Dräger Polytron Ex, Инструкция по эксплуатации, Страница: 30 / 39

Design and Mode of Operation
Page 30 of 40
9023744 - 3rd edition - October 2005
Design and Mode of Operation
The transmitter Polytron Ex consists of a housing, an Ex-sensor and the corres-
ponding electronics.
The transmitter Polytron Ex R consists of a housing with electronics and a sensing
head including an Ex-sensor, installed at a maximum distance of 10 m.
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 obtained from the monitored air. The detector
element is additionally heated by the resultant heat of reaction. This heat in turn cau-
ses 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 the two
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
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