Analytical Industries GPR-3500 MO Portable Oxygen Analyzer - Manual (Page 6)

Advanced Instruments Inc.
6
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. To reduce the possibility of leakage for low ppm measurements, position a
metering needle valve upstream of the sensor to control the flow rate and position a flow indicator downstream of the sensor. 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 the analyzer is 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 ppm range oxygen measurements, an optional external
sampling pump should be positioned downstream of the sensor to draw the sample from the process, by 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 - wipe away.
¾ If the analyzer is equipped with an optional integral sampling pump (positioned downstream of the sensor) and a flow control
metering valve (positioned upstream of the sensor), completely open the flow control metering valve to avoid drawing a vacuum
on the sensor and placing an undue burden on the pump.
Moisture & Particulates: Installation of a suitable coalescing or particulate filter is required to remove condensation, moisture and/or
particulates from the sample gas to prevent erroneous analysis readings and damage to the sensor or optional components. Moisture
and/or particulates do not necessarily damage the sensor, however, collection on the sensing surface can block or inhibit the diffusion
of sample gas into the sensor resulting in a reduction of sensor signal output – and the appearance of a sensor failure when in fact the
problem is easily remedied by blowing on the front of the sensor. Consult the factory for recommendations concerning the proper
selection and installation of components.