Ensure the quality of compressed air and reduce costs with testo dewpoint transmitters

Monitor trace humidity, prevent damage

Air, compressed air and gases are used in all areas of industry. Humidity and moisture are generally undesirable because they can cause damage or impair the quality of the end product. The international standard ISO 8573 defines seven classes of compressed air quality and specifies the humidity, the oil content, the particle content etc. which the compressed air is allowed to have. Class 1 represents the most stringent requirements.

Class 4 is met if, for instance, the dewpoint does not exceed 3 °Ctd or 37 °Ftd or an absolute humidity of 6 g water vapour per m3 or 1083 ppmV (parts per million, based on volume). The main way of ensuring compliance with a quality class involves installing a suitable dryer. This can be monitored and, where necessary, controlled by the Testo trace humidity transmitters (testo 6721, testo 6741-6744, testo 6681 + 6615).

Of course, the main purpose of the testo trace humidity transmitters is to monitor and prevent excessive moisture in the system so as to prevent damage. This damage results in considerable costs, especially if the quality of the end product is affected. Moreover, when using adsorption dryers, the operating costs can be considerably reduced with the aid of testo transmitters.

Dew point transmitter

Trace humidity transmitters at a glance

testo 6721


testo 6740


testo 6681 + 6615


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Dew point transmitters

Overview of dewpoint transmitters


Optimal for trace humidity measurement

The testo humidity sensor is generally calibrated at several points, resulting in minimal deviations. For trace humidity measurement, a calibration is also carried out at -40 °Ctd (dewpoint) with the aid of a high-precision reference measurement (dewpoint mirror) and logged. This provides the user with reliable and accurate readings in the relevant range (down to -60 °Ctd / -76 °Ftd).

Adsorption dryers

If chamber switching is humidity-controlled using Testo trace humidity transmitters rather than being time-controlled (see diagram on the right), the drying phases (blue) are usually much longer than the regeneration phases (red). During this time, no regeneration air needs to be generated, so the compressors can be switched back from 100 % to approx. 85 % volume flow. This results in significant savings in operating costs.

Refrigeration dryers and membrane dryers

Regardless of whether refrigeration or membrane dryers are used, without continuous monitoring of the dryer, damage is almost impossible to prevent. Blocked condensate drains and poorly closing bypass lines result in a measurable increase in humidity in the compressed air line.