DEPARTMENT OF COMMERCE 

National Institute of Standards and Technology 

National Voluntary Laboratory Accreditation Program

ISSUE DATE:

February 19, 2004

(revised February 21, 2007) 

NUMBER: LB-10-2004 LAB BULLETIN LAP: Calibration 

SUBJECT: Thermometry Proficiency Tests

This bulletin applies to calibration laboratories accredited or seeking accreditation by NVLAP for 

Resistance Thermometry. It provides the NVLAP Proficiency Test cutoff values for laboratories 

providing ITS-90 SPRT calibrations. The laboratory will be contacted by a NVLAP Program Manager to 

schedule the appropriate test(s). Laboratories that are providing these calibration services should refer to 

the following information developed by the Thermometry Group in the Process Measurements Division at 

NIST: 

NVLAP Proficiency Test Cutoff Values for Laboratories 

Providing ITS-90 SPRT Calibrations 

When laboratories are seeking or maintaining NVLAP accreditation for their ITS-90 SPRT calibration 

services with expanded uncertainties (k=2) less than or equal to the values given in Table 1, a need is 

indicated for a NIST Measurement Assurance Program (MAP) Test and the direct comparison of fixedpoint cells. For those fixed-point cells that require a direct comparison with the NIST reference 

standards, a subset will be selected by the appropriate NIST Thermometry Group technical staff member. 

The use of the MAP and direct comparison of fixed-point cells as proficiency tests will be performed 

during alternate years. The MAP should be performed once every three years and the direct comparison 

of fixed-point cells once every five years. A single SPRT proficiency test covering the temperature range 

of the accreditation should be performed in the intervening years. 

Table 1. MAP and direct comparison of fixed-point cell cutoff values. Cutoff values are for expanded 

uncertainties (k=2) in mK. 

LN2 1.5 In FP 1.2 

Ar TP 1.5 Sn FP 1.0 

Hg TP 0.6 Zn FP 2.0 

H2O TP 0.3 Al FP 3.4 

Ga MP 0.5 Ag FP 10.0 

When laboratories are seeking or maintaining NVLAP accreditation for their ITS-90 SPRT calibration 

services with expanded uncertainties (k=2) within the range given in Table 2, a need is indicated for a 

NIST Measurement Assurance Program (MAP) Test. The use of the MAP as a proficiency test should be 

performed once every three years with a single SPRT proficiency test that covers the temperature range of 

the accreditation in the intervening years.

NVLAP LAB BULLETIN NUMBER: LB-10-2004 (Revised 2007-02-21) PAGE: 1 of 5 
NVLAP LAB BULLETIN NUMBER: LB-10-2004 (Revised 2007-02-21) PAGE: 2 of 5 

Table 2. MAP cutoff range. Cutoff ranges are for expanded uncertainties (k=2) in mK. 

LN2 >1.5 to 2.3 In FP >1.2 to 1.8 

Ar TP >1.5 to 2.3 Sn FP >1.0 to 2.0 

Hg TP >0.6 to 0.8 Zn FP >2.0 to 3.4 

H2O TP >0.3 to 0.5 Al FP >3.4 to 8.5 

Ga MP >0.5 to 0.6 Ag FP >10.0 to 15.0 

 

When laboratories are seeking or maintaining NVLAP accreditation for their ITS-90 SPRT calibration 

services with expanded uncertainties (k=2) greater than the values given in Table 3, a need is indicated for 

a single SPRT proficiency test that covers the temperature range of the accreditation. The full temperature 

range of accreditation should be tested once every three years. A single SPRT proficiency test covering a 

reduced temperature range of the accreditation should be performed in the intervening years. 

Table 3. Single SPRT proficiency test cutoff values. Cutoff values are for expanded uncertainties (k=2) in 

mK. 

LN2 >2.3 In FP >1.8 

Ar TP >2.3 Sn FP >2.0 

Hg TP >0.8 Zn FP >3.4 

H2O TP >0.5 Al FP >8.5 

Ga MP >0.6 Ag FP >15.0 

The cutoff values given in Tables 1-3 are statistically derived from the ITS-90 SPRT calibration expanded 

uncertainties (k=2) claimed by NMIs from APMP, SIM, EUROMET, COOMET, and SADCMET. 

The current cost of the proficiency tests as indicated in the above text is given in Appendix A (prices 

subject to change). 

A list of additional information to have available for the on-site assessment is given in Appendix B. 

A suggested list of uncertainty components used in assigning an uncertainty to an ITS-90 fixed-point cell 

is given in Appendix C.
NVLAP LAB BULLETIN NUMBER: LB-10-2004 (Revised 2007-02-21) PAGE: 3 of 5 

Appendix A: FY2004 costs of the proficiency tests for those laboratories seeking or maintaining NVLAP 

accreditation for their ITS-90 SPRT calibration services (prices subject to change). Proficiency tests must 

be scheduled through NVLAP. 

Proficiency Test Temperature Range, °C Cost, $ 

MAP -200 °C to 420 °C Please contact NVLAP for current costs 

MAP -200 °C to 661 °C Please contact NVLAP for current costs 

Direct Comparison 

of a Fixed-Point Cell 

to be determined with NIST 

staff 

Please contact NVLAP for current costs 

Single SPRT -200 °C to 420 °C Please contact NVLAP for current costs 

Single SPRT -200 °C to 661 °C Please contact NVLAP for current costs 

Other temperature ranges for each type of proficiency test are available upon request. Costs will be 

reflective of requested temperature range.
NVLAP LAB BULLETIN NUMBER: LB-10-2004 (Revised 2007-02-21) PAGE: 4 of 5 

Appendix B: The following information should be available for the NVLAP on-site assessment of 

laboratories providing ITS-90 SPRT calibration services. Questions concerning the use of this list may be 

directed to Gregory Strouse, (301) 975-4803 or gstrouse@nist.gov. 

This applies to each fixed point (Ar TP, Hg TP, H2O TP, Ga MP or Ga TP, In FP, Sn FP, Zn FP, Al FP, 

and Ag FP). 

Sample Purity: Show analysis of the effect of impurities on the realized temperature of the fixed-point 

cell (Type B, normal distribution). Use method 1 or 2, in combination with method 3: 1) total wt. % 

impurities, 2) total mole fraction impurities, and 3) derivation from analysis of experimental realization 

curves. 

Phase Transition Repeatability: Using the check SPRT, measure multiple realizations (n≥10) of a fixedpoint cell to calculate the phase transition repeatability value (Type A, standard deviation of the 

measurements). Supply a graph of the results. 

Heat Flux: Quantify the heat flux by using the SPRT to measure the immersion profile of the phase 

transition realization of a fixed-point cell (Type B, normal distribution). For the SPRT to be near thermal 

equilibrium, the SPRT must be able to track the ITS-90 hydrostatic head effect over the bottommost 3 cm. 

Phase Transition Realization Curves: Supply at least one freezing (where applicable) and melting 

curve. 

The following should be provided for each measurement system used to measure the SPRTs. 

Measurement System Repeatability: Supply a graph showing measurements of a thermostatically 

controlled reference resistor over at least a 3 h period to determine the repeatability of only the resistance 

ratio bridge. Supply a graph showing measurements of an SPRT in either a water or Ga fixed-point cell 

over at least a 3 h period to determine the repeatability of the measurement system under nominal SPRT 

calibration conditions. 

Non-Linearity: Supply results giving the non-linearity of the resistance ratio bridge. 

Calibration questions: 

1) Prior to calibration, what is the stability requirement of the SPRT R(H2O TP) during the annealing 

cycle? 

2) What is the maximum allowable change of the R(H2O TP) over the calibration of the SPRT? 

Do you verify the calibration of the SPRT with measurements at redundant fixed-points (e.g., Ga MP or 

In FP)? If so, what are the acceptable limits of the non-uniqueness/calibration error [e.g., W(Ga MP) 

predicted from calibrated SPRT coefficients minus W(Ga MP) measured] assigned to the redundant fixedpoint measurements?
NVLAP LAB BULLETIN NUMBER: LB-10-2004 (Revised 2007-02-21) PAGE: 5 of 5 

Appendix C: A suggested list of uncertainty components used in assigning an uncertainty to an ITS-90 

fixed-point cell follows. Questions concerning the use of this list may be directed to Gregory Strouse, 

(301) 975-4803 or gstrouse@nist.gov. 

Bridge Repeatability 

Bridge Resistance Ratio Measurement 

Bridge Quadrature Effects (AC only) 

Reference Resistor Stability 

Phase Transition Realization Repeatability 

Chemical Impurities 

Hydrostatic Head Correction 

SPRT Self-Heating Correction 

Heat Flux 

Gas Pressure 

Slope of Plateau 

Isotopic Variation (TPW only) 

Propagation of TPW