Analytical Industries GPR-1900 Series Trace PPB Oxygen Analyzer - Manual (Page 23)

Advanced Instruments Inc.
23
Zero Calibration
In theory, the oxygen sensor produces no signal output when exposed to an oxygen free sample gas. In reality, expect the
analyzer to 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 only for online analyzers intended for continuous precise measurements
below 5 ppm on the 0-100 ppm range and then only when the sample system connections are modified and when installing a
new oxygen sensor. It is not practical on higher ranges or portable analyzers for the following reasons:
1. Determining the true zero offset reguires the user allow the analyzer approximately 24 hours to stabilize with flowing zero
gas as evidenced by a stable reading with no downward trend on an external recording device. Note: Approximately 24-36
hours is required to assure the sensor has consumed the oxygen that has dissolved into the electrolyte inside the sensor
while exposed to air or percentage levels of oxygen. For optimum accuracy, utilize as much of the actual sample system as
possible.
2. Thus it is not practical to find the true zero offset particularly in the case of applications requiring higher level oxygen
measurements because of the low offset value, normally 50% of the most sensitive analysis range, is not material to the
accuracy of higher level measurements. Nor is it practical to zero a portable analyzer every time it is moved from one
sample point to another.
3. Caution: Prematurely zeroing the analyzer can cause a negative reading in both the ZERO and SAMPLE modes.
4. Satisfying users that the zero offset is reasonably acceptable for their application can be accomplished much quicker.
Unless the zero gas is contaminated or there is a significant leak in the sample connections, the analyzer should read less
than 1000 ppm oxygen within 2-3 minutes after being placed on zero gas thereby indicating it is operating normally.
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 the time required to determine the true zero offset of specific oxygen sensor, analyzer
and sample system combination.
The maximum zero calibration adjustment permitted is 50% of the lowest full scale range available. Accordingly, the analyzer’s
ZERO has not been adjusted prior to shipment because the factory conditions are different from the application condition at the
user’s installation.
1. Access the MAIN MENU by pressing the MENU key.
2. Advance the reverse shade cursor using the ARROW keys to highlight CALIBRATION.
3. Press the ENTER key to select the highlighted menu option.
4. The following displays appear:
MAIN MENU
AUTO SAMPLE
MANUAL SAMPLE
CALIBRATION
CONFIG ALARMS
BYPASS ALARMS
>>>
CALIBRATION
SPAN CALIBRATE
ZERO CALIBRATE
DEFAULT SPAN
DEFAULT ZERO
OUTPUT SPAN
OUTPUT ZERO
5. Advance the reverse shade cursor using the ARROW keys to highlight ZERO CALIBRATE.
6. Press the ENTER key to select the highlighted menu option.
7. The following displays appear: