the amount of water vapor in air. The most common form of humidity measurement is relative humidity. Relative humidity is defined as the partial pressure of water vapor in air divided by the vapor pressure of water at the given temperature. This provides a scale of 0 to 100% where 0% represents air that is completely dry with no moisture content and 100% is air that is completely saturated with water vapor.
Dew Point Temperature Measurement
Dew point temperature is a measurement which also provides an indication of the amount the moisture content in air. The dew point temperature is the temperature at which a mixture of water vapor and air is saturated. If the temperature is decreased below the dew point temperature the water vapor will begin to condense to liquid water or dew. If the dew point temperature is the same as the ambient temperature of the air then the air is completely saturated with water vapor and at 100% relative humidity.
It is important to note that the relative humidity varies with temperature of the air. As the air temperature rises the humidity falls. This means when controlling a process based on relative humidity, the temperature also needs to be monitored. Dew point temperature, however, is independent of the air ambient temperature.
Which Measurement is Better: Humidity or Dew Point Temperature?
Well it depends on the application and industry. Some industries such as the compressed air market typically look for moisture content in terms of dew point temperature while the HVAC industry more often defines moisture content in terms of humidity. If you have either the dew point or humidity the other may be calculated but the calculation is quite complex and probably requires more effort than the casual user wants to take on. There are many references which can be found on the internet which provides these calculations but it may be unnecessary to go through the manual calculation since many measurement instruments have calculation built into their firmware and will provide an output in either humidity or dew point temperature.
Chilled Mirror Hygrometers
Chilled mirror hygrometers are the most accurate, stable and reliable instruments for measuring dew point temperature or humidity in an industrial process. Because they operate on a fundamental principle, they are often used as a calibration standard. Chilled mirror hygrometers operate by thermoelectrically controlling the temperature of an element with a mirrored surface. The temperature of the mirror is adjusted until condensate or dew forms on the mirror’s surface. The condensate is detected by an optical sensor and the temperature at which the condensate forms or the dew point is read by a precision temperature sensor.
Although the initial cost of a chilled mirror hygrometer may be higher than other systems, the long-term cost of ownership makes them very cost competitive with the other systems that have a lower initial purchase price. Chilled mirror hygrometers have a very long life and can last in service up to 8 to 12 years. They do not drift and do not require re-calibration although periodic verification may be advisable. The dew point measurement itself is not impacted by gas temperature changes and chilled mirrors work with all gases containing moisture as long as the gas is not corrosive to the mirrored element. They even work well with gases which contain particulates, although the mirror may require some cleaning in such an environment. The chilled mirror hygrometers offered by IOThrifty and manufactured by Roscid Technologies will let you know when it is time to clean the mirror. The table below shows some of the chilled mirror hygrometers offered by IOThrifty.
|DEWTRAN Precision Chilled Mirror Hygrometers||$2,210|
|DEWTRAN-LC Series HIgh Precision Chilled Mirror Dew PointTransmitter||$1870.00|
|Dewtran-HB High Temperature 2 Stage Chilled Mirror Hygrometer||$6699.00|
*Starting price at the time the article was published.
Capacitance Based Humidity/Moisture Sensors
Capacitance humidity sensors are small capacitors consisting of a hygroscopic dielectric material placed between a pair of electrodes. Most capacitive sensors use a polymer as the dielectric material. At normal room temperature, the dielectric constant of water vapor is much higher than that of the sensor dielectric material. As the dielectric absorbs moisture, its capacitance increases. The change in capacitance can be correlated to moisture content in the air which in turn can be used to calculate humidity. Since the humidity is dependent on temperature as is also is the sensor dielectric properties, automatic temperature compensation must be built into a capacitance-based humidity measurement system to avoid large measurement errors. Capacitance systems generally have a lower initial price than chilled mirror systems, although the chilled mirror systems have longer life which tends to even out the price difference for long term use. In addition to price they are also faster responding than chilled mirors. They are available in compact sizes which makes them well suited for applications which may have space limitations. If exposed to contaminants in the air stream the capacitive sensor can foul and unlike a chilled mirror it cannot be easily cleaned. However, for those cases we do offer field replaceable sensors for our capacitive hygrometers manufactured by Roscid. The table below highlights some of the capacitive sensor humidity and dew point instruments offered by IOThrifty.
|HR200 Dew Point Transmitter/Meter||$795|
|DPXO-70 Low Dew Point Transmitter||$1179|
|RO120 High Accuracy Duct Mount Temperature, Dew Point and Humidity Transmitter||$875|
|RO120-DIS High Accuracy Temperature, Humidity and Dewpoint Transmitter with Remote Display||$1,295.00|
*Starting price at the time the article was published.
Humidity and Dew Point Measurement of Gases Other Than Air
Most of the discussion so far has been about measuring the moisture content in air, however, there are many applications where the moisture of gases other than air needs to be measured. One such application is in the ship building industry. As part of their welding process, the moisture content in Argon gas must be monitored. Both chilled mirrors as well as capacitance sensor based instruments can be used with many other gases. Chilled mirror hygrometers are readily used with other gases as long as the gas is chemically compatible with the wetted parts of the hygrometer. The capacitance instruments can also be used with other gases although their use is more limited with non-air gases than chilled mirrors. One example is chlorine gas which can damage a capacitance based sensor. For applications other than air it is always best to contact us at IOThrifty and we will be happy to discuss your application and whether there is any compatibility issues with the gas you are monitoring.