Accuracy Of Cover; Autosizing Accuracy; Lapped Bars - Elcometer protovale P330 Operating Instructions Manual

Accuracy of cover

The displayed cover will only be correct if:
(a) the head is exactly over the centre-line of the bar;
(b) the head is exactly parallel to the bar;
(c) the instrument has been correctly zeroed (with AutoZero, this is no
problem);
(d) the head is not over, or near, any other bars, parallel or transverse;
(e) the correct bar size has been set (for the main display; not necessary for
AutoSizing).
Note to (d): if neighbouring bars are too close, they will cause the main display
to read too low a cover (and cause the AutoSizing to over-estimate bar
diameter); the main display of cover can only read too high if the actual bar
size is smaller than the diameter you have set.

AutoSizing accuracy

Since the instrument has to extract more information from the raw signal data
than in the simpler case of computing the cover to a bar of known size, it
stands to reason that the original data must be appropriately more reliable.
Hence the conditions (a) to (d) above must again be carefully met (but not (e)
of course), and furthermore the bar must not be too deep (otherwise the signal
may be too weak to be reliable); but in the majority of cases, the first layer of
bars will be well within range.
If the head is moved away from a bar, the "ordinary" cover reading will indicate
a higher value, correctly indicating that the bar is further away; but when the
head is not over a bar, the AutoSizing results are meaningless and should be
ignored. At deeper covers, the AutoSizing will lose accuracy, and this will be
apparent as a fluctuating value of estimated diameter; but the deduced cover
will usually be stable and correct. At very deep covers, AutoSizing is not even
attempted, and blanks are shown between the parentheses in the display.
In congested situations, the AutoSizing will tend to over-estimate bar diameter
— correctly reflecting the fact that there is a greater mass of metal present than
a single bar — but will nevertheless continue to give an essentially correct
value for the cover. In the case of lapped bars, AutoSizing will typically show
double the true diameter of a single bar, but the correct cover — which is
generally the desired result.
It should be stressed that the AutoSizing facility is intended to determine the
cover to a bar of unknown size; the value of diameter it gives is an estimation
only, which is however available immediately with no user-intervention
required; when you require more accurate determination of diameter, you
should use SIZE mode.
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