Analytical Industries GPR-2800 AIS ATEX Oxygen Analyzer - Manual (Page 10)

Advanced Instruments Inc.
10
Calibrating either a % or percent analyzer with ambient air (with the exception of Oxygen Purity Analyzers intended to measure
elevated oxygen levels ranging from 50-100% oxygen) on the CAL or 0-25% range meets the 80% criteria discussed above. Air
calibrate the analyzer (with the exception of Oxygen Purity Analyzers intended to measure elevated oxygen levels ranging from
50-100% oxygen) when operating a percent analyzer, installing and replacing a % oxygen sensor, to verify the oxygen content
of a certified span gas or when a certified span gas is not available to calibrate a % analyzer (immediately following air
calibration reintroduce a gas with a low oxygen concentration to expedite the return to % level measurements).
Materials: Assemble the necessary zero, purge and span gases and optional components such as valves, coalescing or
particulate filters, and, pumps as dictated by the application; stainless steel tubing is essential for maintaining the integrity of
the gas stream for % and percentage range (above or below ambient air) analysis; hardware for mounting.
Temperature: The sample must be sufficiently cooled before it enters the transmitter and any optional components. A coiled
10 foot length of ¼” stainless steel tubing is sufficient for cooling sample gases as high as 1,800ºF to ambient.
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).
Sample systems and/or flowing gas samples are generally required for applications involving oxygen
measurements at levels other than ambient air and when the pressure exceeds ambient. 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.
Flow Through Configuration: The sensor is exposed to sample gas that must flow or be drawn through
metal tubing inside the transmitter. The internal sample system includes 1/8” compression 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. To reduce the possibility of leakage for low %
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 equipped with a H2S scrubber, inlet pressure must not exceed 30 psig.
Application Pressure - Atmospheric or Slightly Negative: For accurate % 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.