Psychrometer

Aspiration psychrometer according to Assmann

A psychrometer (from the Greek psychrós - frosty, cool, cold), also an aspiration hygrometer or aspiration psychrometer , is a meteorological measuring device for determining the humidity or the wet-bulb temperature .

Physical basics

On the surface of a volume of liquid water, water molecules always pass from the liquid compound into the surrounding air by evaporating. The energy required for this ( latent heat , evaporation enthalpy) is taken from the thermal energy content of the water surface, which therefore cools down (evaporative cooling). On the other hand, water molecules from the air always hit the water surface and condense there, whereby the latent heat used to evaporate each molecule is released again and heats the water surface (condensation heat). It depends on the condensation rate and thus on the density of the water molecules in the air, to what extent the evaporation cold is compensated by condensation heat. The subcooling of an evaporating water surface below the air temperature is therefore a measure of the air humidity.

Working principle

Psychrometer with reading board

Psychrometer combination of two temperature sensors

The psychrometer consists of two thermometers , one of which, the wet thermometer , is wrapped in a moist material, for example cotton fabric moistened with water . The drier the air , the faster the liquid evaporates , the more evaporation cold is caused and the greater the temperature difference between the two thermometers. The relative humidity and other parameters can be determined from the temperature difference with the aid of psychrometer formulas or tables. The psychrometric measuring principle is one of the most precise and is therefore used in weather stations , where precise measurements are important, or in reference devices. Precondition for an accurate measurement is that the humidity thermometer has sufficient air flow around it so that evaporation is not hindered by the water vapor that has already formed.

From the observed psychrometer difference, the vapor pressure e of the air can be calculated to a good approximation using the following psychrometer formula , or Jump's formula : ${\displaystyle \vartheta _{L}-\vartheta _{f}}$

${\displaystyle e=E_{f}-\gamma (\vartheta _{L}-\vartheta _{f})}$

Denote:

• ${\displaystyle e\,}$ the water vapor partial pressure of the ambient air
• ${\displaystyle E_{f}\,}$the saturation vapor pressure at the temperature of the moist surface
• ${\displaystyle \vartheta _{L}\,}$ the air temperature
• ${\displaystyle \vartheta _{f}\,}$ the temperature of the wet surface (wet temperature)
• ${\displaystyle \gamma \,}$ the psychrometer constant: ${\displaystyle \textstyle \gamma ={\frac {pc_{P}}{\mu \lambda }}}$
  • with the current air pressure ${\displaystyle p}$
  • the specific heat capacity of the air ${\displaystyle c_{P}}$
  • the molar mass ratio of water and air and ${\displaystyle \mu \approx 0{,}622}$
  • the heat of vaporization of water . ${\displaystyle \lambda }$

Can be used at heights of up to 500 m . For substances other than air: ${\displaystyle \gamma \approx 0{,}666}$

${\displaystyle \gamma _{\text{Wasser}}\,=\,6{,}53\cdot 10^{-4}\cdot (1+0{,}000944\,\vartheta _{f})\,p\,}$ on the damp thermometer on or near water

${\displaystyle \gamma _{\text{Eis}}\,=\,5{,}75\cdot 10^{-4}\,p\,}$ on the moist thermometer on ice

The psychrometer constants mentioned apply only to the artificially ventilated aspiration psychrometer according to Assmann. A detailed description and further formulas for calculating the water vapor pressure and the relative humidity can be found in Chapter 4 of the WMO CIMO Guide 8, Edition 2014 .

For the derivation of this formula it is assumed that in the state of equilibrium the temperature is set in such a way that the vapor diffusion flow caused by the vapor pressure difference consumes precisely the latent heat during evaporation that the heat flow caused by the temperature difference delivers from the air to the moist thermometer. ${\displaystyle \vartheta _{f}}$ ${\displaystyle e-E_{f}(\vartheta _{f})}$ ${\displaystyle \vartheta _{L}-\vartheta _{f}}$

Minor dependencies of latent heat and the specific heat capacity of the air on temperature and humidity were ignored. The heat flow through the thermometer neck and the long-wave heat radiation were also neglected because of their insignificance. If necessary, they can be taken into account using correction factors.

The psychrometer formula was first established by Adolf Sprung (1888). The relative humidity can easily be read from the tables or graphical psychrometer tables derived from this.

Execution types

A prerequisite for a correct air humidity measurement is that the evaporation takes place into the air to be examined, not into the air humidified by the psychrometer itself through evaporation. It must therefore be ensured that sufficient fresh air is always supplied. This is the case if the ventilation speed is at least 2 m / s, if possible more.

Aspiration psychrometer

→ Main article : Aspiration psychrometer according to Assmann

The aspiration psychrometer according to Richard Assmann is equipped with its own ventilator (aspirator) for this purpose . When used correctly, it achieves a measurement accuracy of ± 0.5 to 1%. It comes particularly close to the idealizations assumed for the derivation of the psychrometer formula, since due to the narrow shape the heat supply via the thermometer neck is particularly low and long-wave heat radiation is well prevented by the double chrome-plated cover.

Slingshot psychrometer

Slingshot psychrometer

For off-road use there are sling psychrometers , where the necessary ventilation is achieved by flinging the connected thermometers around on a cord or a handle.

See also : hygrometer

Determination of the relative humidity in other gases

In principle, the Aßmann psychrometer method can also be used to determine the relative humidity in other gases and gas mixtures. However, it must be taken into account that the psychrometer constant z. T. changes greatly. While pressure, temperature and the heat of evaporation of water flow equally into the constant for all gases, deviating molar masses (or atomic masses for noble gases) and the substance-specific heat capacities must now be specifically taken into account.

In the case of gas mixtures, there can now be considerable deviations from the constants for pure gases.

The following table shows the calculated psychrometer constants at 1013 hPa, 20 ° C and 50% _{relative humidity} for some arbitrarily selected gases and gas mixtures:

Gas (mixture *)	constant
Sulfur hexafluoride (SF 6 )	0.384
neon	0.477
argon	0.484
Helium (He)	0.489
krypton	0.505
xenon	0.529
Carbon monoxide (CO)	0.667
air	0.668
nitrogen	0.669
Pulmonary function gases (9% to 19% He)	0.67 ... 0.98
oxygen	0.673
Nitric oxide	0.685
N 2 + 10% CO 2	0.701
Hydrogen (H 2 )	0.706
methane	0.816
Carbon dioxide (CO 2 )	0.848
Natural gas L	0.852
Ethane	1.061
Forming gas (90% N 2 + 10% H 2 )	1.357
propane	1.678
SF 6 + 10% He	1,859
n-butane	2.207
SF 6 + 10% H 2	4,509

[*] Composition of the gas mixtures in mol% (= molar proportion)

literature

• H. Häckel: Meteorology . 4th edition. Ulmer, Stuttgart 1999, ISBN 3-8252-1338-2
• A. Sprung: About the determination of the air humidity with the help of the Assmann aspiration psychrometer. In: Z. Angew. Meteorol. Das Wetter, 5 (1888), pp. 105-108.

Web links

Commons : Psychrometer  - Album with pictures, videos and audio files

Example of a graphical psychrometer table

WMO CIMO Guide No. 8th

Individual evidence

1. ↑ Stefan Hesse and Gerhard Schnell: Sensors for process and factory automation: Function, execution, application , Vieweg + Teubner, 2011, ISBN 978-3-8348-0895-0 , p. 241.
2. ↑ Lecture notes ( memento of the original from May 9, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. for an introduction to meteorology at the Ruhr University Bochum 
3. ^ H. Häckel: Meteorologie . 4th edition. Ulmer, Stuttgart 1999, ISBN 3-8252-1338-2 , p. 369f.