Analytical Industries GPR-3500 MO Portable Oxygen Analyzer - Manual (Page 10)

Advanced Instruments Inc.
10
Zero Calibration
In theory, the 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 a zero gas due to:
¾ Contamination or quality of the zero gas
¾ Minor leakage in the sample line connections
¾ Residual oxygen dissolved in the sensor’s electrolyte
¾ 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.
As part of our Quality Control Certification process, the zero capability of every ppm analyzer is qualified prior to shipment. However,
because the factory sample system conditions differ from that of the user, no ZERO OFFSET adjustment is made to analyzer by the
factory
Recommendations:
¾ Zero calibration 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 0.05 ppm on the 0-1 ppm range, 0.5 ppm on the 10 ppm range, or
below 0.1% (1000 ppm) with a percent analyzer.
¾ Determining the true Zero Offset requires approximately 24 hours to assure the galvanic fuel cell sensor has consumed the oxygen
that has dissolved into the electrolyte inside the sensor while exposed to air or percentage levels of oxygen. Allow the analyzer to
stabilize with flowing zero gas as evidenced by a stable reading or horizontal trend on an external recording device. For optimum
accuracy, utilize as much of the actual sample system as possible.
¾ Zero calibration is not practical and not recommended for portable analyzers or measurements on higher ranges. However,
satisfying these users that the zero offset is acceptable for their application without the 24 hour wait can be accomplished by
introducing a zero gas (or sample gas with a low ppm oxygen concentration) to the analyzer. Unless the zero gas is contaminated
or there is a significant leak in the sample connections, the analyzer should read less than 100 ppm oxygen within 10 minutes after
being placed on zero gas thereby indicating it is operating normally.
¾ Zero calibration should precede span calibration.
¾ Initiate the DEFAULT ZERO and DEFAULT SPAN procedures before performing either a ZERO or SPAN CALIBRATION.
¾ Caution: Prematurely initiating the ZERO CALIBRATION function can result in negative readings near zero.
¾ Once the zero offset adjustment is made, zero calibration is normally not required again until the sample system connections are
modified, or, when installing a new oxygen sensor.