If possible, the sensor is fitted inside a measuring chamber and allocated to a measuring point in the compressed air network at which the humidity content is to be determined. One should avoid fitting the measurement sensor directly into the piping, in order to avoid reading distortions caused by pollution. Contamination in the piping such as carbon or oil should, in the first instance, be regarded as additional carriers of stored moisture and lead to a slowing down of the indicating response time. This makes it practically impossible to obtain readings of very low dewpoints. In such cases, the piping would have to be purged. In individual cases, it should be checked which solvent is most suitable for dissolving polluting elements. A sample extraction system (Fig. 8.5.1.1) should always be designed when measuring humidity. Sample extraction is, as far as possible, branched from the centre of the pipe, as moisture is distributed unevenly within the volume flow of compressed air.
Figure 8.5.1.1
Fitting the measurement sensor (item 4) in a measurement sensor chamber of small total volume (item 1) is therefore an essential precondition. The measurement ducting (item 2) should be built without connectors, fittings and dead corners, in order to avoid interference by stored moisture and condensate. The path between pipe network and sample extraction system should be kept as short as possible, as othewise
- The response time of measurement is affected by the conveyance delay of the compressed air
- The walls of the measurement duct tubing act as an additional storage, influencing the indicating response time
- Temperature fluctuations in the measurement duct system exert an influence on the level of adsorption and desorption
- Long thin measurement lines with high flow velocity and resulting pressure drop falsify the value recorded.
