29
M
EASURING
SIZE
OF
REINFORCEMENT
BARS
(
ORTHOGONAL
R
13. If you are satisfied with the bar size, press [
] to set the Covermeter to use this size. If you are
not satisfied with the bar size, press [menu/esc] to return to the reading screen without making
any changes.
If the orthogonal size calculation fails (e.g. because of incorrect search head positioning or because
the bar is too deep), an error is reported with the option to retry the procedure.
9.3 Accuracy
In common with all bar-sizing methods, the orthogonal SIZE function is sensitive to errors in search
head positioning and also to the proximity of neighbouring bars (the pitch of the bars).
The net result of the error-generating influences listed below is that the estimated bar size (and
cover) will be too high. Hence the displayed result should be taken as an upper limit, never as a
lower limit.
Head-positioning
When taking the parallel reading (Figure 16), the search head should be getting the maximum
possible signal from the bar; any misalignment of the search head in either angle or position can
only reduce this signal.
When taking the orthogonal reading (Figure 17), the search head should be getting the minimum
possible signal from the bar; any misalignment of the search 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
estimated 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 search
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
significant) 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 search head is rotated to the orthogonal
position, such bars will become nearer to the ends of the search head than they were to its sides.
Welded mesh
Orthogonal sizing should not be attempted on welded mesh fabric; 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 ‘SIZE MODE ERROR’ message).
331.book Page 29 Thursday, March 12, 2009 9:46 PM
