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Institute for Measurement Technology
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Bachelor Theses.

Measuring the relative humidity using an aspiration psychrometer

Lukas Heindler

To begin with, the constructed measurement system serves to determine the relative humidity using two common temperature sensors. Exposed to sufficiently strong air flow, the sensor covered by a wet sheath cools down to the cooling border temperature. From the temperature difference between the wet and dry bulb sensor, the relative humidity can be resolved.


The built up measurement system consists of two temperature sensors, an electronic board, an integrated humidity sensor, a Raspberry Pi and a fan. An integrated humidity sensor serves as validation instrument for the executed measurements. To achieve valid measurements, the fan has to attain high enough flow speeds. Otherwise the measurements exhibit large errors since the wet bulb would not attain its limiting temperature.

Figure 1: The built measure during a measurement.

Additionally the coded Python program measures the sensors' temperatures, processes the data and displays them on a graphical user interface. The program allows user friendly working and displays a clear overview of every important information.


Figure 2: The last output of the program displaying all three results of the relative humidity.

The thesis portrays the relevant basics of humidity measurements and describes various hygrometers. Furthermore it outlines the constructed measurement system, its results and validity.
Finally the accuracy and validity of the performed measurements, despite the simple method and build, is notable. Hence the measurement system could be a base for further works without any restrictions.

Keywords: aspiration psychrometer, hygrometer, humidity

August 17th, 2020