Advanced Instruments Inc.
Calibration of Analyzer
The electrochemical oxygen sensors generate an electrical current that is linear or proportional to the oxygen
concentration in a sample gas. In the absence of oxygen the sensor exhibits an absolute zero, i.e., the sensor does
not generate a current output in the absence of oxygen. Given the properties of linearity and an absolute zero, a single
point calibration is possible.
The analyzer is equipped with “Zero Offset” feature. However, as described below, zero calibration is recommended
only when the application (or user) demands optimum accuracy of below 5% of the most sensitive or lowest range
available on the analyzer. For example, if the user requires analysis of a sample gas below 0.5 PPM, zero calibration is
highly recommended.
Span calibration, in one of the forms described below, is necessary to adjust the analyzer sensitivity for accurate
measurements of oxygen. As a rule of thumb, zero calibration should be carried out after span calibration.
Zero Offset
Despite the absolute zero inherent in the electrochemical oxygen sensors, the reality is that analyzers may display an
oxygen reading even when sampling a zero gas (oxygen free gas) due to:
1.
Contamination or questionable quality of the zero gas
2.
Minor leakage in the sample line connections
3.
Residual oxygen dissolved in the sensor’s electrolyte
4.
Tolerances of the electronic components
The maximum zero offset of every analyzer is checked prior to shipment. However, due to the fact that the factory
sample system conditions differ from that of the user, no ZERO OFFSET adjustment is made at 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. The frequency of calibration varies with the application, e.g., the degree of
accuracy required by the application and the quality assurance protocol 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
10-15 minutes. However, the time required to bring analyzer back on-line (within 0.01% of original value) after span
calibration 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 influence the calibration procedures.
All electrochemical sensor based analyzers require periodic calibration, e.g. weekly intervals to a 3 month maximum, to
ensure accuracy and ascertain the integrity of the sensor. Although, the sensor signal remains relatively constant
throughout the useful life of the sensor, some components in a gas stream, e.g., sulfides, can adversely affect the
sensor causing the sensor to loose its sensitivity with time. Hence it is highly recommended to verify/adjust the
sensitivity of the sensor by performing span calibration.
For optimum accuracy, calibrate the analyzer at or close to the temperature and pressure of the sample gas
The priority users place on getting or keeping an analyzer online is “the” most significant factor involved in calibration
and troubleshooting issues. The time it takes an analyzer to come down to a specific level after exposure to high O2
concentrations or air is significantly different if a sensor is being installed than if the sensor had been in-service at low
oxygen levels for more than 1 week
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