TEXT
1.
INTRODUCTION.
a. According
to current regulations,
calibration
standards
are
certified at fixed intervals.
The standards may be used daily between
certification dates.
Calibration technicians using these standards must
make periodic checks to determine their conditions.
This procedure is
called cross-check.
b. As defined by TB 750-25 and TB 9-4931-402-50 a cross-check is a
functional comparison between instruments to verify that no significant
departure from assigned tolerance has occurred. If the values listed in the
appropriate TB are exceeded, the suspected instrument will be completely
recalibrated.
c. The primary reference facility (USASL) establishes recall schedules
for the calibration of secondary reference standards equipment. To assure
continuity of operations while secondary reference equipment is being
calibrated, duplicate items are provided in the reference standards sets.
These duplicate items are also calibrated and certified by the USASL, but on
alternating
and
noninterfering
schedules.
Those
duplicated
are
intercompared against their counterparts.
This provides an indication of
the reliability and operation of both items. Secondary transfer standards
are calibrated by a secondary reference facility on a schedule established
by the reference facility.
Unlike the secondary reference set, the
secondary transfer set does not contain duplicate items.
Therefore on a
secondary transfer team you cannot compare like items directly, but must
compare parameters that are similar but of different accuracies. This cross
check of similar parameters provides an indication of the reliability and
operation of both items.
d. In this lesson we will be concerned with the secondary transfer
standards cross-checks, the concept of cross-checks, theory, and procedures
of cross checks.
2.
CROSS-CHECK CONCEPTS.
a. Cross-check procedures for secondary transfer standards differ from
calibration in that adjustments are forbidden.
Cross-checks are performed
with like instruments having like accuracies; therefore, when an out-of-
tolerance condition is apparent, it must be determined which instrument has
deviated from its assigned accuracy and it will be recalibrated.
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