Overview of the Humidity Sensors
Humidity sensors (also called ‘hygrometers’) are manufactured in a range of different designs employing different technologies; but all share in common the ability to measure the quantity of moisture in an atmosphere.
Essentially, humidity sensors actually measure the alterations induced by atmospheric moisture in a material which is sensitive to water content. As humidity is correlated to temperature (the higher the temperature of the air, the more moisture it can contain in the form of a suspension of tiny droplets), most humidity sensors detect ‘relative humidity’ – the proportion of actual water vapour in an atmosphere compared to the maximum possible saturation of that atmosphere with water vapour at any given temperature, expressed as a percentage.
Most modern devices actually measure alterations in a material’s electrical capacitance, electrical resistance, or mass as it absorbs water from the atmosphere. Some electrical devices measure the exact temperature at which condensation occurs (the dew point).
There are two broad categories of humidity sensor: those that measure relative humidity (RH sensors –) and those that measure absolute humidity (AH or moisture sensors). Absolute humidity refers to the total mass of water vapour in a unit volume of an air and water vapour mixture. Atmospheric air at 30°C reaches total saturation at 30g of water vapour per cubic meter of air.
History
Possibly the first humidity sensor in human history came into the world in 1450, when the German philosopher and astronomer constructed a hair hygrometer, in a design which was subsequently drawn by Leonardo da Vinci in 1480. The science-and-technology-obsessed Ferdinand de Medici II, the Grand Duke of Tuscany, built the first condensation hygrometer in 1657, and in 1853, the French physicist August Bravais invented a humidity sensor he called “the whirling psychrometer”, which consisted of one wet bulb and one dry bulb thermometer mounted on an arm. The latter could be rotated through the air on an axis which acted as a handle, detecting moisture as it moved. The moisture content was calculated by means of a formula based on the different readings on the two thermometers.
The Finnish company Vaisala developed and patented the “Humicap” humidity sensor in 1973, a thin film of water vapour-sensitive polymer which changed its electrical capacitance in response to fluctuations in atmospheric moisture. Since then, technological advances have yielded a wide range of different forms of humidity sensor, amongst them:
- Saturated lithium chloride sensors (in which the salt acts as the moisture sensing medium).
- Phosphorous pentoxide electrolytic sensors (the desiccant phosphorous pentoxide converts an electrolytic process upon an applied voltage into a reading of water vapour content).
- Colour-change sensors (in which a material such as cobalt chloride will change colour in relation to atmospheric water content).
- Spectroscopic sensors (which rely on the infrared absorbance of water droplets in the atmosphere).
- Adiabatic expansion sensors (which detect the dewpoint when air cools during expansion and produces a fog).
- Acoustic sensors (which detect alterations in the transmission of sound in air as a reflection of changes in humidity).
Technical aspects
Electronic humidity sensors are usually calibrated (or “characterised”) along on a nine-point scale at an atmospheric temperature held constant at 25°C. The atmosphere is subject to a total of nine rising and falling levels of humidity (0%, 25%, 53.2%, 75.3%, 93.8%, 75.3%, 53.2%, 25% and 0%) and the DC output of the device is taken at each point, following which a Best Fit Straight Line (BFSL) is plotted and a second order non-linear curve.
Each individual sensor will have its own particular calibration curves. A sensor’s accuracy is calculated using the BFSL and the second order non-linear curve and is usually expressed as a percentage (e.g. plus or minus 2 percent).
All sensors are tested for stress in order to obtain a measure of their reliability. If a sensor drifts to such an extent that it yields readings outside its specified RH range under these accelerated stress tests, it is considered “failed”. The tests produce figures for each individual humidity sensor known as the FIT score (the number of failures per billion hours of operation) and MTTF interval (mean-time-to-failure).
All humidity sensors are subject to slow moving air tests (air flowing at below 5 meters per second) in order to determine their response times. Usually, response time refers to the time the sensor takes to either fall to 37 per cent of its final value or to climb 63 percent when subject to a sudden drop or increase in atmospheric humidity.
Because at any given RH, temperature affects the voltage output of a humidity sensor, many are designed to take account of heat when computing the humidity level – a process known as “temperature compensation.”
Where the Humidity Sensor is used in manufacturing
Many industrial processes can be affected by humidity levels, as indeed can human comfort and wellbeing. Humidity sensors are frequently used in medical applications whose functioning can be distorted by excess or insufficient moisture; these include incubators, respiratory equipment, and sterilisers. They are also used to monitor humidity levels in the preparation of various pharmaceuticals and to control humidity during the purification of chemical gases.
These sensors are also used to control humidity in the production of paper and textiles, in food processing and in film desiccation processes. They have numerous applications in agriculture, too, from dew prevention in sensitive crops to the monitoring of the moisture content of soil. Atmospheric moisture control is essential during the manufacture of semiconductors, and humidity sensors play a critical role during wafer processing in this industry.
Domestically, these sensors are widely used to control atmospheric humidity in buildings, and they play a vital part in microwave oven cooking control.
How Humidity Sensors differ from other sensors
Humidity sensors are designed specifically to measure the water vapour content in ambient atmospheres, as opposed to lighting levels, motion, temperature or strain.
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