GLOSSARY

Humidity


Absolute humidity

Indicates how many grams of water vapour are included in one cubic meter of air. (at 0 °C max. 5 g/m3, at 20 °C approx. 17 g/m3)

Capacitive measurement element

An arrangement in which a change in the rel. humidity leads to a change in the electrical capacity. For example the capacity of a polymer film on a carrier material changes when water vapour is absorbed.

Dew point

Temperature , to which the mixture of air and water vapour has to cool down, so that the air is just saturated with the available amount of water vapour, and condensation is starting.
At 15 °C and 50% rel. humidity the dew point is about 5 °C, at 80% rel. humidity about 12 °C, and at a humidity of 100% the dew point corresponds to the current temperature of 15 °C.

Dry bulb temperature

The ambient temperature measured on the dry ventilated thermometer of a psychrometer.

Hygro transmitter

General term for humidity measurement instruments with an electrical measured value output.

Hygrograph

Measurement instrument which mechanically records the rel. humidity as a function of time.

Hygrometer

General term for indicating humidity measuring instruments.

Hygrostat

Humidity-dependent switching instrument to regulate moistening or dehydrating devices or to trigger warning signals indicating too little or too much moisture in moisture-sensitive installations.

Measurement element H

Specially prepared human hairs expand under the influence of humidity, thus changing in length. This change in length is a measure of relative humidity. The range of application lies between 10 and 100% rel. humidity in temperatures ranging from -60 ... +70 °C. Hair measurement elements must be regenerated.

Measurement element K

Under the influence of humidity, specially prepared synthetic fibers change in length. This change in length is a measure of relative humidity. The range of application lies between 0 and 100% rel. humidity in temperatures ranging from 0 ... +100 °C.

Psychrometer

A measurement instrument with which the humidity of the atmosphere can be measured by measuring the dry bulb temperature, and the wet bulb ­temperature, and applying the psychrometric equation. Owing to the good measurement accuracy attainable, it is also used as a reference instrument.

Pt 100 Sensor

A temperature sensor with a measurement coil made of platinum wire. The temperature-dependent resistance-change of the platinum wire is used as measure for the temperature. 100 Ω for 0 °C is used as basic value. The resistance-change is defined in the DIN IEC 60751 standard. Pt 100 sensors are applied for ex. in psychro transmitters.

Relative humidity (rel. humidity)

Indicates the ratio in percent of the instantaneous content of water vapour in the atmosphere to the maximum possible content of water vapour at the same temperature.

Tensiometer

Measurement instrument to measure the saturation potential of the soil (water requirement of soils). Important to determine irrigation requirements.

Wet bulb temperature

Temperature, arising from evaporation (humidity temperature). Wet bulb ­temperature is measured at the moisturized thermometer of a psychrometer. The wet bulb temperature results from the chilling because of the evaporation at the moisturized thermometer.
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