DH Instruments PPCK PLUS, Operation And Maintenance Manual

Page: 124 / 150

PPCK+™ Operation and Maintenance Manual
©2001 DH Instruments, Inc .
Page 116
5.2 CALIBRATION OF REFERENCE PRESSURE
TRANSDUCERS
5.2.1 PRINCIPLE
PPCK+ has reference pressure transducers (RPT) that is the source of precise pressure
measurement for the system. The RPT has three independent ranges.
To calibrate a range, pressures from a standard are applied to the RPT at ascending and
descending points over the range. The pressure defined by the standard and the
corresponding RPT readings are recorded at each point. After all of the pressures have been
applied and recorded, adjustments are made to fit the RPT pressure readings to the
standard. Fitting the readings means performing a linear regression to arrive at the lowest
value of the residuals of errors of the transducer relative to the standard. The transducer
readings are adjusted by user settable coefficients: PA (an adder or offset) and PM (a
multiplier or span set) (see Section 5.2.1.1).
The calibration process is performed independently on each RPT range to arrive at the
optimal fit for each range. This technique allows reduced measurement uncertainty for
individual ranges lower than the RPT’s maximum range by taking into account specific
transducer performance characteristics, in particular localized non-linearity and excursion
dependent hysteresis. Independent range calibration also makes it possible to calibrate only
certain ranges if ranges are on different calibration intervals or not all ranges are needed.
The normal calibration process has a second step which is the determination of the value of
ZNATERR (see Section 5.2.1.2). This added step is necessary to get the full benefit of the
autozero function between calibrations (see Section 3.4.1) if the RPT will be operated in
absolute measure mode.
CalTool for RPTs software provided with the PPCK+ supports the calibration process of
PPCK+ RPTs. CalTool and its documentation is provided on a General Accessories Disk with
the new PPCK+ accessories. Most users should use CalTool software to assist in the
calibration of PPCK+.
PPCK+ is delivered with an interactive reference pressure transducer calibration utility that
steps the operator through the complete RPT calibration procedure including applying the
necessary pressures to each range, collecting data automatically, calculating new PA and
PM values, previewing the results of the new calibration, determining ZNATERR and
activating the results of the new calibration (see the CalTool for RPTs manual on the General
Accessories Disk). PPCK+ also provides complete front panel and remote access to RPT
calibration parameters so that RPT calibrations can be performed without using CalTool
software if desired (see Section 5.2.7).