Analytical Industries GPR-1600 Series Trace PPB Oxygen Analyzer - Manual (Page 34)

Advanced Instruments Inc.
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Zero Calibration Offset
Adjustment Capability: Designed to facilitate precise analysis below 5% of the most sensitive or lowest range
available on the analyzer, this feature enables users’ to compensate for background offsets,
as defined above, of up to 50% of the most sensitive or lowest full scale range available on
the analyzer and bring analyzers online faster.
As described below, accomplishing either objective places a degree of responsibility on the
user. Determining the true offset requires the user to wait, see Online Recovery Time
section, until the analyzer reading is no longer trending downward (best evidenced by a
constant horizontal trend on an external recording device). Bringing the analyzer online
faster, basically the same as choosing not to wait for the stable horizontal trend reading,
requires the user to repeat the ZERO CALIBRATION function. The frequency of which is at
the user’s discretion, hourly is recommended but at least when the reading goes negative.
Advanced Instruments’ oxygen analyzers are capable of zero offset adjustments of 50% of
the most sensitive or lowest range available on the analyzer. Since every analyzer is QC
tested to 1% of the most sensitive or lowest range available, exercise CAUTION when
initiating the ZERO CALIBRATION function at 50% (prematurely) of the most sensitive or
lowest range available on the analyzer. If the anticipated O2 analysis level is less than the
offset value or if adequate time is not allowed for the analyzer to establish the true offset,
the analyzer will in all probability display a negative reading.
Note: From the SYSTEM menu option “Display Negative (Reading)” users can toggle
between ON and OFF choose by pressing the ENTER key and control whether
analyzer displays negative readings.
Type of Analyzer:
Online ppb or ppm process analyzers: Analysis below 5% of the most sensitive or
lowest range is normally limited to these analyzers. However, such analysis is possible with
portable analyzers from Advanced Instruments due to their 60 day duty cycle and/or ability
to operate during battery charging.
Portable analyzers: Typically used intermittently moving between different sample
points/systems for trend analysis above 5% of the most sensitive range and, therefore, they
fall into the “all other analysis” category requiring only span calibration.
Percentage analyzers: Generally used above 5% of the most sensitive range and,
therefore, fall into the “all other analysis” category requiring only span calibration.
Online Recovery Time:
The priority users place on getting or keeping an analyzer online is “the” most significant
factor involved in calibration and troubleshooting issues. The time it takes an analyzer to
come down to a specific level after exposure to high O2 concentrations or air is significantly
different if a sensor is being installed than if the sensor had been in-service at low ppm
levels for more than 1 week as illustrated below:
Sensor Calibration at Install In-service Calibration
ppm Fuel Cell
Air to 100 ppm < 10 min
Air to 10 ppm < 1 hr
Air to 1 ppm < 6 hrs
Air to .1 ppm < 16 hrs
Air to 1 ppm < 30 min
80 ppm to .1 ppm < 10 min
8 ppm to .05 ppm < 5 min
The above times assume the introduction of a purge gas, the lower of the available zero or
sample (if known and constant) gas, with a ppm level O2 concentration less than 0.5 ppm is
introduced to the analyzer following span calibration to purge (accelerate the reaction of)
the O2 that has dissolved into the electrolyte inside the sensor. If zero gas is not available,
substitute the sample gas and expect slightly longer recovery time.