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Defining Your Calibration Requirement
Okay, so you've convinced the accountant that
the consequence of not having your test gear calibrated is too risky
and also believe you've selected a calibration vendor that meets
your organization's initial requirements.
But what steps can be taken when actually placing
the order to ensure you get a "proper" calibration? Here's
a suggestion.
How to Take the Uncertainty Out of Buying a
Service
The level of knowledge possessed by purchasers of calibration services
varies significantly from company to company. Some organizations
employ an experienced metrologist to make calibration purchases
while others entrust this complex decision to a non-technical purchasing
function. The result of this is that the validity of purchasing
decisions vary from exceptionally good to extremely inappropriate.
Extremely inappropriate decisions invariably cost the company more
money; either because they are buying unnecessary or unnecessarily
detailed calibration or, more commonly, because they are buying
insufficient or insufficiently detailed calibration. While the insufficient
option may be cheaper, it often results in far greater eventual
cost to the company through rework and incorrect decisions relating
to product quality and customer satisfaction.
To justify unnecessary calibration, a detailed knowledge
of the precise application of the equipment is required as part
of the analysis but adoption of simple measures can enable a purchaser
to overcome a tendency to buy inadequate calibration. This
is particularly necessary for final test instruments and those responsible
for end-product quality. Inputs from some of the most knowledgeable
technical buyers in the UK have been used to construct the following
purchase order statement which should help the less well-informed
calibration purchaser avoid the inadequacy trap:
Calibration shall be undertaken against full manufacturers’
specifications. Where equipment is found to be out of specification,
full performance test results, in the as-received condition, must
be obtained before any adjustment or repair action is taken. On
completion of the calibration work a certificate of calibration
is required, signed by your authorized representative, containing
a statement confirming that the calibration can be demonstrated
to be traceable to National or International standards and stating
the item’s full compliance with its performance specifications.
Full performance test results, taking the form of measured values,
are to be supplied. Any omissions from the full calibration are
to be notified to us and shall be agreed in writing before a certificate,
clearly annotated "Limited Calibration" is issued. A declaration
of measurement uncertainty values shall be included with all test
results.
But What Does it Mean?
This may seem a lengthy statement but it encompasses many of the
important points missed by orders which simply require "Calibration".
It is because there is no standard for what is provided under "a
calibration" that such an explicit purchase statement is necessary.
A breakdown of each element of the statement follows with explanation:
"Calibration shall be undertaken against full manufacturers’
specifications."
- Requesting that the calibration be to
"full manufacturers’ specifications" offers protection
from a supplier who may normally test just the simple parameters
or those which can be checked without investing in expensive test
gear. Where the original manufacturer (or recognized standard)
has provided guidance on the tests necessary for high confidence
in the product's overall performance, these recommendations provide
some practical distinction between significant and less important
aspects of the specification.
"Where equipment is found to be out of specification, full
performance test results, in the as-received condition, must be
obtained before any adjustment or repair action is taken."
- Many service providers perform adjustments
during the performance test process. Due to the interactivity
of circuitry in many complex instruments (adjusting one parameter
has a knock-on effect on other parameters) this immediately
destroys the history and the "as-received" state of
any parameters tested after the first adjustment can never be
known. The metrologically correct approach is to fully test every
parameter (noting the values which will form the "pre-adjustment
data"), then make necessary adjustments and finally perform
the tests again to have confidence in the integrity of the whole
set of post-adjustment test data.
"On completion of the calibration work a certificate of
calibration is required, signed by your authorized representative,
containing a statement confirming that the calibration can be demonstrated
to be traceable to National or International standards…"
- Clearly a calibration must have a direct and
demonstrable traceability path to the appropriate standards.
"…and stating the item’s full compliance with
its performance specifications."
- Test equipment users often assume that having
their instrument calibrated means that it now meets specification.
However, the international definition allows calibration to be
simply a comparison with standards of known accuracy, not necessarily
making adjustment. Also, for some applications it is undesirable
to alter the product's characteristics or in other cases adjustment
may be impossible. But by making this a specific request the supplier
has a clear instruction to restore performance to meet the original
manufacturers published specification if possible.
"Full performance test results, taking the form of measured
values, are to be supplied."
- Measured values are more useful than "tick"
or check boxes or "pass/fail" indications. They provide
greater visibility of what was tested and better confidence that
each tested parameter really did fall within the accepted tolerances.
Where appropriate, provision of actual figures also enable the
user to make measurement corrections.
"Any omissions from the full calibration are to be notified
to us and shall be agreed in writing before a certificate, clearly
annotated "Limited Calibration" is issued."
- There is no definition of what should
be performed to achieve a "full calibration" so if taken
out of context this term is indefinite. However, as part of a
statement in which the extent of calibration has been defined
(as in the first sentence of this statement) the phrase can be
meaningful. In this case, the purchaser has made it clear that
reducing the degree of testing to save time or reduce the need
for test gear investment is not desirable and that any such compromise
should be clearly pointed out before the work is undertaken. To
ensure that all users are aware of the limitation, the calibration
certificate should disclose any recommended tests that have been
omitted which consequently reduce confidence in the product's
overall specification conformity.
"A declaration of measurement uncertainty values shall
be included with all test results."
- In the case of an accredited calibration
(e.g. UKAS, DKD, NVLAP, etc.) the uncertainties of measurements
for which the laboratory is accredited should be provided. For
non-accredited calibration, it is necessary to include remarks
affirming the adequacy of measurement accuracy so as to qualify
the specification compliance statement. This commonly takes the
form of a "test accuracy ratio", being (simplistically)
the ratio of the accuracy of the reference instrument to that
of the instrument being tested. Traditionally, a ratio of at least
3-to-1 has been recognized as providing acceptable confidence.
Conclusion
This all serves to highlight a shortfall in the calibration industry.
Without specific demands from the customer, it is left to the service
supplier to define how much (or little) of a particular instrument
is checked during "a calibration". By carefully stipulating
your calibration requirement on purchase orders it is possible to
identify (and eliminate) the suppliers that will not meet your explicit
needs. All calibration is not equal.
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