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Pressure Systems, Inc. 98RK-1 & 9816 User’s Manual©
Page 38 www.PressureSystems.com
3.1.5.3 Calibration Adjustment of Offset/Gain Correction Coefficients
All NetScanner
™
System Intelligent Pressure Scanners have built-in software commands (and
pneumatic hardware) to perform a periodic zero and span calibration adjustment of its internal
or attached pressure transducers. Use of these periodic adjustments result in the highest
possible data accuracy. The result of these calibrations is a new set of internal offset and gain
coefficients. These correction coefficients are over and above those factory-determined and
unchanging thermal correction coefficients stored in each transducer's non-volatile memory.
The factory coefficients provide basic engineering unit conversion capability, while also
correcting for various non-linear effects, including temperature effect compensation. The offset
and gain correction coefficients provide for fine “linear fit” adjustment of the factory calibration of
each transducer. If used properly, the periodic zero and span calibration adjustment
should be the only calibration required to maintain specified performance throughout the
life of the Intelligent Pressure Scanner.
It is generally necessary for the transducer to have real “zero” and “span” pressure points
(specified as 2 or more values) applied when calibration adjustment is required These pressure
values may be generated by secondary pressure standards, such as the Model 903x calibrator
module or by other external means provided by the customer (such as a dead weight calibrator).
For the more common zero-only calibration adjustment, zero differential pressures can typically
be provided without the need for external pressure generators. All 9816 models have built-in
pneumatic inputs (CAL side inputs) and calibration manifolds required for directing the
generated pressures to the various channels of the module(s) being calibrated. Refer to
Chapter 4 of this manual for detailed background and procedures for periodic calibration
of the Intelligent Pressure Scanners. A summary of the commands used for calibration
purposes is included below.
The Calculate and Set Offsets ('h') command is executed only when a suitable “minimum”
(e.g., zero) pressure has been applied to all channels of the module. The new offset
coefficients that result from execution of this command are stored in the module’s volatile (or
temporary) engineering-unit conversion database. They are also returned to the host in the
command’s response.
The Calculate and Set Gains ('Z') command should be executed only when “full-scale” (or
other suitable specified up-scale) pressure has been applied to the appropriate channels of a
module. The new gain coefficients that result from this command are stored in the module’s
volatile (or temporary) engineering-unit conversion database. They are also returned to the host
in the command’s response.
In modules using firmware version 2.24 or later, a Configure/Control Multi-Point Calibration
('C') command is provided. This command (actually 4 sub-commands) is an improvement over
the single calibration commands ('h' and 'Z') described above. Though 'C' provides for the
adjustment of the same offset and gain correction coefficients already described above, it does
so with two or more applied pressure calibration points. The final linear fit (i.e., new offset and
gain correction coefficients) is a “least squares” correction fit between all the calibration points
specified. This 'C' command is particularly useful in calibrating differential transducers over
their entire negative-to-positive range.
Although the calculated offset and gain correction coefficients simply remain in volatile memory
following execution of the calibration commands (for use by all subsequent EU conversions),
they may be optionally stored in non-volatile transducer memory with the Set Operating
Options ('w') command (Index 08 and 09).