Measuring Humidity in Practice

Author: Hans-Jürgen Schwarz

English Translation by Sandra Leithäuser

back to Air Humidity Measurement

Abstract

In households, air humidity measurements were previously carried out using devices with hair harps or similar sensors. Today, small and compact electronic devices are used. Electronic measuring devices are usually used for surveys. These devices are made to cater for a very specific field. In general capacitive polymer sensors are used for normal, everyday measuring tasks. These take up moisture from the air and therefore require some adjustment time. The exact digital display of such devices may mislead into overestimating their accuracy. These devices also need calibrating and the manufacturers usually supply a sensor check.

Chilled mirror hygrometers (very expensive) and psychrometers, however, perform accurate measurements without the need for calibration. Psychrometers have the advantage to provide exact measurements over a long period of time, but also require some care with the handling. Firstly, the humidity sensor needs to be permanently humidified during the measurement process, secondly the gradual build up of dirt-crusts needs to be taken into consideration.

Long-term measurements are often still carried out using mechanical thermohygrographs, i.e. measuring devices that measure temperature and relative humidity simultaneously and write onto a paper strip. These simple devices are set to daily, weekly or monthly intervalls. The temperature sensor (usually bi-metal) and the humidity sensor (usually a hair measuring element) determine the values and these are written onto a recording drum. Due to the above mentioned disadvantages of the measuring process, the thermohygrograph needs checking and re-adjusting repeatedly. Yet another disadvantage limits the use of these devices. In many cases not only an indication of the conditions of a specific point in the room is needed, but several points or different rooms. The humidity is usually stable and needs to be measured only at one specific point, even in large rooms, whereas the air temperature varies greatly, even in small rooms. Therefore the setting up of several devices is required and the evaluation is encumbered by many single pieces of paper. In such cases it is best to store the data with special devices, which allow to conveniently evaluating the data on the computer. A wide variety of data acquisition points can be connected to these devices, making possible to evaluate parameters like surface temperature, rainfall, air currents and the timing of heating systems. The advantage of electronic measuring techniques, e.g. for capacitve humidity sensors and resistance thermometers becomes particularly clear.

Literature

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an official verification of measurement accuracy of instruments.