Analytical Industries GPR-35 Oxygen Analyzer - Manual (Page 7)

Advanced Instruments Inc.
7
Pressure & Flow
All electrochemical oxygen sensors respond to partial pressure changes in oxygen. The sensors are equally capable of analyzing
the oxygen content of a flowing sample gas stream or monitoring the oxygen concentration in ambient air (such as a confined
space such in a control room or an open area such as a landfill or bio-pond).
The GPR-35 designed for in-situ ambient or area monitoring and has no sample system
because the sensor is intended to be exposed directly to the surrounding ambient atmosphere
which it is sampling and to operate at atmospheric pressure, however, slightly positive
pressure has minimal effect on accuracy.
A certified span gas is recommended for calibration purposes, part number A-2344 Flow
Through Adapter is supplied with the GPR-35 and provides: a hose connection for piping the
calibration gas (see positive pressure see below) to the sensor, an o-ring seal to isolate the
sensor from the atmosphere being monitored and outlet to vent the calibration gas.
Further, applications situations may dictate that the sample be transported from a semi-sealed
area to a safe area where the analyzer is located. In these cases the analyzer can be readily adapted to include optional pumps,
tubing and connection fittings. Users interested in adding their own sample conditioning system should consult the factory.
Analyzers designed for flowing samples under positive pressure or pump vacuum (for samples at atmospheric or slightly
negative atmospheres) that does not exceed 14” water column are equipped with bulkhead tube fitting connections on the side
of the unit (unless otherwise indicated, either fitting can serve as inlet or vent) and are intended to operate at positive pressure
regulated to between 5-30 psig although their particular rating is considerably higher. In positive pressure applications the vent
pressure must be less than the inlet, preferably atmospheric.
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 provided 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.
Application Pressure - Atmospheric or Slightly Negative: An optional external sampling pump should be positioned
upstream of the sensor to draw the sample from the process, introduce it at a predetermined flow rate of 2 SCFH to the sensor
and out to atmosphere. A flow meter is generally not necessary to obtain the recommended flow rate with most sampling
pumps.
Caution: If the analyzer is equipped with an optional flow indicator with integral metering valve or a metering flow control
valve upstream of the sensor - open the metering valve completely to avoid drawing a vacuum on the sensor and placing an
undue burden on the pump.
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.
Recommendations to avoid erroneous oxygen readings and damaging the sensor:
¾ 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 (thus voiding
the sensor warranty).