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Since the difference in indicated cover for two adjacent DIAM settings is typically only 2 or 3mm, and the discrepancy between the true cover to a bar of (say) Ø18, and the indicated covers with DIAM set to either 16 or 20, will be only half of this, it will not normally be necessary to double the time required for a survey by taking two measurements on every bar:-- rather it is recommended that the whole survey be performed with the DIAM knob set to the nearest size below the actual diameter, (16 in this example), and duplicate measurements (with DIAM at 16 and 20) only made on three selected bars, representative of shallow, typical, and deep cover respectively.
This will enable the (small) correction to be evaluated, to be added to all the results in the survey.
Of course, if the purpose of the survey was to confirm that all bars had more than the design minimum cover, and all the indicated (but slightly under-estimated) covers turned out to be greater than or equal to that minimum, then the adequacy of cover will have been confirmed without the need to apply any correction at all; correction will only be necessary if bars had less than minimum cover, or if the survey was required to record the amount by which minimum cover was exceeded.
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Older (pre-metrification) structures in the U.K. will contain reinforcement specified in inches rather than millimetres. Fortunately, due to the fact that the cover as indicated by the CM52 does not vary much with bar-size, exact conversion between actual imperial size and metric DIAM setting is not critical (and some conversions are exact anyway: for example, the difference between 5/8 inch and 16mm is only one eighth of a millimetre or 30 thou!).
The table on the right shows the suggested equivalent metric DIAM settings for a range of imperial diameters. Where the size of the bar is unknown and the Bar-Sizing technique as described in the User Manual (or Application Note #1) has been used, the deduced bar diameter will be in millimetres and this metric value should be used thereafter; this table can still be used to deduce the imperial equivalent if desired. |
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The DIAM knob should be set fully anticlockwise to 5.
The CM52 will continue to read down to (actual) covers of less than the usual limit of about 30mm.
On DEPth, the display will not show true cover, but will over-estimate.
To obtain true cover, the user will need to construct a conversion/calibration graph or table.
Measurements for this should ideally be done using a sample of the wire or bar (of at least 500mm length) and a set of non-metallic spacers of known thickness; if no such sample is available, the calibration measurements will have to be done on a number of buried wires/bars whose true cover has been determined by breaking out. In either case, the instrument should be zeroed (on CALib) in the usual way before the measurements.
There are two possible ways to construct a calibration graph (or table) :
(a) keeping the MODE switch at CALib:
This is the traditionally preferred method (and, incidentally, the reason why that position is called "CALib"); for each actual cover, a reading in milliVolts is obtained, recorded, and plotted.
Note that increasing covers give decreasing signal strengths, and vice versa; also that these signal strengths cover a wide "dynamic range", from only a few milliVolts at deep covers up to over one thousand milliVolts at shallow covers.
The graph is completed by drawing a smooth curve through the measurement points plotted.
To use the graph, measurements of signal strength are taken over wires/bars of unknown cover, and the deduced actual cover is read off the graph.
(b) with the MODE switch at DEPth:
Here the "readings" taken will be of indicated (but incorrect) cover, and the graph will be one of indicated cover versus true cover. This graph will then be used as in (a) above.
If the readings are tabulated rather than plotted, it will be helpful to create a third column of difference between true and indicated cover-- this will be a value to be subtracted from the indicated cover to arrive at true cover.
The main disadvantage of this method is that during recording of site readings, there is a risk of confusing the readings of indicated cover with true cover, which is unlikely to occur in method (a). However, this may be outweighed by the fact that the graph is easier to construct and use. Also, if a table is generated instead of a graph, it may be found that over the range of covers of interest the difference between indicated and true covers is approximately constant, ie there may exist a single value of correction which can be applied to all readings.
(Note that by recalibrating with DIAM at greater than 5, covers of less than 30mm can be measured; but for shallow covers, the CM52 Midget Head should be used in preference to the standard head)
The user will need to prepare a calibration/conversion graph (or table).
This is constructed in exactly the same way as in section 3 above (except that if method (b) is used with tabulated results, the difference correction is to be added to the indicated cover).
(Note that by recalibrating with DIAM at less than 40, covers greater than 100mm can be measured)
(Note that the CM52 can also detect aluminium alloy reliably if required; however stainless steel is a special case -- refer to Application Note #25)
(a) the target is too massive, and even with DIAM at 40, the reading is too shallow:
in this case it will be necessary to construct a conversion/calibration graph as in §4 earlier.
(b) no one single setting of DIAM gives the correct cover over the whole range of covers to be measured, but there exist a set of three or four adjacent settings, each of which works over a narrow range of covers:
this occurs when the target is not rod-shaped -- again it will be necessary to construct a calibration graph, using just one of the near-optimum settings of the DIAM knob referred to.
Other specific differences are detailed below.
If constructing calibration graphs, it will be noticed that the CM9 does not appear to have a "CALib" setting. In fact, this facility can be obtained by holding down the "pi" symbol on the front panel when switching on, which causes the instrument to enter "Test mode". The instrument should be zeroed by pressing the [<>] button, after which the signal strength is shown at the right-hand end of the top line; the concept of "diameter" is not relevant to this mode.
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