Application Note # C10

CM5/52 -- Verification of Calibration

Note
Quick Check
Precautions
Recommended method
Using a concrete prism
Frequency of calibration
Calibration vs Recalibration

1. Note:

Please note that all CoverMasters are shipped calibrated for ribbed High-Tensile steel bars, as this is the type most commonly used. The discrepancy when measuring round mild-steel bars may be up to ±5%; this means that in practice such round bars can usually be measured with adequate accuracy, but mild-steel bars should never be used for absolute calibration.

2. Quick Check:

After zeroing the instrument with the head in air, place a length of high-tensile rebar (of diameter as set on the DIAM knob) on the white face of the head. The indicated cover should be 30 ±2mm (typically ±1).

3. Precautions:

As stated in (1) above, bars used for testing must be of high-tensile steel if the test measurements are to be valid when applied to actual reinforced concrete.
The length of each bar must be at least 500mm, and measurements must be made at the centre of the bar.
If the instrument has recently had a full charge, the battery voltage may be well over the nominal value (see Battery Charging section in User Manual). In this case, the instrument must be left switched on for at least one hour before making any test measurements, and for at least two hours (total) before performing any re-calibration.
The measurements must be made in a metal-free area.
The zero setting (with MODE at CALib) must be checked before each cover reading (MODE at DEPth).

4. Recommended method:

4a: Systematic check:

Prepare a set of spacers of non-metallic, non-magnetic material (plastic being recommended, or wood as an alternative) of known thickness covering the range of 30mm up to the maximum distance to be checked. Prepare a set of high-tensile (eg ribbed) bars, covering the range of bar diameters to be checked and including T16, each of length at least 500mm. Place the head on a non-metallic surface, with black face uppermost.

Then systematically repeat the following sequence:

set DIAM to the size of the bar to be measured,
place a spacer of thickness equal to the cover to be measured on the head,
set MODE to CALib, and use ZERO to bring display to ±.000,
place the bar on the spacer, ensuring that it is central over the head and aligned parallel to it,
set MODE to DEPth, and read and record the indicated cover.

4b: Interim Check:

A single measurement using a T16 bar and a 60mm spacer (otherwise as per 4a), is the most sensitive way of verifying that the calibration for covers of 30-69mm has not changed since the last full test.

Note that the "quick check" in (2) above verifies operation at 30mm cover (without needing a test spacer); and there also exists a method of verifying operation in the region of 90mm:

When the CoverMaster is zeroed (accurately) and then switched to DEPth, a number is displayed, typically between 110 and 120. The exact value of this "infinity" representation is not part of any specification, but is characteristic of that particular CoverMaster, and should be noted down, as it is a sensitive indicator of any change in calibration for deep covers (only). The indicated value is also sensitive to inaccurate zeroing, and, more to the point, to the effect of battery over-voltage (as referred to in the Precautions above). In that latter case, the over-voltage effect causes the "infinity" reading to be less than usual; such a discrepancy can be taken as an upper limit to the extent that measurements of covers greater than 80mm may also indicate low, and also an indication that no test or calibration measurements should be performed until the voltage has dropped to normal.

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5. Using a concrete prism (as described in BS 1881:204):

Please note that some of the details of the prism are a "typical example", not mandatory dimensions. In particular: the bar must be high-tensile, not mild steel (see (1) above); the diameter need not be 10mm -- it makes more sense to use a size (or sizes) representative of the sizes to be measured in practice; the prism cross-section need not be 100mm nor even square -- indeed a prism of only 100mm square would not allow the CoverMaster to be tested to its full depth; the length of the prism is not in fact critical, but the length of the bar must be at least 500mm.

All the other precautions noted in (3) above must also be adhered to, particularly with regard to warm-up time and zeroing (note that the procedure in 4a keeps the head stationary, and so small amounts of metal near the head can be tolerated because they will be zeroed out; but if the head is zeroed in one position and then moved to the test position, it is essential to ensure that there is no extraneous metal near the test prism).

The remaining guidelines in BS 1881:204 may then be followed, noting the requirement that the concrete should have no magnetic aggregates; although the effect of such aggregates on the CoverMaster is negligible in practice for covers of 60mm or less, the presence of any such aggregates nevertheless invalidates the test.

Finally, if a contract misguidedly specifies that the cover meter shall be tested exactly as the example in BS 1881:204, right down to the use of a smooth round mild steel bar, and a prism containing a mild steel bar of sufficient length is available, then the best solution is to prepare a second prism containing a suitable high-tensile bar. The second prism is then used to verify the absolute accuracy of the instrument when measuring reinforced concrete, and the recorded results from the mild steel bar are ideally suited to monitoring that the calibration does not change over periods of time. Indeed, if the correlation between the mild-steel and high-tensile results are documented and recorded, the high-tensile-bar prism may be used thereafter as a working standard, as the National Accreditation requirements for its "traceability" will have been met.

6. Frequency of calibration:

Due to its speed and simplicity, the "quick check"(2) can be performed as frequently as desired (even daily); though it would be inappropriate to document every such occasion, as this is really an "operator confidence" check.

The "interim check"(4b) with a 60mm spacer, being also a simple test, could be performed on a regular basis, for example at two- or three-month intervals. Whether such checks are recorded or not will depend on the policy of the user's quality control documentation procedures.

The "systematic check"(4a) will form the basis of the user's regular calibration control, and should therefore be performed at fixed regular intervals. We recommend that the interval should be not more than two years (as long intervals complicate scheduling), nor less than six months (as no noticeable change in calibration is to be expected in such a short term).
However, the full calibration check should be brought forward if any of the following reasons occur:
(a) either the quick check or the interim check indicate a significant change or error;
(b) the instrument has sustained any damage or needed any repair;
(c) the face of the search head has become seriously worn away;
(d) the search head has been replaced.

7. Calibration versus Recalibration:

"Calibration", in the context of these notes, refers to systematically-performed measurements of bars at known distances in order to document the repeatability of an instrument, tabulate any corrections necessary to indicated readings, or document that no corrections to indicated readings are necessary for the instrument to be used within the permitted tolerances of its specification and the BS requirement.
However, from the manufacturer's point of view, "calibration" is the process whereby the instrument is internally-adjusted to ensure that all readings are within permitted tolerance of the true values; and if an instrument is returned for the purpose of receiving a maintenance service, this would be called "re-calibration".

Therefore, if the user's full calibration check indicates that an instrument is outside specification, it must be returned for these "factory-only" adjustments to be made.
However, as the face of the search head wears away, the instrument will start to read consistently low; and as the head nears the end of its life, this discrepancy will increase to -2mm. Under these circumstances, we recommend that any observed discrepancy should be recorded as a correction to be applied, and the instrument should not be submitted for recalibration until such time as a new replacement search head is obtained (as otherwise the instrument would, in effect, become mis-adjusted).

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