Dräger PEX 3000, Руководство пользователя, Страница: 29 / 100

Principles of operation
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Dräger PEX 3000
3 Principles of operation
3.1 Feature description
The PEX 3000 transmitter consists of a 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 combustible
gases and vapours present in the atmosphere. It functions according to the
heat-of-reaction principle. The monitored ambient air is diffused through the
wire mesh disc into the ex-sensor. This is where the combustible gases and
vapours are catalytically burned at a heated detector element (pellistor). The
oxygen required for combustion is taken from the ambient air. The combustion
heat generated heats the detector element up further. This heating up results in
a change in the resistance of the detector element. This change is proportional
to the partial pressure of the combustible gases and vapours.
In addition to the catalytically active detector element, the ex-sensor
accommodates an inactive compensator element, which is also heated. Both
elements are part of a Wheatstone bridge. Environmental influences, such as
temperature, air humidity or thermal conductivity of the ambient air to be
monitored, affect both elements in the same way, so that these influences have
no significant effect on the measured signal.
The voltage measured on the ex-sensor is amplified by the electronics,
indicated on the display and converted into a 4 to 20 mA signal which is
transmitted to the controller.
Operating principle
1. Measuring chamber
2. Detector element
3. Sintered metal
4. Compensator element
3.2 Intended use
The PEX 3000 transmitter is intended to be used for the stationary, continuous
monitoring of mixtures of combustible gases and vapours with air under
atmospheric conditions. The full-scale value is 100 % or 10 % of the lower
explosive limit (LEL). The transmitter is connected to a suitable controller by
means of a 3-core-cable, the measured signal during normal operation is
between 3.8 and 20.5 mA.
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