Principle Of Measuring - Dräger VarioGard IR EX Gebrauchsanweisung

Principle of Measuring

Ex-Transmitter
The VarioGard IR Ex Transmitter is a measuring transformer for monitoring
the concentration of explosive gases in the ambient air according to the
principles of absorption of infra-red radiation.
The ambient air to be monitored diffuses and accesses the optical ex-
sensor which is mounted on a printed circuit board.
The printed circuit board contains a microcontroller for processing signals,
an adjustable pre-amplifier for the detector signals and a 4 to 20 mA
interface for connection to a controller.
The light from the two lamps 1 and 2 in the sensor is evenly distributed via
a beam splitter 3 onto the measuring detector 4 and reference detector 5.
Parts of the gases to be detected in the ambient air cause an absorption of
the beam at the measuring wavelength and, thus, to a signal reduction of
the measuring detector.
The light from the two lamps is modulated at different frequencies. The light
from Lamp 1 serves for measuring the gas concentration, that from Lamp 2
for referencing the detectors. Thus, a superimposed signal from both lamps
is produced in the detectors which is evaluated in the microprocessor. As a
result, four individual signals are generated.
The gas concentration is determined based on the ratio of these signals.
The ratio of the signals is independent of the intensity of the lamps and
sensitivity of the detectors. This ensures a long service life and temperature
stability of the sensors.
In order to rule out high relative humidity influencing the measured signal, a
heating source is installed on the outside on the cell which raises the
temperature inside the cell above that outside.
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