b. When you calibrate you compare a standard of a known accuracy
against an unknown, Under ideal conditions, the standard should be 10 times
more accurate than the unit under test. When you cross-check you only check
the probability of standards remaining within their stated tolerance. One
cannot assure that a standard is in tolerance by cross-checking; this is
done only by a proper calibration. However, you can compute the probability
error between two standards whose tolerances are basically the same. This
is determined by using the formula: Square root of the sums of the
individual accuracies squared, as shown below:
%Ep = [(%E1)2 ----(%En)2]1/2
where:
%Ep
=
Percent
of
probability error
%E1
=
Percent
of
error of standard 1
%E2
=
Percent
of
error of standard 2
%En
=
Percent
of
error of all other standards to the nth standard
Note: The probability of the standard being within tolerance is greatly
increased as more readings and standards are cross-checked on the same
parameters. In post cases %Ep is based on intercomparison of two standards.
3.
REASONS FOR CROSS-CHECKS.
a. General.
The theory of cross-checking is a functional comparison
between instruments to verify that no significant departure from assigned
tolerance has occurred.
The cross-check is a method to detect any change
(drift) in transfer standards that may establish trends and to assist in
preventing out-of-tolerance standards from being used in the performance of
calibration.
Also, the cross-check increases confidence of operator
personnel in their equipment. Secondary transfer standards are more subject
to out-of-tolerance condition because they are mobile and used in
uncontrolled
environments,
causing
premature
failure
and
aging
of
components.
b. Aging of components. As a piece of equipment is used, it is subject
to current surges as it is turned on and off, or due to input line voltage
changes. After a piece of equipment has been operating continuously for 8
hours or more, which is not uncommon for a calibration standard, the heat
will affect some components by some small amount.
When this happens day
after day for months, the component's value will change, causing the value
of the standard to change.
By making periodic cross-checks of our
standards, we may detect it drifting out of tolerance.
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