FAQ

ITS-90 FAQs:

1. What are the ITS-90 fixed-point cells and their associated assigned temperatures, Wr(T90) and dWr(T90)/dT90 values for the SPRT temperature range?

2. What are the ITS-90 temperature subranges available for a NIST calibration of a long-stem SPRT?

3. What are the ITS-90 temperature subranges available for a NIST calibration of a capsule SPRT?

4. What are the ITS-90 temperature subranges available for a NIST calibration of an HTSPRT?

5. What are the hydrostatic head and pressure corrections for the ITS-90 fixed-point cells?

6. How does an error in the TPW measurement propagate in the ITS-90?

7. What does a freezing curve for a fixed-point cell look like?

8. What does a melting curve for a fixed-point cell look like?

SPRT FAQs:

9. What are the NIST annealing procedures for SPRTs?

10. What does an immersion profile of an SPRT in a fixed-point cell during a freezing curve realization look like?

11. What are some items to consider when creating an uncertainty budget for SPRTs calibrated by ITS-90 fixed-point cells?

IPRT FAQs:

12. What equations should be used for industrial platinum resistance thermometers (IPRTs)?

OR:

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1. What are the ITS-90 fixed-point cells and their associated assigned temperatures, Wr(T90) and dWr(T90)/dT90 values for the SPRT temperature range?

Material
T90/K
t90/°C
Wr(T90)
dWr(T90)/dT90 / mK
 
 
 
 
 
e-H2 (TP)
13.8033
–259.3467
0.001 190 070
2.41E-07
e-H2 (VP)
17
–256.15
 
 
e-H2 (VP)
20.3
–252.85
 
 
Ne (TP)
24.5561
–248.5939
0.008 449 740
1.23E-06
O2 (TP)
54.3584
–218.7916
0.091 718 040
3.90E-06
Ar (TP)
83.8058
–189.3442
0.215 859 750
4.34E-06
Hg (TP)
234.3156
–38.8344
0.844 142 110
4.04E-06
H2O (TP)
273.16
0.01
1.000 000 000
3.99E-06
Ga (MP)
302.9146
29.7646
1.118 138 890
3.95E-06
In (FP)
429.7485
156.5985
1.609 801 850
3.80E-06
Sn (FP)
505.078
231.928
1.892 797 680
3.71E-06
Zn (FP)
692.677
419.527
2.568 917 300
3.50E-06
Al (FP)
933.473
660.323
3.376 008 600
3.21E-06
Ag (FP)
1234.93
961.78
4.286 420 530
2.84E-06
 
 
 
 
 
TP: triple point
MP: melting point
FP: freezing point
VP: vapor pressure point

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2. What are the ITS-90 temperature subranges available for a NIST calibration of a long-stem SPRT?

ITS-90 Temperature Subranges
Fixed-Points
Coefficients
 
 
 
83.8058 K to 273.16 K
AR, Hg, TPW
a4, b4
234.3156 K to 302.9146 K
Hg, TPW, GA
a5, b5
273.15 K to 302.9146 K
GA, TPW
a11
273.15 K to 429.7485 K
In, TPW
a10
273.15 K to 505.078 K
Sn, In, TPW
a9, b9
273.15 K to 692.677 K
Zn, Sn, TPW
a8, b8
273.15 K to 933.473 K
Al, Zn, Sn, TPW
a7, b7, c7
     
     
25.5 W SPRT from 83.8058 K to 692.677 K: mica former, borosilicate sheath   
25.5 W SPRT from 83.8058 K to 933.473 K: SiO2 or ceramic former, SiO2 or Inconel sheath   
     
SPRTs must meet ITS-90 specifications:  
W(302.9146 K) ³ 1.118 07 or W(234.3156 K) £ 0.844 235   
W(T90) = R(T90)/R(273.16 K)  

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3. What are the ITS-90 temperature subranges available for a NIST calibration of a capsule SPRT?

ITS-90 Temperature Subranges
Fixed-Points
Coefficients
 
 
 
13.8033 K to 273.16 K
H2, 17 K, 20 K, NE,O2, AR, Hg, TPW
a1, b1, c1, c2,
54.3584 K to 273.16 K
O2, AR, Hg, TPW
a3, b3, c3
83.8058 K to 273.16 K
AR, Hg, TPW
a4, b4
234.3156 K to 302.9146 K
Hg, TPW, GA
a5, b5
273.15 K to 302.9146 K
GA, TPW
a11
273.15 K to 429.7485 K
In, TPW
a10
273.15 K to 505.078 K
Sn, TPW
a9, b9
     
SPRTs must meet ITS-90 specifications:  
W(302.9146 K) ³ 1.118 07 or W(234.3156 K) £ 0.844 235   
W(T90) = R(T90)/R(273.16 K)  

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4. What are the ITS-90 temperature subranges available for a NIST calibration of an HTSPRT?

ITS-90 Temperature Subranges
Fixed-Points
Coefficients
     
273.15 K to 933.473 K
Al, Zn, Sn, TPW
a7, b7, c7
273.15 K to 1234.93 K
Ag, Al, Zn, Sn, TPW
a6, b6, c6, d
     
0.25 W to 2.5 W HTSPRT: SiO2 former and sheath    
     
SPRTs must meet ITS-90 specifications:    

W(302.9146 K) ³ 1.118 07

    
or W(234.3156 K) £ 0.844 235    
     
For above 933.473 K    
W(1234.93 K) ³ 4.284 4    
     
W(T90) = R(T90)/R(273.16 K)    

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5. What are the hydrostatic head and pressure corrections for the ITS-90 fixed-point cells?

Material 
T90/K  
mK/atm (pressure)
mK/(meter of liquid) (hydrostatic head)
e-H2 (TP)
13.8033
34
0.25
NE (TP)
24.5561
16
1.9
O2 (TP)
54.3584
12
1.5
AR (TP)
83.8058
25
3.3
Hg (TP)
234.3156
5.4
7.1
H2O (TP)
273.16
-7.5
-0.73
GA (MP)
302.9146
-2
-1.2
In (FP)
429.7485
4.9
3.3
Sn (FP)
505.078
3.3
2.2
Zn (FP)
692.677
4.3
2.7
Al (FP)
933.473
7
1.6
Ag (FP)
1234.93
6
5.4

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6. How does an error in the TPW measurement propagate in the ITS-90?

Graph of ITS-90 Calibration Error Propagation

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7. What does a freezing curve for a fixed-point cell look like?

Graph of Freezing Curves for In 96-6

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8. What does a melting curve for a fixed-point cell look like?

Graph of Melting Curves for In 96-6

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9. What are the NIST annealing procedures for SPRTs?

  • Capsule SPRTs do not get annealed
  • 83.8058 K to 692.677 K
    • SPRTs cleaned with acetone/alcohol
    • 4 hours at 450 °C - 480 °C
  • up to 933.473 K (Al)
    • SPRTs cleaned with acetone/alcohol
    • from 500 °C to 675 °C over 1 hour
    • 675 °C for 2.5 hours
    • from 675 °C to 500 °C over 3 hours
    • remove to ambient
  • up to 1234.93 K (Ag)
    • HTSPRTs cleaned with acetone/alcohol
    • from 500 °C to 975 °C over 1 hour
    • 975 °C for 2 hours
    • from 975 °C to 500 °C over 5 hours
    • remove to ambient
      • must be handled with clean plastic/rubber gloves
      • protection from metal ions in pre-heat furnaces at t > 700 °C

 

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10. What does an immersion profile of an SPRT in a fixed-point cell during a freezing curve realization look like?

Graph of Immersion Test

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11. What are some items to consider when creating an uncertainty budget for SPRTs calibrated by ITS-90 fixed-point cells?

Measurement Instruments:

  • non-linearity of instrument
  • difference in resistance unit
  • AC vs DC Resistance bridge
  • difference in lead resistance
  • number of readings
  • excitation current

SPRT:

  • immersion
  • self heating
  • protection from light piping
  • protection from contamination for HTSPRTs
  • mechanical and thermal shock
  • insulation resistance
  • corrections to make to T (ESH)
  • measurement equipment operating correctly

Fixed Point Cells and Furnaces:

  • impurities in fixed-point cell
  • immersion profile
  • length of freezing or melting curve
  • temperature corrections(HH,P)
  • furnace gradient
  • furnace temperature
  • furnace control

Use check thermometers for measurement assurance

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12. What equations should be used for industrial platinum resistance thermometers (IPRTs)?

Below 0 °C:

Rt=R0*[1+A*t+B*t^2+C*(t-100)*t^3]

t=1/(alpha)*(Rt/R0-1)+(delta)*(t/100-1)*t/100+(beta)*(t/100-1)*(t/100)^3

Above 0 °C:

Rt=R0*[1+A*t+B*t^2]

t=1/(alpha)*(Rt/R0-1)+(delta)*(t/100-1)*t/100

Where:

B=-(alpha)*(delta)/100^2C=-(alpha)*(beta)/100^4A=(alpha)*(1+(delta)/100)

(beta)=-100^4*C/(A+100*B)(delta)=-100^2*B/(A+100*B)

  • ASTM E1137 and IEC751 have adopted values for the coefficients A, B and C for ITS-90 over the range from -200  °C to 850 °C
    • A90 = 3.9083 x 10-3
    • B90 = -5.775 x 10-7
    • C90 = -4.183 x 10-12

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Last Updated on: 2/25/04

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