Ambient humidity usually surpasses the moisture content of the dry gas to be monitored many times over. For this reason, the slightest leaks into the system always cause incorrect measurements. The large difference between the water vapour partial pressures inside and outside the piping cause the moisture to penetrate into gas carrying piping, even though the latter is under pressure. Even now, piping systems for compressed air still display leakages of up to 20%. Flanged and threaded connections, as well as push-in couplings all contribute to leakage of compressed air lines. At such connecting points, ambient moisture diffuses into the system against the operating pressure. Less so with short lines with few joints than with correspondingly longer piping or plug-in lines.
Figure 8.5.5.1
Leakages in a gas line are normally regarded as approximately constant. Changes in the moisture content indicate a leakage. With a high throughput, this influence regarding the moisture content diminishes, and at slower flow velocity the moisture share increases. If no compressed air is consumed and the flow velocity within the system thus equals nought, the diffusion of moisture at the connecting point has a stronger effect. The influence of leakages at maximum flow velocity of compressed air diminishes through dilution of the intruded humidity over a larger quantity of compressed air. Ambient moisture can also penetrate into the sample extraction system from the measurement outlet and against the direction of flow. There should, therefore be a length of pipe of at least 2m length and of spiral shape to be fitted to the measurement chamber outlet, so that the compressed air has to flow through this before entering the atmosphere. The measurement sensors are suitable for measurements where extreme demands of leaktightness are important. The specified rate of leakage is in the range of <10-7 bar L/s.
