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2.
Measurement Process Characterization
2.3. Calibration 2.3.4. Catalog of calibration designs
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| Tie to the defined unit of length | The unit of length in many industries is maintained and disseminated by gauge blocks. The highest accuracy calibrations of gauge blocks are done by laser intererometry which allows the transfer of the unit of length to a gauge piece. Primary standards laboratories maintain master sets of English gauge blocks and metric gauge blocks which are calibrated in this manner. Gauge blocks ranging in sizes from 0.1 to 20 inches are required to support industrial processes in the United States. | ||
| Mechanical comparison of gauge blocks | However, the majority of gauge blocks are calibrated by comparison with master gauges using a mechanical comparator specifically designed for measuring the small difference between two blocks of the same nominal length. The measurements are temperature corrected from readings taken directly on the surfaces of the blocks. Measurements on 2 to 20 inch blocks require special handling techniques to minimize thermal effects. A typical calibration involves a set of 81 gauge blocks which are compared one-by-one with master gauges of the same nominal size. | ||
| Calibration designs for gauge blocks | Calibration designs allow comparison of several gauge blocks of the same nominal size to one master gauge in a manner that promotes economy of operation and minimizes wear on the master gauge. The calibration design is repeated for each size until measurements on all the blocks in the test sets are completed. | ||
| Problem of thermal drift | Measurements on gauge blocks are subject to drift from heat build-up in the comparator. This drift must be accounted for in the calibration experiment or the lengths assigned to the blocks will be contaminated by the drift term. | ||
| Elimination of linear drift | The designs in this catalog are constructed so that the solutions are immune to linear drift if the measurements are equally spaced over time. The size of the drift is the average of the n difference measurements. Keeping track of drift from design to design is useful because a marked change from its usual range of values may indicate a problem with the measurement system. | ||
| Assumption for Doiron designs |
Mechanical measurements on gauge blocks take place successively
with one block being inserted into the comparator followed by a second
block and so on. This scenario leads to the assumption that the
individual measurements are subject to drift
(Doiron).
Doiron lists designs meeting this criterion which also allow for:
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| Properties of drift-elimination designs that use 1 master block | The designs are constructed to:
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| Caution | Because of the large number of gauge blocks that are being intercompared and the need to eliminate drift, the Doiron designs are not completely balanced with respect to the test blocks. Therefore, the standard deviations are not equal for all blocks. If all the blocks are being calibrated for use in one facility, it is easiest to quote the largest of the standard deviations for all blocks rather than try to maintain a separate record on each block. | ||
| Definition of master block and check standard |
At the National Institute of Standards and Technology (NIST), the
first two blocks in the design are NIST masters which are designated
R1 and R2, respectively. The
R1 block is a steel
block, and the R2 block is a chrome-carbide block. If the test
blocks are steel, the reference is R1; if the test blocks are
chrome-carbide, the reference is R2. The check standard is
always the difference between R1 and R2 as estimated from
the design and is independent of R1 and R2. The designs
are listed in this section of the catalog as:
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| Properties of designs that use 2 master blocks |
Historical designs for gauge blocks
(Cameron and Hailes)
work on the assumption that the difference measurements are
contaminated by linear drift. This assumption is more restrictive and
covers the case of drift in successive measurements but produces fewer
designs. The Cameron/Hailes designs meeting this criterion allow for:
and assign equal uncertainties to values of all test blocks. The designs are listed in this section of the catalog as: |
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| Important concept - check standard | The check standards for the designs in this section are not artifact standards but constructions from the design. The value of one master block or the average of two master blocks is the restraint for the design, and values for the masters, R1 and R2, are estimated from a set of measurements taken according to the design. The check standard value is the difference between the estimates, R1 and R2. Measurement control is exercised by comparing the current value of the check standard with its historical average. | ||
