Advanced Instruments GPR-2800 IS, Руководство пользователя, Страница: 25 / 52

Advanced Instruments, Inc
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NOTE: With oxygen reading above 110% of the selected range, the mA signal output will increase but will
freeze at a maximum value of 24 mA. After the oxygen reading falls below the full scale range, the mA signal
will become normal.
Calibration of Transmitter
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.
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. 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 Calibration
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:
•
Contamination or questionable quality of the zero gas
•
Minor leakage in the sample line connections
•
Residual oxygen dissolved in the sensor’s electrolyte
•
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 a PPM analyzer back on-line 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
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