Design And Measuring Principle; Principle Of Operation - Dräger PEX 3000 XTR 0000 Notice D'utilisation

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 sintered metal 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 obtai-
ned from the monitored air. The detector element is additionally heated by the resul-
tant 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 con-
verted 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
Page 24 of 42
9023810 - 2nd edition - September 2005