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Calibration
Automatic Zero and Span Adjustment
Automatic Zero and
Span Adjustment
4-14
Model 48i Trace Level-Enhanced Instruction Manual
Span Error, % = (((A
80
Where:
Z = Recorder response obtained at the last calibration for zero air in % scale.
[CO] = Span concentration
URL: The current full-scale range setting
For more information on regulatory requirements regarding reporting of
zero and span errors, please consult the latest copy of the Quality Assurance
Handbook for Air Pollution Measurement Systems
in setting up a quality assurance program, refer to the Code of Federal
Regulations and the EPA Handbook on Quality Assurance.
While the gas-filter correlation IR measurement method used by this
analyzer can provide highly specific, accurate and precise data, the ability to
make continuous measurements at low ppm concentrations may be limited
by gradual changes in electronic baseline, or zero background. This drift
occurs primarily due to slight changes in environmental parameters, such as
temperature. The enhanced trace level version of the 48i CO analyzer uses
both hardware and software compensation to limit the impact of changes in
the instrument's environment. One key feature that is included in the
instrument design is an auto-zeroing function that allows the instrument to
automatically check and adjust the zero background on a periodic basis.
The auto-zero feature includes a solenoid valve that re-routes the sample
stream through an internal CO scrubber at an interval that is programmed
by the operator. Typically, the auto-zero will have a duration of ten
minutes and will be programmed to occur once every two hours.
While the auto-zero function greatly improves instrument stability, it is not
intended to replace the routine calibration procedures described in this
chapter.
In addition to the automatic zero function, the Model 48i Trace Level-
Enhanced also includes the capability for fully automated span testing
and/or adjustment. In order to use this feature, the span gas cylinder must
be connected directly to the Span In bulkhead on the rear panel of the
instrument. The span gas pressure should be set to 10 ± 2 psi and the
operator should verify that the flow rate of span gas is enough to provide a
small excess over what the instrument pump will draw.
Although the automatic zero and span functions are designed to operate
from the same internal timer, the choice of whether to adjust or simply
- Z)*(URL/100)) - [CO]) * (100 / [CO])
1
and for detailed guidance
Thermo Fisher Scientific
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