How
well do coating thickness gauges work?
What
does "accuracy" mean?
A key decision on
the overall selection of a suitable coating thickness gauge is how accurate
do the readings need to be?
There is a progression
from moderately accurate to very accurate gauges, which is mirrored
by the price of the gauge. In addition the coating application process
and other factors affect the variability of the coating thickness on
a particular surface and the skill and knowledge of the coating thickness
gauge operator also influences results.
The basic measure
of a coating thickness gauge's performance is the accuracy with which
the gauge takes readings. That is the difference between the reading
and the true coating thickness. Elcometer quote accuracy for gauges
in the form:
± X% of reading or
±Y µm, whichever is the greater.
This means that a
gauge with a specification of ± 1% or ±2.5 µm, whichever is the greater
will give readings within 2.5 µm of the actual value from zero to 50
µm and above 50 µm the reading will be within 1% of the actual value.
Testing
For Accuracy.
In order to test
the accuracy of a particular gauge it is important to have traceable
coating thickness standards. With the gauge adjusted to zero on an uncoated
smooth substrate and set to a known thickness standard at, or near to
the maximum thickness, intermediate thickness standards are measured
and the readings compared to the actual thickness of the standard.
The errors are the
differences between the values of the reading and the value of the standard.
These are most conveniently expressed as a percentage of the reading.
Misleading
Accuracy Statements.
The topic of coating
thickness gauge accuracy is a minefield of misleading, inappropriate
and unclear statements.
In sales literature,
the commendable objective of keeping things simple can result in accuracy
statements that are not clear and understandable.
For example, there
are gauges on the market whose accuracy performance is expressed in
terms of fixed and variable tolerance. In this case these two parameters
have to be added together to obtain the gauge's accuracy performance.
It is too easy for
the unsuspecting reader to assume that as the variable tolerance is
stated as 1% that the reading will be within 1% of the true value. In
practise the variable tolerance of 1% (1 µm at 100 µm) has to be added
to the fixed tolerance (2 µm over the range 50 to 1,500 µm) making at
total of 3 µm or 3% at 100 µm.
Other examples of
accuracy statements that can all too easily be misunderstood include:
-
Accuracy is X%
of full scale deflection
-
Accuracy is typically
Y%
In the first case
this is a style of statement that goes back to the days of analogue
meters that carried the scale for the gauge. An example would be a 0
to 500 µm range gauge with an accuracy of ±1% of full-scale deflection
i.e. ± 5 µm across the range. At 100 µm the accuracy, in Elcometer
terms would be ± 5% of reading!
In the second case
the word "typically" is being used to mask the fact that not all gauges
of this type achieve Y% or that Y% does not apply at all points on the
full range of the gauge.
The strong message
is, ask your supplier what his accuracy statement means and ask him
to demonstrate it to you using measured thickness standards and where
possible a sample of the substrate material for which the gauge is required.
|
