Analytical Industries GPR-1000 ATEX Portable Trace PPM Oxygen Analyzer - Manual (Page 7)

7
Pressure & Flow
All electrochemical oxygen sensors respond to partial pressure changes in oxygen. The inlet pressure must always
be higher than the pressure at the outlet vent which is normally at atmospheric pressure.
Flow Through Configuration:
The sensor is exposed to sample gas that must flow or be drawn through
metal tubing inside the analyzer. The GPR-1000 internal sample system
includes a quick disconnect female inlet fitting, a stainless steel sensor
housing with an o-ring seal to prevent the leakage of air and another quick
disconnect female vent fitting. Mating male quick disconnect fittings are
provided for introducing sample and calibration gases, a third male fitting
is provided for the vent line.
Flow rates of 1-5 SCFH cause no appreciable change in the oxygen
reading. However, flow rates above 5 SCFH generate backpressure and
erroneous oxygen readings because the diameter of the integral tubing
cannot evacuate the sample gas at the higher flow rate. The direction the
sample gas flows is not important, thus either female fitting can serve as
the inlet or vent – just not simultaneously.
A flow indicator with an integral metering valve upstream of the sensor is
recommended as a means of controlling the flow rate of the sample gas. A
flow rate of 2 SCFH or 1liter per minute is recommended for optimum
performance.
Caution: Do not place your finger over the vent (it pressurizes the sensor)
to test the flow indicator when gas is flowing to the sensor. Removing your
finger (the restriction) generates a vacuum on the sensor and may damage the sensor (voiding the sensor
warranty).
To avoid generating a vacuum on the sensor (as described above) during operation, always
select and install the vent fitting first and remove the vent fitting last.
Application Pressure - Positive:
A flow indicator with integral metering valve (GPR-1000M option) positioned upstream of the
sensor is recommended for controlling the sample flow rate between 1-5 SCFH.
If necessary, a pressure regulator (with a metallic diaphragm is recommended for optimum
accuracy, the use of diaphragms of more permeable materials may result in erroneous
readings) upstream of the flow control valve should be used to regulate the inlet pressure
between 5-30 psig.