SADP_Operating_ Instructions_0114
3.0 Installing the Air/Gas Sampling System
The piping installation schematic diagram (see section 3.2) shows all components, which could be used
in a dry gas measurement application although not all the items shown will be required for every
installation.
Care should be taken to ensure that the sample presented to the SADP is not contaminated with any
component that will damage, contaminate or affect the SADP in a way that will impair the unit’s
accuracy.
It is strongly recommended that the sample should not contain particulate matter, oil, hydrocarbon or
any other condensate. If these components contaminate the sample system and/or the measuring
sensor, the SADP response time will be lengthened, although the sensor calibration will not be effected.
The flow rate, although not critical to the sensor measurement, should be low enough to avoid abrasion
to the sensor surface without being so low as to extend the SADP response time to an unacceptable
level. In general, a flow rate of between 5 and 8 litres/min at NTP will give the right balance.
The sensor used in the SADP is a variable capacitor, which is directly affected by changes in partial
pressure of water vapour, and these changes, that are proportional to the dew/frost point temperature,
are displayed on the instrument indicator.
3.1 Gases to Avoid
The moisture sensors are suitable for many different industrial and research applications. Most gases
can be checked for their moisture content with no need for the calibration to be altered when changing
between different gases, as the sensor operates only with reference to the water vapour content. There
are, however, some gases that must be avoided, as they are not compatible with the material of
construction of the sensor. Ammonia (NH
3
), Ozone(O
3
) and chlorine (Cl) must be avoided at all times,
even in small quantities. Hydrogen chloride (HCl) also attacks the sensors very quickly. Some, less
aggressive, acidic gases, such as sulphur dioxide (SO
2
), can be monitored, as long as the moisture
content is low, generally less than 100ppm(v). If in doubt, please ask your supplier.
Sulphur hexaflouride (SF
6
) has no effect on the sensor. If the gas has been exposed to arcing,
however, it is possible that various acidic species will have been formed that will corrode the sensor.
When testing SF
6
that may have been arced, therefore, an acidity test should be carried out first; if the
gas proves to be acidic then the moisture test should not be carried out.
