Design And Measuring Principle; Principle Of Operation - Dräger PEX 3000 Gebrauchsanweisung

Design and measuring principle

Design and measuring principle
The transmitter PEX 3000 consists of housing, an Ex-sensor and corresponding
electronics.
The housing made of conductive plastic prevents electrostatic charge-up and is
resistant to solvents.
The Ex-sensor is a transducer for measuring the partial pressure of flammable
gases and vapours present in the atmosphere. They use the heat-of-reaction
principle.
The monitored air is diffusing through the wire mesh disc into the Ex-sensor. This is
where the mixture of flammable gases and vapours are catalytically combusted at a
heated detector element (pellistor). The oxygen required for combustion is obtained
from the monitored air. The detector element is additionally heated by the resultant
heat of reaction. This heat in turn causes a change in the resistance of the detector
element, which is proportional to the concentration of the flammable gas or vapour.
In addition to the catalytically active detector element, the Ex-sensor
accommodates an inactive compensator element, which is likewise heated. Both
elements are parts of a Wheatstone bridge. Environmental effects such as
temperature, humidity or heat conductivity of the ambient air to be monitored, have
identical effects on both the elements, whereby these effects on the measuring
signal are almost completely compensated.
The sensor signal is amplified by the electronics, indicated on the display and
converted into a 4 to 20 mA signal which is passed to the central controller.

Principle of Operation

1 Measuring chamber
2 Detector element
3 Sintered metal
4 Compensator element
4 2
1
3
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