Measurement Principles and Sensors for Humidity Measurement
Sensors and Measurement Principles
Mechanical Humidity MeasurementPsychometric Humidity Measurement
Electronic Humidity Measurement
Comparison of the different procedures
Other Humidity Measurement Procedures
Relative Humidity
Unit: %
By relative humidity 
we refer to the ratio of water vapour actually present in the air to the
maximum possible mass of water vapour in the air. Relative humidity is
usually expressed as a percentage.
Note: Since maximum humidity is dependent on temperature, relative humidity changes with temperature, even when absolute humidity remains constant.
Absolute Humidity
Unit: g/m3
By absolute humidity fabs we refer to the volume of water vapour actually present in a specific quantity of air. Absolute humidity is the ratio of the weight of the water contained in the air and the volume of this moist air.
Water Vapour Partial Pressure, Saturation Vapour Pressure
Unit: hPa
By saturation vapour pressure eS(t) we refer to the maximum possible pressure of the water vapour at a certain temperature. The saturation vapour pressure of water dependent on temperature is presented in the chart below. The water vapour partial pressure e(t) varies between 0 (dry air) and 30hPa. The saturation vapour pressure determines the upper limit.
Saturation humidity, maximum humidity,
saturation quantity
Unit: g/m3
By maximum humidity fmax we refer to the maximum possible water vapour quantity in one cubic meter of air at a certain temperature. The moisture absorption capacity of air increases with increasing temperature. If maximum humidity is exceeded, the excess water vapour condenses in the form of condensate (droplet formation).
Dewpoint
Unit: °C, °F, K
By dewpoint temperature ttp we refer to the temperature at which the cooling of moist air leads to condensate formulation. This means that when moist air is cooled down to dewpoint the relative humidity rises to 100%.
Frost Point
Unit: °C, °F, K
In some cases where dewpoint temperatures are below 0°C we refer to the frost point temperature. Other descriptions are also dewpoint temperature above ice or freezing point.
Specific Enthalpy (heat content)
Unit: kJ/kg
By specific enthalpy Espez we refer to the quantity of heat necessary to raise the temperature of a gas (or gas mixture) from one temperature to another at constant pressure.
Or:
By specific enthalpy we refer to the quantity of heat that is available in the air mass relative to a certain air condition. The enthalpy of unsaturated air is the sum of the enthalpy to vaporise the water content present, plus the enthalpy to heat the water vapour mixture to the corresponding temperature.
Mixing Ratio
Unit: g/kg
By mixing ratio m we refer to the ratio of water vapour mass to the mass of dry air.
Wet Bulb Temperature
Unit: °C, °F, K
By wet bulb temperature tF in psychrometric measurement we refer to the temperature output or indicated by a thermometer wrapped in a moist sock. Due to the latent heat of evaporation, this temperature dependent on the relative humidity lies below the air temperature.
Sensors and Measurement Principles
The mechanical procedure is based on the expansion and contraction of various (mostly organic) measuring elements. Such measuring elements are, for example: hair, durometers, gut strings, etc. The measuring elements mostly used are hair elements or the so-called Durotherm, an artificial, humidity-sensitive measuring element. The change in length of the measuring element is transferred to the indicator by means of a mechanism.
Hair hygrometers require regular service and maintenance. To avoid drying-out and the associated drift, hair hygrometers must be regularly regenerated. To do this, the hair harp is wrapped in a cloth moistened with distilled water, or sprayed with distilled water, such that saturation occurs. After approximately one hour a measurement value of ca. 98%RH is obtained. A single point adjustment can be carried out on most units using an adjustment screw.
Psychrometers function with two identical, very accurate thermometers, along which the air to be measured is led at a defined speed or over which the air circulates.
The first thermometer measures
the ambient air, the second the so-called wet bulb temperature. For this
purpose the measurement head of the thermometer is covered by a cotton
wick and moistened with distilled water. Both thermometers are placed
in an airflow or in moving air and are protected from radiant heat. Due
to the latent heat of evaporation the temperature of the wet thermometer
falls, and indeed the drier the air, the more so. After a short time (1
- 2 min) the temperature of the wet thermometer remains constant and the
measurement values of the wet and dry thermometers can be read. With these
two temperatures the relative humidity can be determined in accordance
with the following formula :
where A is the so-called psychrometer constant. This depends on the air velocity and is presented in the table below.
|
Ventilation Speed |
0,0 |
1,0 |
1,5 |
2,3 |
3,4 |
4,4 |
|
|
A: 106 in Degrees-1 |
1140 |
675 |
674 |
678 |
682 |
704 |
Capacitive Procedure
The humidity-sensitive condenser consists of two flat electrodes, between which is located an electrically isolating, hygroscopic synthetic coating (dielectric). This dielectric can absorb the water present in the air. With increasing air humidity the capacity of the humidity-sensitive condenser also increases.
The following applies: 
,
where e r is the dielectrical
constant of the humidity-sensitive condenser.
Dewpoint Mirror Hygrometer
The dewpoint mirror is a very precise measurement procedure for reading relative humidity, wherein the condensation of water vapour is evaluated as it falls below dewpoint. The temperature of a reflected surface (mirror) is cooled down to the point where it begins to be covered in condensation. The temperature measured at this moment by a Pt100 resistance thermometer corresponds to the dewpoint temperature, from which the relative humidity may be calculated by means of the saturation pressure with the aid of the measured air temperature. A Peltier element is installed for cooling, and the mirrored surface is evaluated using an optoelectronic procedure (see drawing).
In contrast to psychrometers the moisture content of the atmosphere is virtually unchanged by the measurement. Dewpoint mirror instruments may therefore also be installed in closed systems such as environmental cabinets.
The mirrors must be cleaned regularly with a suitable liquid, such as trichloroethylene for example.
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Procedure |
Advantages |
Disadvantages |
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Mechanical humidity measurement |
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Psychrometric humidity measurement |
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Capacitive procedure |
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Dewpoint mirror hygrometer |
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The following is a list of other humidity measurement procedures (detailed observations are not made here):
- Resistive procedure (determination of the impedance of the alternating current resistance of an hygroscopic element)
- Lithium chloride dewpoint hygrometer (a measurement procedure based on the hygroscopic characteristic of lithium chloride)