Analytical Industries GPR-2500 Series Oxygen Transmitter - Manual (Page 24)

24
Zero Calibration
In theory, the oxygen sensor produces no signal output when exposed to an oxygen free sample gas. However, the
transmitter 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% on the 0-1% range, 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.
Refer to Span Calibration below for the detailed procedure. Differences include the displays illustrated below,
substituting a suitable zero gas for the span gas and allowing the transmitter 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).
Thus, for the reasons above, it is not practical to zero a percent transmitter. 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 transmitter 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 using the ARROW keys to highlight CALIBRATE.
3. Press the ENTER key to select the highlighted menu option.
4. Repeat to select ZERO CALIBRATE.
5. The following displays appear:
MAIN MENU
AUTO SAMPLE
MANUAL SAMPLE
CALIBRATE
24.5 C 100 KPA
>>>
CALIBRATION
SPAN CALIBRATE
ZERO CALIBRATE
DEFAULT SPAN
DEFAULT ZERO
24.5 C 100 KPA