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The most commonly accepted definition for traceability is found in the International Vocabulary of Basic and General Terms in Metrology, 1993, as:
"..the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties."
Troxler Electronic Laboratories, Inc., maintains a set of Primary Calibration
Standard Blocks for density calibration These blocks are identical in construction
and composition to the Secondary Calibration Standard Blocks that are used
by Troxler Electronic Laboratories (and laboratories that are approved for
calibration by Troxler Electronic Laboratories) to calibrate the density system
our surface moisture/density gauges. The densities of the Primary Calibration
Standard Blocks are traceable to master standards of the National Institute
of Standards and Technology (NIST), Gaithersburg, MD, USA.
Troxler Electronic Laboratories maintains permanent custody of all pertinent
documentation provided by the certifying agencies (including measurement uncertainty)
related to the traceability of measurements made on these Primary Calibration
Standard Blocks. Troxler also maintains all documentation pursuant to the
establishment of the densities of these Primary Calibration Standard Blocks,
including test data, procedures, internal measurement assurance policies,
and uncertainty computations. These documents present clear evidence that
Troxler meets the requirements for traceability claims regarding the Primary
Calibration Standard Block density values.
The Primary Calibration Standard Blocks are used to establish the density
values of the Secondary Calibration Standard Blocks that are manufactured
by Troxler Electronic Laboratories. To accomplish this task, a high precision,
high-accuracy calibration of a Troxler surface moisture/density gauge is performed
on the Primary Calibration Standard Block. Once this calibration has been
performed and confirmed, the gauge is used to measure a Secondary Calibration
Standard Block. After these measurements are complete, the data collected
are used to assign an average gravimetric density value to the Secondary Calibration
Standard Block.
Troxler Electronic Laboratories maintains permanent custody of the calibration
certificates for this surface moisture/density gauge for each time it is calibrated
on the Primary Calibration Standard Blocks. This gauge is reserved and maintained
solely for the purpose of measuring the densities of Secondary Calibration
Standard Blocks. Troxler Electronic Laboratories also maintains permanent
custody of density certification information for each Secondary Calibration
Standard Block it produces and provides the customer with a traceability certificate.
Troxler Electronic Laboratories also maintains all documentation pursuant
to the establishment of the densities of these Secondary Calibration Standard
Blocks, including test data, procedures, internal measurement assurance policies,
and uncertainty computations. These documents present clear evidence that
Troxler meets the requirements for traceability claims regarding the Secondary
Calibration Standard Block density values.
Surface moisture/density gauges that are calibrated by Troxler Electronic
Laboratories (and laboratories that are approved for calibration by Troxler
Electronic Laboratories) are always calibrated on Secondary Calibration Standard
Block that have been certified in the aforementioned manner. Troxler Electronic
Laboratories maintains permanent custody of the calibration certificates for
each surface/moisture density gauge that it calibrates and provides the customer
with a copy of this document.
All calibration certificates and reports provided by Troxler Electronic Labs
conform to the format specified by NIST Handbook 150, "NVLAP Procedures
and General Requirements", 2001.
Because of their careful treatment and stable structure and composition, one
would not anticipate the density values of the Primary Calibration Standard
Blocks to change appreciably with time. Nonetheless, to maintain the highest
degree of accuracy and traceability in its measurements and standards, Troxler
Electronic Laboratories periodically constructs new Primary Calibration Standard
Blocks in addition to re-measuring and re-certifying its Secondary Calibration
Standard Blocks on a regularly scheduled basis. When the new Primary Calibration
Standard Blocks are constructed, the current Primary Calibration Standard
Blocks are removed from service. This practice is consistent with the following
recommendation from NBS Handbook 145, "Handbook for the Quality Assurance
of Metrological Measurements":
"Routine recalibration of standards, particularly volumetric standards, even when a change of value is not anticipated, should be made with sufficient frequency to affirm their continued accuracy.."
For more information on traceability, visit the NIST Traceability web site
at www.nist.gov/traceability/.