Analytical Industries GPR-2900 Series Oxygen Analyzer - Manual (Page 8)

Advanced Instruments Inc.
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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 analyzer 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 analyzer 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. Except for Oxygen Purity Analyzers intended to measure elevated oxygen levels ranging from 50-100%
oxygen, calibrating either a ppm or percent analyzer with ambient air on the CAL or 0-25% range meets the 80% criteria
discussed in Recommedation 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).