Annex
9323A_002-476e-11.24 Page 69
This requirement is found mainly in repetitive
measurements in production processes, where good
repeatability is usually sufficient for process monitoring.
Accuracy primarily plays only a subordinate role, when
measurements can be directly related to
conforming/nonconforming parts.
For good repeatability, piezoelectric measuring technology
offers the particular advantage that the charge can be
discharged with <Reset> before every measurement,
enabling the zero point to be re-determined. Errors due to
zero drift caused by influences changing with time, such as
the temperature, are thus basically excluded.
With Kistler piezoelectric measuring chains, a typical
repeatability within 0,1 % FSO can be assumed.
10.3 Linearity
Sensor linearity
The quartz crystal produces an electric charge, which is
exactly proportional to the load. However, certain
unavoidable deviations occur due to the mechanical
construction of the sensor. Linearity represents the
maximum deviation between ideal and actual output signal
characteristics in relation to the measurand in a specific
measuring range. It is expressed in percentage of the
particular measuring range limit and is defined according to
ANSI/ISA-S37.1 as the closeness of the calibration curve to
a "best straight line" passing through the zero point:
"Best Straight Line“ – A line midway between the two
parallel straight lines closest together and enclosing all
Output vs. Measurand values on a Calibration Curve.
The best straight line can be determined as follows:
Best straight line – geometric definition
A line midway the two parallel straight lines closest
together and enclosing all output versus measurand values
on a calibration curve. In addition, it must pass through the
zero point based on the assumption that zero measurand
results in zero output signal.
Note: The slope of this center line corresponds to the
sensitivity of the sensor. Half the distance between the two
parallels (measured in the ordinate direction) is the linearity
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