Analytical Industries GPR-2900 Series Oxygen Analyzer - Manual (Page 25)

Advanced Instruments Inc.
25
Span Calibration
Maximum drift from calibration temperature is approximately 0.11% of reading per °C. The analyzer has been calibrated at the
factory. However, in order to obtain reliable data, the analyzer must be calibrated at the initial start-up and periodically
thereafter. The maximum calibration interval recommended is approximately 3 months, or as determined by the user’s
application.
Calibration involves adjusting the analyzer electronics to the sensor’s signal output at a given oxygen standard, e.g. a certified
span gas with an oxygen content (balance nitrogen) approximating 80% of the next higher full scale range above the intended
measuring range is recommended for optimum accuracy, see Calibration and Accuracy.
Recommendation – based on the inherent linearity of the galvanic oxygen sensor air calibrate the analyzer as described
below when installing and replacing the oxygen sensor (exception UHP and MS versions of the Pico Ion Sensor); or, to verify the
oxygen content of a certified span gas; or, when a certified span gas is not available (immediately following air calibration
reintroduce a gas with a low oxygen concentration to expedite the return to ppm level measurements).
Caution: Prematurely initiating the SPAN CALIBRATION key before the galvanic fuel cell sensor based analyzer reading has
stabilized can result in erroneous readings.
For example, to assure an accurate air calibration when installing a new ppm oxygen sensor from its packaged oxygen free
atmosphere allow the oxygen sensor 2-3 minutes to reach equilibrium with the oxygen content of the ambient air surrounding it
before pressing the SPAN CALIBRATE key. A ppm oxygen sensor that has not been allowed to reach equilibrium will generate a
lower current output than a ppm oxygen sensor that has reached equilibrium. Pressing the SPAN CALIBRATE key before the
ppm oxygen sensor has reached equilibrium forces the micro-processor to prematurely read the (erroneous low) current output
of the ppm oxygen sensor and introduce larger (erroneous) than required electronic gain adjustment and display (also
erroneous) CALIBRATION SUCCESSFUL message to the user. The error will become evident when a zero gas with a low oxygen
concentration is introduced into the ppm analyzer to purge it down below 10 ppm. The analyzer reading may stop and appear
to stabilize as high as 1800 ppm – giving the user the (erroneous) impression there is a problem with the ppm oxygen sensor
when in fact the problem lies with the user’s failure to follow the recommended calibration procedure.
Required components: Refer to Installing Span Gas section above.
1. Certified span gas cylinder with an oxygen concentration, balance nitrogen, approximating 80% of the full scale range
above the intended measuring range.
2. Regulator to reduce pressure to between 5 and 30 psig.
3. Flow meter to set the flow between 1-5 SCFH,
4. Suitable fittings and a 4-6 ft. in length of 1/8” dia. metal tubing to connect the regulator to the flow meter inlet
5. Suitable fitting and a 4-6 ft. in length of 1/8” dia. metal tubing to connect to the flow meter vent
6. 1/8” male NPT to tube adapter fitting to connect the 1/8” dia. metal tubing from the flow meter vent to the mating male
fitting supplied with the GPR-2900.
Procedure:
The user must ascertain that the oxygen reading (actually the sensor’s signal output) has reached a stable value within the
limits entered below before entering the span adjustment. Failure to do so will result in an error.
This procedure assumes a span gas under positive pressure and is recommended for a analyzer without an optional sampling
pump, which if installed downstream of the sensor should be placed in the OFF position and disconnected so the vent is not
restricted during calibration. To assure an accurate calibration, the temperature and pressure of the span gas must closely
approximate the sample conditions.
For calibration purposes, use of the AUTO SAMPLE mode is recommended. However, the user can select the full scale MANUAL
SAMPLE RANGE for calibration as dictated by the accuracy of the analysis required – for example, a span gas with an 80 ppm
oxygen concentration with the balance nitrogen would dictate the use of the 0-100 ppm full scale range for calibration and a 0-
10 ppm measuring range. Select as described above.
1. Access the MAIN MENU by pressing the MENU key.
2. Advance the reverse shade cursor using the ARROW keys to highlight AUTO SAMPLE.
3. Press the ENTER key to select the highlighted menu option.
4. The following displays appear: