Elcometer protovale P330 Operating Instructions Manual page 20

Head-positioning:
When taking the Parallel reading, the head should be getting the maximum
possible signal from the bar: any mis-alignment of the head can only reduce
this signal; when taking the Orthogonal reading, the head should be getting
the minimum possible signal from the bar: any mis-alignment of the head–
in either angle or position– can only increase this signal.
Therefore, either form of positioning error will always result in the diameter
being over-estimated (and so too will be the cover).
Neighbouring bars:
Any other metal within range of the search head will increase both the
(orthogonal and parallel) readings by an unknown amount — but not
necessarily the same for both readings. In practice, since the orthogonal
signal is the weaker of the two, it is more susceptible to "interference", and
so the deduced diameter will probably again be too high.
Although, in normal use, transverse bars have comparatively little effect on
the cover obtained, these same transverse bars will have a serious effect
on this bar-sizing method: when the head is rotated to take the orthogonal
reading, the signal from the bar under investigation decreases, but the
signal from any nearby transverse bars increases.
Again, the result is a (possibly serious) over-estimation of bar diameter.
In particular, orthogonal sizing should never be attempted over the
crossing-point of transverse bars.
Neighbouring parallel bars, whilst not causing such drastic effects as
transverse ones, should nevertheless be taken into consideration; when the
head is rotated to the orthogonal position, such bars will become nearer to
the ends of the head than they were to its sides.
Welded mesh:
As explained in the earlier section on joined bars, the orthogonal reading
will receive an additional signal from the rectangular loop of bars. This will
result in a gross over-estimate of bar-size (or even the "does not compute"
message).
The net result is that all error-generating influences always result in the
deduced
bar-size (and cover) being too high.
Hence the displayed result should be taken as an upper limit, never as a
lower limit.
18