Analytical Industries GPR-1800 AIS ATEX Trace PPM Oxygen Analyzer - Manual (Page 8)

Advanced Instruments Inc.
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Zero Calibration: In theory, the electrochemical galvanic fuel cell type oxygen has an absolute zero meaning it produces no
signal output when exposed to an oxygen free sample gas. In reality, expect the analyzer to generate an oxygen reading when
sampling oxygen free sample gas due to contamination or quality of the zero gas; minor leakage in the sample line connections;
residual oxygen dissolved in the sensor’s electrolyte; and, tolerances of the electronic components. The Zero Offset capability of
the analyzer is limited to 50% of lowest most sensitive range available with the analyzer.
Recommendation 1: Zero calibration, see Determining True Zero Offset below, is recommended only for online analyzers
performing continuous analysis below 5% of the lowest most sensitive range available with a ppm analyzer, e.g. analysis below
0.5 ppm on the 10 ppm range, or below 0.1% (1000 ppm) with a percent analyzer.
Note 1: Once the zero offset adjustment is made, zero calibration is not required again until the sample system connections are
modified, or, when installing a new oxygen sensor. As a result, zero calibration is not practical and therefore not recommended
for higher ranges or portable analyzers.
Determining True Zero Offset: Allow the transmitter approximately 24 hours to stabilize with flowing zero gas as evidenced by a
stable reading or horizontal trend on an external recording device.
Note 2: 24 hours is required to assure the sensor has consumed the oxygen that has dissolved into the electrolyte inside the
sensor while exposed to air or percentage levels of oxygen. For optimum accuracy, utilize as much of the actual sample system
as possible.
Span Calibration: Involves adjusting the transmitter electronics to the sensor’s signal output at a given oxygen standard.
Regardless of the oxygen concentration of the oxygen standard used, a typical span calibration takes approximately 10 minutes.
Note 3: The amount time required to get the analyzer back on line for normal use is influenced by a.) the level of oxygen
analysis anticipated during normal operation (also determines the initial analyzer selection), and, b.) whether the sensor is new
or has been in service for at least two weeks. General guidelines for analyzers to come online following span calibration and
exposure to a zero/purge/sample gas with an oxygen content below the stated thresholds:
¾ measurements above 1000 ppm or 0.1% require less than 3 minutes
¾ measurements above 100 ppm (parts-per-million analyzer) require less than 10 minutes
¾ measurements below 10 ppm (part-per-million analyzer) require 20 minutes if the sensor has been in service at ppm levels
for at least two weeks, and, 60 minutes if the sensor is new assuming the zero/purge/sample gas has an oxygen
concentration below 1 ppm
Recommendation 2: For 'optimum calibration accuracy' calibrate with a span gas approximating 80% of the full scale range
one or two ranges higher than the full scale range of interest (normal use) to achieve the effect illustrated on Graph A and
Example 1. Always calibrate at the same temperature and pressure of the sample gas stream.
Note 4: Calibrating with a span gas approximating 10% of the full scale range near the expected oxygen concentration of the
sample gas is acceptable but less accurate than ‘optimum calibration accuracy’ method recommended – the method usually
depends on the gas available. Calibrating at the same 10% of the full scale range for measurements at the higher end of the
range results in magnification of errors as discussed in Graph A and Example 1 and is not recommended. Of course the user can
always elect at his discretion to accept an accuracy error of +2-3% of full scale range if no other span gas is available.
Air Calibration: Based on the inherent linearity of the electrochemical galvanic fuel cell type oxygen sensor enables the user to
calibrate the analyzer with ambient air (20.9% oxygen) and operate the analyzer within the stated accuracy spec on the lowest
most sensitive range available with the analyzer – there is no need to recalibrate the analyzer with span gas containing a lower
oxygen concentration. Calibrating either a ppm 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 in Recommendation 2.
Recommendation 3: 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 ppm oxygen sensor,
to verify the oxygen content of a certified span gas or when a certified span gas is not available to calibrate a ppm analyzer
(immediately following air calibration reintroduce a gas with a low oxygen concentration to expedite the return to ppm level
measurements as per Note 3).