Humidity Sensors for Industrial Applications

It could be argued that humidity plays a part in every industrial production process. The very fact that our own atmosphere contains water vapour bears witness to this fact even if it is only that the end product is likely to be stored and eventually used in our environment; therefore, the product’s potential performance under varying conditions of humidity must be known. The extent to which humidity plays a part in any given production process may vary but in many cases it is essential that, at the very least, it is monitored and, in most cases, controlled. It may also be said that humidity is a more difficult property to define and measure than associated parameters such as temperature and pressure. Indeed, it is a truly analytical measurement in which the sensor must contact the process environment, in contrast to pressure and temperature sensors, which are invariably insulated from the process by a thermowell and a diaphragm respectively. This of course has implications for contamination and degradation of the sensor to varying degrees depending on the nature of the environment.
This paper reviews various humidity sensor technologies and their typical applications in context of the measurement ranges to which they are best suited. The effects of contamination, highly significant in view of the analytical nature of the measurement, are briefly assessed. In conclusion, it is suggested that, if initial cost is not the prime consideration, the chilled mirror, optical dew point hygrometer offer the most accurate, repeatable and reliable method of humidity measurement with the widest possible range.
Humidity Measurement Applications in a Range of Industries
Table 1 shows an A to Z of industries where humidity measurement plays a part. Whilst the list is by no means exhaustive, it does serve to illustrate the extremely wide range of applications with which a supplier of humidity instrumentation may be confronted. Indeed, these applications cover six orders of magnitude when considered in terms of Parts Per Million (PPM) by volume of water vapour, equivalent to an overall range of -85 to +100°C dew point. It is of course very unlikely that one measurement technique can cover the entire range but, if initial cost is not the prime consideration, the chilled mirror, optical dew point hygrometer can probably be said to come closest to achieving this.
In practice, a variety of commercial and technical criteria will dictate which measurement technology is used for any particular application. Table 2 shows the most common humidity measurement parameters used within the industries referenced, depending on application, and Table 3 illustrates how certain sensor technologies are associated with specific industries as dictated by the commercial pressures and technical demands of the measurement. These aspects are themselves invariably influenced by the criticality of the measurement.
Two important points to note are that different units are used for different parts of the measurement range and that the measurement units an industry uses are very often a good indicator as to the type of sensor technology they should be employing. Humidity measurement determines the amount of water vapour present in a gas. This gas can be a mixture, such as air, or it can be a pure gas, such as nitrogen or argon. While there are many measurement techniques, the most common parameters are Relative Humidity (RH), Dew/Frostpoint (D/F PT) and Parts Per Million (PPM).
Relative Humidity Measurement (RH)
An RH measurement is the ratio of the partial pressure of water vapour present in the gas to the saturation vapour pressure of the gas at a given temperature. Thus, RH is a function of temperature. The measurement is expressed as a percentage.
The human body is sensitive to, and can experience varying RH in terms of the contrast between a dry and a muggy Summer day.
Dew/Frost Point Measurements (D/F PT)
Dewpoint is the temperature (above 0°C) at which the water vapour in a gas condenses to liquid water. Frost point is the temperature (below 0°C) at which the vapour crystallises to ice. D/F PT is a function of the pressure of the gas but is independent of temperature and is therefore defined as fundamental.
We can all observe the dew point phenomenon in our bathrooms. On a cold day, when the temperature of the surface of a mirror or a polished metal surface such a tap, is below that of dew point of the atmosphere, a dew or condensation layer will form on its surface.
Parts Per Million (PPM)
Expression of water vapour content by volume fraction (PPMv) or, if multiplied by the ratio of the molecular weight of water to that of air, as PPMw.
This parameter is more difficult to conceive as it is beyond the ability of the human body to detect changes of this magnitude in the atmosphere. However a practical example of its application in an industry is that of medical gases: those gases such as nitrous oxide, carbon dioxide and oxygen when used in surgical operations should have a moisture content lower than 60ppm and are regulated in this regard.
The Consideration of some Sensor types and their Application
As previously stated, the wide range of humidity measurement required by the various industries described in Table 1 precludes any one sensor technology from being suitable for all applications. In view of this, and the subject matter constraints on this paper, what follows is a summary of some of the sensor technologies typically used in the industries referenced.

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