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Each subsequent level of the hierarchy is derived by calibration against the standard at the
previous higher level. Table 1.3 lists an example of such a lineage for standards from a primary or
reference standard maintained at a national standards lab down to a working standard used in a
typical laboratory or production facility to calibrate everyday working instruments. If the facility
does not maintain a local (laboratory or working) standard, then the instruments must be sent off and
calibrated elsewhere. In such a case, a standards traceability certificate would be issued for the
instrument.
As one moves down through the standards lineage, the degree of exactness by which a standard
approximates the primary standard deteriorates. That is, increasing elements of error are introduced
into the standard as one moves from one level of hierarchy of standard to the next. As a common
example, an institution might maintain its own working standard (for some application) that is used
to calibrate the measurement devices found in the individual laboratories throughout the institution.
Periodic calibration of the working standard might be against the institution’s well-maintained local
standard. The local standard would be periodically sent off to be calibrated against the NIST (or
appropriate national standards lab) transfer standard (and traceability certificate issued). NIST will
periodically calibrate its own transfer standard against its reference or primary standard. This is
illustrated for a temperature standard traceability hierarchy in Table 1.4. The uncertainty in the
approximation of the known value increases as one moves down the hierarchy. It follows, then, that
since the calibration determines the relationship between the input value and the output value, the
accuracy of the calibration will depend in part on the accuracy of the standard. But if typical working
standards contain errors, how is accuracy ever determined? At best, this closeness of agreement is
quantified by the estimates of the known uncertainties in the calibration. And the confidence in that
estimate depends on the quality of the standard and the calibration techniques used.
Table 1.3 Hierarchy of Standards
a
Primary standard
Maintained as absolute unit standard
Transfer standard
Used to calibrate local standards
Local standard
Used to calibrate working standards
Working standard
Used to calibrate local instruments
a There may be additional intermediate standards between each hierarchy level.
Table 1.4 Example of a Temperature Standard Traceability
Standard
Level
Method
Uncertainty [
C]
a
Primary
Fixed thermodynamic points
0
Transfer
Platinum resistance thermometer
Æ0.005
Working
Platinum resistance thermometer
Æ0.05
Local
Thermocouple
Æ0.5
a Typical combined instrument systematic and random uncertainties.
1.5 Standards 29
15:40:35 Page 29
Each subsequent level of the hierarchy is derived by calibration against the standard at the
previous higher level. Table 1.3 lists an example of such a lineage for standards from a primary or
reference standard maintained at a national standards lab down to a working standard used in a
typical laboratory or production facility to calibrate everyday working instruments. If the facility
does not maintain a local (laboratory or working) standard, then the instruments must be sent off and
calibrated elsewhere. In such a case, a standards traceability certificate would be issued for the
instrument.
As one moves down through the standards lineage, the degree of exactness by which a standard
approximates the primary standard deteriorates. That is, increasing elements of error are introduced
into the standard as one moves from one level of hierarchy of standard to the next. As a common
example, an institution might maintain its own working standard (for some application) that is used
to calibrate the measurement devices found in the individual laboratories throughout the institution.
Periodic calibration of the working standard might be against the institution’s well-maintained local
standard. The local standard would be periodically sent off to be calibrated against the NIST (or
appropriate national standards lab) transfer standard (and traceability certificate issued). NIST will
periodically calibrate its own transfer standard against its reference or primary standard. This is
illustrated for a temperature standard traceability hierarchy in Table 1.4. The uncertainty in the
approximation of the known value increases as one moves down the hierarchy. It follows, then, that
since the calibration determines the relationship between the input value and the output value, the
accuracy of the calibration will depend in part on the accuracy of the standard. But if typical working
standards contain errors, how is accuracy ever determined? At best, this closeness of agreement is
quantified by the estimates of the known uncertainties in the calibration. And the confidence in that
estimate depends on the quality of the standard and the calibration techniques used.
Table 1.3 Hierarchy of Standards
a
Primary standard
Maintained as absolute unit standard
Transfer standard
Used to calibrate local standards
Local standard
Used to calibrate working standards
Working standard
Used to calibrate local instruments
a There may be additional intermediate standards between each hierarchy level.
Table 1.4 Example of a Temperature Standard Traceability
Standard
Level
Method
Uncertainty [
C]
a
Primary
Fixed thermodynamic points
0
Transfer
Platinum resistance thermometer
Æ0.005
Working
Platinum resistance thermometer
Æ0.05
Local
Thermocouple
Æ0.5
a Typical combined instrument systematic and random uncertainties.
1.5 Standards 29