Installing
4.5 Purging
SITRANS SL
64
Operating Instructions, 12/2010, A5E01132948-04
4.5
Purging
The easiest way to avoid condensation and dust deposits on the sensor windows or
excessively high thermal load of the windows and the sealing material as well as the sensor
electronics is to purge them, e.g. with air. Purging must be selected depending on the
application. The sensors can therefore be configured for the respective situation.
There are two different types
●
Purging of optical process interfaces
●
Purging of sensors.
Note
Not all purging options are available to all applications. Please refer to the specification of
each application for details.
4.5.1
Sensor side purging
For some applications (e. g. oxygen) purging of the sensors is necessary. In this case a
needle valve and an exit aperture must be mounted on the sensors. The exit aperture is
used to build up a slight overpressure inside the sensor to enable the purging gas to
penetrate all relevant areas within a short time during all measurements.
The purging gas must not contain any concentrations of the measurement component. The
presence of the gas in the sensor heads or the purging tubes may influence the measured
concentration. In oxygen measurements it is necessary to purge the sensor with an oxygen
free gas, such as nitrogen.
When measuring CO also purging of the sensor may be considered. Normally the CO
content in standard air is lower than the detection limit of SITRANS SL. Nevertheless, the
CO content may be higher inside buildings and industrial plants due to exhalation or
combustion processes. In this case sensor purging is necessary.
CAUTION
When the sensor purging is connected to the process purging, it may be necessary to use
non-return valves to ensure no process gas can enter the purging gas line in the event of
failure of the purging gas supply. This applies especially in the case of cascaded process
and sensor purging where there is otherwise the danger that, for example, corrosive
process gases could enter the sensor enclosure.