Closely-Spaced And Lapped Bars; Lapped Bars - Elcometer Protovale CoverMaster CM52 Operating Instructions Manual

5.6 Closely-spaced bars
If the head is scanned lengthwise along a bar whose location has already been accurately
determined, the signal should stay constant unless the head is moving over either a
transverse bar or a lapped join. Section 5.4 dealt with transverse bars, which have a
characteristic double-humped response (Figure 8). Any other marked increase in signal
strength when scanning along a bar is likely to indicate a lapped join between two touching
bars (see section 5.7).
The following procedure can be used to identify whether a single audible indication is in
fact being caused by two bars running parallel to the surface, quite close together but not
touching.
1. Set the DIAM control to the anticipated bar size, or a smaller size if the cover is
deep.
2. Set MODE to CALibrate, and zero the instrument in 'free air' as normal. The
digital display is now indicating signal strength, and will be more precise than the
audible indication.
3. Align the search head parallel to the bars and scan sideways, watching the digital
display carefully.
4. The digital display will show a maximum when the centre-line of the head is over
each bar, with a small dip when the centre-line is between the bars. This change
may not be noticeable on the audible tone.
5. With the centre-line of the head located accurately between the two bars, turn the
ZERO control slowly anticlockwise until the digital display shows a small but still
positive value.
differences in the digital display (and possibly also the audible tone) will be more
pronounced.
6. If no dip is observable in steps 4 and 5, the bars may be lying one above the
other.
A smaller search head is available to provide better resolution at cover depths of 5-35mm
– see section 5.12 for details, or contact Protovale for availability.

5.7 Lapped bars

Two touching lapped bars represent the most extreme case of irregular or multiple-bar
reinforcement, so the simple advice in section 4.3 (to move the DIAM control one click
clockwise) may not give adequate compensation for error in cover measurement.
If the two lapped bars are known to be lying side by side, parallel to the surface, set DIAM
to twice the diameter of either bar, If the bars are known to be lying one above the other,
measure the cover with DIAM set to one size larger than either bar, If the lie of the lap is
unknown, setting DIAM to two sizes larger should give an answer within about ±2mm.
When the head is now scanned across the two bars, any
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