Analytical Industries GPR-1200 MS ATEX Portable PPB Oxygen Analyzer - Manual (Page 18)

18
Zero Calibration
In theory, the oxygen sensor produces no signal output when exposed to an oxygen free sample gas. However, the
analyzer will generate an oxygen reading when sampling oxygen free sample 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
Recommendation: Zero calibration is recommended for measurements below 1 ppm only, as it is not practical on
higher ranges as described below.
Procedure:
Zero calibration should precede the span calibration and once performed should not have to be repeated with
subsequent span calibrations. Normally, zero calibrations are performed when a new sensor is installed or changes
are made in the sample system connections, especially impractical in the case of a portable analyzer.
Refer to Span Calibration below for the detailed procedure. Differences include substituting a suitable zero gas for
the span gas and allowing the analyzer 24 hours with flowing zero gas to determine the true zero offset (a stable
reading evidenced by a horizontal trend on an external recording device) of the system before conducting the zero
calibration. Note: 24 hours is required for the sensor to consume the oxygen that has dissolved into the electrolyte
inside the sensor (while exposed to air or percentage levels of oxygen).
Note: Thus, it is not practical to zero a portable analyzer every time it is moved from one sample point to another.
Finding the true zero offset is not always necessary particularly in the case of applications requiring higher level
oxygen measurements because of the low offset value, normally < 0.1 ppm, is not material to the accuracy of
higher level measurements.
Note: Prematurely zeroing the analyzer can cause a negative reading in both the ZERO and SAMPLE modes.
1. Access the MAIN MENU by pressing the MENU key.
2. Advance the reverse shade cursor to highlight CALIBRATION.
3. Press the ENTER key to select the highlighted menu option.
4. The following displays appear:
MAIN MENU
SPAN CALIBRATE
ZERO CALIBRATE
DEFAULT SPAN
DEFAULT ZERO
OUTPUT SPAN
OUTPUT ZERO
>>>
0.000 PPM
ZERO
CALIBRTION
ENTER TO CALIBRATE
ESCAPE TO ABORT
5. Press the ENTER key to calibrate, refer to the Span Calibration section for the detailed procedure.
Both the Zero Calibrate and Span Calibrate functions result in the following displays:
PASSED
CALIBRATION
OR
FAILED
CALIBRATION