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 |
|
—Uncertainty—
|
| ISO17025 requires that measured values
and measurement uncertainty is reported on a certificate. |
This is true if the certificate does not include
a statement concerning the equipment's compliance to a stated
specification. In this case, section 5-10-4 says that the results
and uncertainty must be maintained by the lab. |
| We need to determine our own measurement
uncertainty so need to know the calibration lab's uncertainty. |
If the calibration confirmed that the instrument met the
manufacturer's specification, the effect of uncertainty on
that status decision has already been taken into account (as
required by ISO17025, para.5-10-4-2). In this case, the user's
own uncertainty budget starts with the product specification
and the calibration uncertainty is not included again.
If the calibrated item does not have a specification (i.e.
the certificate provides only measured values) then the cal
lab's uncertainty will need to be included in the user's own
uncertainty analysis.
|
| The need to know "uncertainty"
is new. We've been certified against ISO9001:1994 for years
and have never been asked before. |
You've just been lucky or were satisfactorily meeting the
requirement without realizing it !
Look again at clause 4-11-1; it clearly states that "...equipment
shall be used in a manner which ensures that the measurement
uncertainty is known and is consistent with the required measurement
capability."
For the majority of instrument users, the requirement is
readily satisfied by referring to the equipment specifications.
In general terms, the specification is the user's uncertainty.
|
| The uncertainties that an accredited
lab will report on a certificate are published in their Scope/Schedule. |
The published capability represents the best
(smallest possible) measurement uncertainties, perhaps applicable
to particular characteristics and types of tested equipment.
It's very unlikely that those figures would be assigned to all
calibrations made assuming a wide variety of models are seen.
Until measurements are made, it may not be possible for the
cal lab to estimate the uncertainty that will be assigned because
the unit-under-test contributes to the uncertainty. |
|
Published "best measurement uncertainty" can never
be achieved because it assumes an ideal unit-under-test.
|
In the past there have been different practices allowed by
the various conformity assessment schemes. However, the European
co-operation for Accreditation publication EA-4/02 (refer
to Uncertainty Resources in this Basics
section) recognizes that harmonization was required and, in
Appendix A, establishes definitions.
This means that, certainly within Europe, best measurement
uncertainty (BMC) must include contributions associated with
the normal characteristics of equipment they expect to calibrate.
For example, it's not acceptable to base the uncertainty of
an attenuation measurement on a device having an assumed perfect
match. Some BMC's are qualified with the phrase "nearly
ideal" regarding the test item but this means that the
capability does not depend upon the item's characteristics
and that such perfect items are available and routinely seen
by the lab.
|
| Calibrations without
uncertainty are not traceable. |
It is true that the internationally
agreed definition of traceability includes a need for the uncertainty
of the comparisons to be stated. However, it doesn't mean that
a calibration certificate must include uncertainty (or measured
values), as is allowed by ISO17025 and other standards if a
specification compliance statement is used, although this information
must be maintained by the lab. |
| By using a correction based on the
instrument's error as determined by calibration, the working
specification can be tightened. This effectively minimizes the
user's own measurement uncertainty to that of the calibrating
lab. |
The equipment manufacturer specifications cannot be ignored.
For instance, they include allowances for drift over time
and environmental conditions. In contrast, the calibration
represents a performance assessment at that time and in particular
conditions. Yet the myth dangerously assumes that the "error"
is constant despite these variables.
|
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Calibration
Myths — EXPLODED !
