Humidity Calculations for the Assmann Psychrometer

The Assmann Psychrometer

The Assmann psychrometer is an ingenous device for measurement of relative humidity in the atmosphere (in- and outdoor), invented back in the 19th century and technically almost unchanged since then. The instrument may look rather archaic with its spring motor driven aspirator (ventilator), but it is based on a very clever design with apparent love to detail while thriving for simplicity and robustness. The psychrometer working principle can be described rather stabily by empirical formulas connected to the physics of gas, and it therefore provides very good precision, repeatability, and long-term stability, not needing calibration (other than checking the wick, motor, and thermometers).

Nowadays electronic humidity sensors have largely replaced the Assmann psychrometers. These tiny and cheap devices are in use in large quantities, and they allow simple, continuous and unattended humidity measurements. However it is not easy to check their accuracy and drift over the years. Then it helps to have an old Assmann psychrometer still around in good working condition for comparison.

By the way, it is really fun to wet the wick, wind up the spring motor, and watch and hear (!) the instrument in smooth operation. And it has a high educative value as well. Used Assmann psychrometers appear frequently on ebay, even new ones are still being built.

In the pre-computer aera, deriving the humidity from the two temperature readings of the dry and wet bulb within the psychrometer required lookup of multi-page psychrometric tables or psychrometric charts. Nowadays it is much easier and faster to calculate the humidity from the dry and wet bulb temperatures by a computer. Various psychrometric formulae can be found in the literature, see e. g., the short list below.

Lua Scripts for Calculation of the Relative Humidity and Dew Point

For demonstration here are four sets of psychrometric formulae, which were taken from the literature and typed into the interpreter language Lua ( The literature is referenced within the formulae files. These calculations obviously provide almost identical results.

The simple driver files allow to calculate the humidity and dew point from the dry and wet bulb temperatures given in the command line. Each driver file includes the corresponding formulae file. E. g., under Linux, for temperatures 20.2 deg. C (dry bulb) and 14.6 deg. C (wet bulb), a program call would be:

and the answer should be a relative humidity of 54.39 percent and a dew point of 10.70 deg. C. The digits after the decimal point suggest a precision well beyond the practical accuracy of the instrument; these figures are there only for comparison between the different formulas.

Here are the Lua program files:

Beware: The scripts are most likely not working for the phase over ice!

Beware: Files may be buggy! Use on your own risk!

Psychrometric Table/Aspirations-Psychrometer-Tafel

Here is a Lua program to calculate a psychrometric table:

And this is a ready psychrometric table as PDF file, after running the above Lua script through Lua, and the resulting LaTeX source through pdfLaTeX:

Literature and Links

This page and programs are from a private hobby activity, after having bought an old Assmann psychrometer.

This page first put online 8 January 2014.

Copyright © 2001 - 2017 by Hartmut Henkel

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