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Appendix A
Summary of Specifications
Accuracy
Specifications of pressure transducer instrumentation can be divided into three categories:
Input Specifications, General Specifications, and Performance Specifications. Each of
these categories in turn consists of parameters which are usually specified by minimum
and/or maximum numeric limits. Almost all of these parameters can have an effect on
what is generally referred to as the instrument’s “accuracy.” Therefore, the accuracy of
pressure instrumentation can be varied either beneficially or detrimentally by controlling
the Input Specifications, operating within the General Specifications, or knowing the
actual Performance Specifications.
For example, if Input Specifications have not been met for the line voltage, the unit may
not have a catastrophic failure, but errors may be present in the transducer measurement.
As another example, if the requirement for the Pressure Source Flow Capacity has not
been met, the RUSKA 7350 may not be able to achieve a final steady state controlled
pressure within the settling time specification. Finally, if the RUSKA 7350 is
commanded to a pressure outside of the Applicable Control Pressure Range, the
nonlinearity in the pressure output may be greater than that specified.
Performance Specifications give the user the most flexibility and control over his
“accuracy claims.” The term accuracy is defined by ISA-S37.1 as either the ratio of the
error to the full-scale output (%FS) or the ratio of the error to the reading (%RDG). Note
that the definition of accuracy is not the summation of some or even all of the possible
error source maximum limits.
The true accuracy of an instrument is relative to the actual error introduced by the
calibration transfer standard plus the actual error not eliminated from the instrument’s
indicated output. Therefore, an instrument’s accuracy can be manipulated by introducing
more or less actual error through the choice of a calibration standard; or its accuracy can
be varied by the elimination of actual errors inherent in the instrument. For example, if an
instrument has a known error due to being used in an attitude, or tilt, the %FS zero shift
error can be eliminated by re-zeroing the instrument in the tilted position. Even %RDG
sensitivity shifts can be eliminated mathematically or by controlling the attitude of the
instrument during its calibration.