5.
5.5.
5. ACCURACY AND LINEARITY
5.1 INSTRUMENT TESTS AND CHECKS
The SI9500 has been designed to provide maximum stability and linearity
given the nature of the test. Each unit is tested for linearity using highly
stable test capacitors that are traceable to national standards and each unit
is shipped with its unique test certificate giving the results of these tests.
As the instrument is a comparator it is impractical to state absolute accuracy
in terms of film weight. It is also difficult for the user to check for linearity
without extensive test equipment.
For companies operating to quality assurance standards or procedures that
require periodic instrument testing Sencon will undertake a linearity and
stability test for a nominal charge. For further details please contact your
appropriate sales centre.
5.2 FACTORS AFFECTING ACCURACY
The instrument is effectively measuring capacitance formed at the probe tip
as described in section 1.2. Unfortunately the probe creates a relatively
unstable capacitor resulting in instability in the reading being displayed. This
is particularly noticeable if the unit is set to display 2 decimal places.
The following factors all influence the stability and repeatability of the probe
which is reflected into display readings;
A) Pressure applied to probe tip
B) Angle of probe tip to sample
C) Firmness of surface below sample
D) Dirt and contamination of probe tip or sample
E) Softness of probe tip
F) Damage to probe tip
G) Humidity of atmosphere
H) Ambient temperature
It is for these reasons that periodic re-calibration to the set piece is required -
not because the instrument itself has drifted or changed.
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