It seems that the calibration world also abounds with myths and
legends but, fear not, we shall banish this folklore from the land
and dispel these misconceptions !
The myths are grouped into 3 broad topic areas (on separate pages
because there are so many ! ) :
| MYTH |
TRUTH |
|
Standards
|
| ISO17025 states that it's equivalent
to ISO9000 so ISO9000 must be equivalent to ISO17025. |
ISO17025 does indeed state, in its Introduction and in paragraph
1-6, that compliance with the standard means that the laboratory's
quality system for their calibration or testing activities
also meets the criteria of ISO9001/2. Two points to emphasize
though:
- The activities of many service providers extends beyond
just calibration or testing (e.g. repair, supply of parts,
training, etc.) where 17025 does not apply.
- The equivalence is to the 1994 version of the ISO9000
standards which was superseded in late 2000.
|
| My factory's quality system complies
with ISO9000 so all my equipment must be calibrated "Before
& After"adjustment. |
A calibration service that provides assessment of the product's
performance on-receipt and, if necessary, after adjustment
or repair has been completed has two purposes.
- It enables analysis of the equipment's stability over
time.
- More significantly, if the on-receipt performance did
not meet the user's accuracy requirements, an investigation
of its impact can be triggered that may result in product
or work recall.
These possibilities need only apply to equipment affecting
the quality of the factory's product or service, for example
that used for alignment or end-of-line inspection. Understanding
the distinction can save a lot of money !
|
| Accreditation agencies define the
extent of testing for various products so that users can have
confidence in their equipment's overall performance. |
In some countries there are national and regulatory standards
that are applicable to some measuring equipment. These usually
relate to legal metrology (i.e. measurements made in the course
of consumer trade) or statutory codes (e.g. safety) or certain
sectors of industry.
However, accreditation bodies do not stipulate that these
must be used although labs would generally do so where applicable.
Also, there are no standards concerning the typical general
purpose instruments that may be used in the electronics industry,
for example.
Although accreditation criteria includes a need for
calibration certificates to draw attention to limitations
in the scope of testing performed versus the product's capability,
it is left to the client and supplier to agree the content
of the service. Whether the calibration utilizes any recommendations
of the equipment's manufacturer is part of this negotiation.
|
| My calibration supplier is ISO17025
accredited so all the calibrations they undertake meet that
standard. |
The results of a calibration performed under the scope of
the accreditation are reported on a certificate bearing the
authorized brand-mark of the accreditation program.
For commercial reasons, most accredited laboratories offer
at least two calibration service levels -- a certificate with
the accreditation logo or a company-proprietary
certificate.
The processes used to undertake the calibration and the extent
of testing may be the same in both cases, or may differ. Some
accreditation programs allow the inclusion of (a minority
of) measurements which are not within the lab's accredited
capability, providing they are clearly identified as non-accredited.
|
| Results which are simply reported
as "Pass" or "Fail" are not acceptable. |
Recording of numerical measurement data is not relevant for
some tests. This may be because it's of the "go, no go"
type (e.g. checking a bore using a plug gauge) or because
the test procedure establishes known test conditions and looks
for satisfactory response in the unit-under-test (e.g. checking
input sensitivity of a frequency counter by applying a signal
whose amplitude equals the specified sensitivity and noting
whether stable triggering is observed).
To summarize, pass/fail is valid where the decision criteria
is defined (i.e. specification limits).
|
| A supplier that has an ISO9000 certificate
is good enough. |
This may be reasonable but questions concerning
the scope of the certification should be asked. If the quality
system that was assessed related to a company's pressure sensor
manufacturing operation in Chicago, how much assurance does
that endow on micrometer service at their Dallas repair office?
Possibly none! The scope of registration is explicit in coverage. |
| Only accredited calibrations are
traceable to national standards. |
Traceable measurements are those supported by records that
can demonstrate an unbroken series of calibrations or comparisons
against successive standards of increasing accuracy (reducing
uncertainty) culminating in a recognized national metrology
institute.
Measurement traceability is, of course, also reviewed as
part of an ISO9000 quality system certification.
|
| My own testing laboratory is accredited
against ISO17025 so our instruments must be calibrated at an
accredited lab. |
This may depend upon the interpretation of the standard by
the particular accreditation body. Clause 5-6-2-1-1 of ISO17025
does not actually stipulate that traceability must only be
obtained from an accredited facility only that the supplier
"can demonstrate competence, measurement capability and
traceability".
The British accreditation agency has confirmed that it will
not add supplementary requirements to the 17025 criteria.
It also accepts the possibility of traceability to a non-accredited
source provided that sufficient evidence is available to UKAS
to confirm that the supplier complies with the standard and
that the lab being audited by UKAS has the critical technical
competence to make such an assessment.
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