Technical NotesMoisture and Humidity Measurement

General information about moisture and humidity measurements1.Foreword2.Air moisture and relative humidity3.Absolute air moisture4.Saturation vapour pressure5.Dew point

When dealing with moisture measurements, one is confronted with many terms and concepts without knowing their exact definitions or being able to distinguish between them. In the last years the measurement of moisture and humidity becomes more and more important for a wide range of industry domains. In order to make such measurements easier for the user, B+B Thermo-Technik GmbH gives you in this article a better understanding of the fundamental concepts of humidity measurements.The air moisture, also called humidity, is an admeasurement of the amount of water vapour in the earth's atmosphere or generally in gas mixtures. Mostly when speaking about humidity one means the relative humidity given in percent (%). It characterizes the ratio of the actual amount of water vapour in the ambient atmosphere and its maximally possible amount multiplied with 100. Usually one gives the relative humidity followed by the abbreviation RH% for relative humidity (in German % r.F. for relative Feuchte, in French HR% for humidité relative). For example 46 RH%. A 100% RH does not mean that the air is composed exclusively of water steam, but rather can no more absorb watersteam. The absolute air moisture or humidity, also called water steam (or vapour) density, designates the mass of the water steam in a defined air volume. This means, we are dealing with the concentration of water vapour in the air. The absolute humidity is usually given in gram per cubic meter, for example 6,62 g/m. It is limited by the maximal humidity that reigns in a saturated environment. It depends also so hardly on the ambient temperature and pressure, that without mentioning this two parameters a comparison with other values makes no sense.The value 100 RH% means that the air is no more able to incorporate water vapour. One says the state of saturation vapour pressure is reigning. In the nature this state of completely saturated air occurs, for example, when having fog and mist. If the temperature falls while this state, water will condensate and then appears as rain.The dew point designates the condensation point of water. This means it is given by the conditions that reign during the transformation from vapour (gas) to water (liquid). In this state the relative humidity amounts to 100 RH% and saturation vapour pressure is reigning. The dew point is a quantity that depends on pressure and temperature. The dew point, given as a measure for the humidity, is a derivative, not really regnant temperature. It is usually smaller or equal the ambient temperature. In the case of equality we have the saturation vapour pressure state. The dew point is a measure of humidity because of its hard dependence on the amount of water vapour in ambient air. If a water vapour saturated air is cooled down the dew point, condensation occurs and appears as fog, dew or generally as precipitation.
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6. Reference values table Ambient temperatureRelative HumidityAbsolute HumidityDew Point

20,0 °C60,0 % 10,42 g / m12,0 °C
3 20,0 °C40,0 % 6,95 g / m6,0 °C
3 20,0 °C20,0 % 3,47 g / m-3.6 °C
3 20,0 °C10,0 % 1,73 g / m-12.5 °C
3 20,0 °C5,0 % 0,87 g / m-20.8 °C
3 20,0 °C1,0 % 0,17 g / m-38.0 °C
3 0 °C60,0 % 2,92 g / m-6,8 °C
3 subject to technical changes 0 °C40,0 % 1,94 g / m-12,0 °C
3 0 °C20,0 % 0,97 g / m-20,3 °C
3 0 °C10,0 % 0,48 g / m-28,1 °C
3 0 °C5,0 % 0,24 g / m-35,3 °C
3 0 °C1,0 % 0,05 g / m-50,5 °C
3 Values apply at an air pressure of 1013,25 hPa.
0141 0035-50/02 2007 - Technische Änderungen vorbehalten /

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