Protocol for Establishing and Maintaining the Calibration - NPL ...

4 Good Practice Guide 93 Chapter 1
relevant National Metrology Institute (NMI). In the UK, the NMI is the National Physical
Laboratory in Teddington. The NMI confirms the accuracy of its standards by comparisons with
the NMIs of other countries: these are organised by the Bureau International des Poids et Mesures
(BIPM), the world measurement standards authority. This process is generally described as the
establishment of Equivalence (see Figure 1.2):
Figure 1.2
Traceability and equivalence
← E Q U I V A L E N C E →
NIST
(USA)
⇔
NPL
(UK)
Secondary
Standards
Laboratory
Reference
Instrument
Field
Instrument
⇔
PTB
(DE)
⇔
LNHB
(FR)
⇔
⇔
↑
T
R
A
C
E
A
B
I
L
I
T
Y
↓
There are a number of ways in which traceability can be established and the uncertainty associated
with the calibration factor will depend to a large degree on the calibration method used. Two
possible calibration methods used by manufacturers are:
(a)
(b)
The master system is calibrated for each radionuclide using primary standards.
The manufacturer determines of the calibration factors for a typical production calibrator for a
restricted range of radionuclides using traceable standards. The response-energy curve is then
calculated and calibration factors for all radionuclides are calculated from knowledge of their
radiative emissions and the response curve. Additional uncertainties then need to be included
to allow for manufacturing differences between the “typical” calibrator and all other
production models.
These two examples might be regarded as the extremes in terms of the resultant uncertainties on
individual calibration factors. There are other options and these can also be supplemented by
subsequent calibrations and comparisons that will provide opportunities to reduce these
uncertainties.
A practical method to ensure traceability to the national standard is to calibrate, at least annually,
the routine instrument (field instrument) against a reference instrument. This can be a “secondary
standard” instrument for which nationally traceable measurements are available for a