Intermediate Checks; Traceability To Bs1881:204; Recalibration With Midget Head - Elcometer Protovale CoverMaster CM52 Operating Instructions Manual

6.3 Intermediate checks

1. Carry out the quick check as described in section 6.1.
2. Use a T16 high-tensile bar and only a 60mm spacer, carry out part of the full
check (section 6.2).
3. Use the 60mm spacer and the additional 30mm spacing of the white face of the
search head for a third check at a total spacing of 90mm.
4. When the CM52 CoverMaster is zeroed accurately and then switched to DEPth,
the digital display shows a number which is typically between 110 and 120.
Although not part of any specification, this 'infinity' reading is a sensitive indication
of the stability of calibration of that particular instrument. If a long-term record is
kept of the number indicated, it can serve as an additional calibration check.
5. The 'infinity' reading is also a sensitive check of the accuracy of zeroing, and more
importantly of the state of charge of the battery. If the correct reading is known
from previous records, it can be used to confirm that the battery voltage is neither
too high nor too low to permit a definitive recalibration.

6.4 Traceability to BS1881:204

Traceability of calibration BS1881:204 is sometimes required as a condition of contract for
site work. Recalibrating by Protovale automatically meets this requirement, but the same
procedure can also be carried out by the user.
The procedure is similar to the full check described above, except that the test bar must
be mild steel rather than high-tensile, and the spacer is a concrete prism cast around the
bar. Consult BS1881:204:1988 for further details.
The smooth mild steel bar required in BS1881:204 is obviously not the best material to
verify the performance of the instrument with the ribbed high-tensile bars that are used in
practice. If the terms of a survey contract misguidedly require the instrument to be tested
"exactly following the examples in BS1881:204", the recommended solution is to prepare
two concrete prisms. The first should contain a mild steel bar, exactly as in the Standard,
and the second should contain a realistic high-tensile bar while conforming to the Standard
in all other respects.
The prism containing the mild steel bar should be used to monitor the long-term stability of
the calibration, and the second prism should be used to verify the accuracy of
measurements on high-tensile bars. In this way, the optimum accuracy can be obtained
for the practical measurements while also meeting the UK National accreditation
requirements for 'traceability'.
6.5 Recalibration with the Midget Head
Note that the checks described above all refer to the standard head only.
It is of course possible to also devise checks for the midget head using spacers of known
thicknesses between 10 and 30mm, and a bar of (preferably) 12mm diameter.
31