Analytical Industries GPR-1600 Series Trace PPB Oxygen Analyzer - Manual (Page 33)

Advanced Instruments Inc.
33
Calibration
The electrochemical oxygen sensors manufactured by Analytical Industries Inc. (dba Advanced Instruments) generate an
electrical current that is linear or proportional to the oxygen concentration in the sample gas. In the absence of oxygen the
sensor exhibits an absolute zero, e.g. the sensor does not generate a current output in the absence of oxygen. Given the
properties of linearity and an absolute zero, single point calibration is possible.
As described below, zero calibration is recommended only when the application (or user) demands optimum accuracy for
analysis below 5% of the most sensitive or lowest range available on the analyzer. Span calibration in one of the forms
described below is sufficient for all other measurements. When employed zero calibration should precede span calibration.
Zero Calibration
Despite the absolute zero inherent in electrochemical oxygen sensors, the reality is that analyzers display an oxygen reading
when sampling a zero 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
The zero capability (low end sensitivity) of every analyzer is qualified prior to shipment. However, because the factory sample
system conditions differ from that of the user, no ZERO OFFSET adjustment is made to the analyzer by the factory
Span Calibration
Involves periodically, see Intervals section below, checking and/or adjusting the electronics to the sensor’s signal output at a
given oxygen standard or span. Maximum drift from calibration temperature is approximately 0.11% of reading per °C. The
frequency of calibration varies with the application conditions (potential for contamination), the degree of accuracy required by
the application and the quality requirements of the user. However, the interval between span calibrations should not exceed
three (3) months.
Note: Regardless of the oxygen concentration of the standard used, the span calibration process takes approximately 10
minutes, however, the time required to bring a ppm analyzer back on-line can vary, see Online Recovery Time below.
Considerations
When it comes to the calibration of oxygen analyzers utilizing an electrochemical oxygen sensor circumstances vary widely from
the ideal conditions that exist at the factory to a variety of differing circumstances users encounter in the field. The following
describes the most common factors and reasons that they influence the calibration procedures.
Factor Reasons
Intervals:
All electrochemical sensor based analyzers require periodic, e.g. weekly intervals to a 3
month maximum, calibration to ensure accuracy and ascertain whether the sensor has been
contaminated or otherwise damaged while in service.
Conditions:
Calibrate at the temperature and pressure of the sample.
Analysis Level Required:
Continuous analysis below 5% of the most sensitive or lowest range available:
ZERO CALIBRATION followed by SPAN CALIBRATION with good quality gases is
recommended (for optimum accuracy) when:
- the analyzer and/or O2 sensor is initially installed,
- the sample system connections are modified,
- the O2 sensor is replaced.
Note: It is not necessary to repeat the ZERO CALIBRATION with subsequent periodic
SPAN CALIBRATION unless desired or one of the above events occurs.
All other analysis: SPAN CALIBRATION is sufficient. Procedure varies with factors.