Analytical Industries GPR-2500 A Oxygen Analyzer - Manual (Page 8)

8
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 or Tygon tubing inside the transmitter. The GPR-2500A internal
sample system includes 1/8” compression tube inlet and vent fittings, a
stainless steel sensor housing with an o-ring seal to prevent the leakage
of air and stainless steel tubing.
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 tube 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 1 liter 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 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.
Caution: If equipped with a H2S scrubber as part of an optional sample conditioning system, inlet
pressure must not exceed 30 psig.
Application Pressure - Atmospheric or Slightly Negative:
For accurate oxygen measurements, an optional external sampling pump should be positioned upstream of the
sensor to draw the sample from the process and push the sample by the sensor and out to atmosphere. A flow
meter is not necessary if the discharge of the sampling pump approximates the recommended 2SCFH flow rate.
If pump loading is a consideration, a second throttle valve on the pump’s inlet side may be necessary to provide a
bypass path so the sample flow rate is within the above parameters.