Analytical Industries GPR-2500 S Oxygen Analyzer - Manual (Page 7)

7
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.
Outlet Pressure: In positive pressure applications the vent pressure must be less than the inlet, preferably
atmospheric.
Sample systems and flowing gas samples are generally required for applications involving oxygen measurements at
a pressure other than ambient air. In these situations, the use of stainless steel tubing and fittings is critical to
maintaining the integrity of the gas stream to be sampled and the inlet pressure must always be higher than the
pressure at the outlet vent which is normally at atmospheric pressure. The sensor is exposed to sample gas that
must flow or be drawn through metal tubing inside the analyzer. The internal sample system includes 1/8”
compression inlet and vent fittings, a delrin (stainless steel is optional) 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 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).
¾ Assure there are no restrictions in the sample or vent lines
¾ Avoid drawing a vacuum that exceeds 14” of water column pressure – unless done gradually
¾ Avoid excessive flow rates above 5 SCFH which generate backpressure on the sensor.
¾ Avoid sudden releases of backpressure that can severely damage the sensor.
¾ Avoid the collection of liquids or particulates on the sensor, they block the diffusion of oxygen into the sensor -