Analytical Industries GPR-1600 Series Trace PPB Oxygen Analyzer - Manual (Page 35)

Advanced Instruments Inc.
35
Span Gas Selection:
The O2 concentration of a span gas should approximate 70-90% of the full scale range
dictated by the span gas, e.g. 80 ppm O2 on the 0-100 ppm range. For optimum accuracy,
the full scale range dictated by the span gas should be at least one range higher than the
intended analysis range. Both of the aforementioned recommendations reduce the error
induced by the tolerance of the electronic components; also, span gases with higher O2
concentrations are more reliable and less expensive.
Conversely, if the recommended span gas is not available and air calibration is not an
option, a span gas of the same full scale range and near the anticipated analysis level
(approximately 10% of full scale) is acceptable with the understanding the accuracy will
suffer slightly.
Use of span gas near 10% of the same full scale range for measurements at the higher end
of the range has the effect of “expanding the error” by moving upscale as illustrated by
Graph A and Example 1 in the Accuracy section above 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.
Type of Sensor:
Galvanic Fuel Cell Sensors: Analyzers utilizing these sensors can be calibrated with
ambient air (20.9% or 209,000 ppm O2) and have been proven capable of precise analysis
below 0.5 ppm (500 ppb).
Note: As described above, these oxygen sensors are capable of one point calibration.
Span Gas vs. Air
(Fuel Cell Sensors only):
Span gas calibration: Recommended for calibration of in-service analyzers, if the priority
is the fastest online recovery time.
Air calibration: Recommended for calibration an analyzer if a new oxygen sensor is being
installed or if the availability of span gas, the cost of span gas or the accuracy of a span gas
is an issue. An air calibrated analyzer can be used to reliably verify a “certified” span gas,
which has frequently been found to be inaccurate. For best results, select a recognized
name supplier.
Note: Galvanic Fuel Cell sensors can be calibrated (using air) and brought online (using
sample gas) without “purchased calibration gases” and without sacrificing accuracy -
provided the analyzer is given enough time to come down to the O2 concentration of the
sample gas.
Span Calibration
Adjustments:
Prematurely initiating the SPAN CALIBRATION function (when there is no intention of
performing a ZERO CALIBRATION) before the analyzer reading has stabilized can result in
erroneous readings as follows:
When purging an analyzer to lower ranges for span gas calibration: If the oxygen
reading reaches less than 2% of the intended calibration range, enter the value of the span
gas. If the oxygen reading is greater than 2% of the calibration range, add the O2 reading
to the value of the span gas (the impact of the offset on accuracy is minor but the addition
allows the oxygen sensor to continue to purge down and avoid negative readings).
Note: If ZERO CALIBRATION has been performed or the analyzer has been in service, the
analyzer reading should already be stable and below 2% of the calibration range.
When installing a new oxygen sensor and calibrating with air: Allow 5-10 minutes
for the sensor to equilibrate in ambient air from storage packaging. Failure to do so can
introduce a positive offset (electronic gain) that prevents the analyzer from displaying low
ppm O2 readings.