RECENT REALIZATIONS OF THE SF6 AND CO2 TRIPLE POINTS AT 

THE NATIONAL INSTITIUTE OF STANDARDS AND TECHNOLOGY

International restrictions on the trade of mercury-based products have created a near-term

need to develop alternative materials for temperature standards. The triple points of SF6 and

CO2 are good candidates for replacing the Hg triple point (TP) as a defined fixed point on the

International Temperature Scale of 1990 (ITS-90). We have constructed a series of immersion

cells for realizations of both the SF6 TP and CO2 TP that are suitable as ‘drop-in-replacements’

for Hg TP cells as currently used in conventional refrigerated baths. These immersion cells are

pressurized, with no external manifold, and refillable via a conventional service valve.

Realizations are performed in a quasi-adiabatic mode inside of a small vacuum chamber that is

immersed in an ethanol bath. We report recent measurements using capsule-type standard

platinum resistance thermometers (SPRTs) that have been calibrated on the subrange 4 of the

ITS-90. Melting plateaus are presented versus both melted fraction F and inverse fraction F

-1

scales. We compare the interpolated T90 temperatures as extrapolated to F=1 from these

realizations with those reported in our prior work and from the published literature.

Abstract / Introduction

Triple Point Cell Design and Construction

Results using SF6

Summary 

• Results using SF6:

• average of four cells (#302, 1202, 801, 401) 2017 [1]

• 223.55587(33) K , F=0.5 ; 223.55607(33) K , F=1 

• cell #803 (immersion type), 2019, preliminary

• 223.5556(5) K , F=0.5 ; 223.55608(50) K , F=1

• Pressure head correction: 0.116(16) mK/cm [1] ; Clapeyron Eqn. : 0.116 mK/cm

• Results using CO2, Cell 810, 2019 preliminary:

• 216.5910(5) K , F=0.5 

• References

• [1] W. Tew and K. Quelhas, J Res NIST, Vol 123, Art. No. 12013 (2018) 

https://doi.org/10.6028/jres.123.013 

Weston L Tew1, Andrea Peruzzi

2

, and Klaus N. Quelhas

3

National Institute of Standards and Technology, Gaithersburg, MD, 20199, USA

Van Swinden Laboratory, Delft, The Netherlands

INMETRO, Xerem, Brazil

email: westew@nist.gov

Ttp ptp rl rs Dhf dpm/dT dT/dz A-1

(K) (MPa) 3 (kg/m ) 3 (kg/m ) (J/cm3) (MPa/K) mK/m uK/ppm

Hg 234.3156 1.7×10-10 13,690 14,184 156 18.5 8.6 199

SF6 223.556 0.231 1845 2281 66.7 1.56 11.6 79

CO2 216.59 0.55536 1178.5 1532 232 4.69 2.47 45

Cell Volume

Vcell

SF6 /CO2

mass, mcell

Density

rcell

CDR

rcell / rcr

Effective 

Immersion

Enthapy

DHf

s/n 3 cm g g·cm-3 cm kJ

302 42 12.9 0.307 0.41 N/A 0.47

401 14 9.1 0.650 0.87 N/A 0.33

801 580 590 1.017 1.37 16 21.3

1202 550 467 0.849 1.14 13 17.0

803 537 573.3 1.068 1.44 18.3 20.7

810 352 68.55 0.195 0.42 16.8 13.5

Five SF6 TP cells and one CO2 TP cell are in this study. Four cells (SF6serial numbers 1202, 

801, 803 and CO2cell 810) are larger immersion types and two others (SF6 Cell serial 

numbers 302 and 401) are smaller ‘adiabatic’ type cells. The cell design parameters are 

summarized in the below Table. The cells are shown in the photographs below.

Results using CO2

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0 20 40 60 80

change in temperature relative to bottom, mK

change in immersion depth, mm

Immersion profiles CO2 TP Cell

F=0.11

F=0.21

F=0.31

Theory

302 801 1202

401

810

803

223.5545

223.5550

223.5555

223.5560

223.5565

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

T90 /K

F

SF6 803 5th Melt SF6 Cell 803 , 2019

Direct Heating Only

Pulse Melting

5 % DF / heat pulse

2 pulses / day

6 hrs. ON, 6 hrs. OFF

Stable over all F values

223.5545

223.5550

223.5555

223.5560

223.5565

1.0 10.0

T90 /K

1/F

Cell # 302, 2016

Cell #1202, 2017

Cell # 801, 2017

Cell # 401, 2017

Cell 803, 2019

2.0 3.0 4.0 5.0 6.0 8.0

Complied SF6 TP Realizations

Cells 302/401: Adiabatic 2016-17

Pulsed Direct Heating 

Kapton Thermofoil Heaters

500 mW , 1000 mW

Cells 801/1202: 2016-17

Pulsed Indirect Heating

Gas-coupled to Bath Heat

<≈ 10 mW

Cell 803: 2019

Pulsed Direct Heating

Kapton Thermofoil Heaters

48 mW

Cell 803 is similar to cell 801, but with the 

addition of an internal set of 30 copper 

baffles (shown below) to equalize radial 

gradients and support the solid SF6 mantle. 

Below: Experimental setup for immersion-type 

cells : capsule-type SPRTs to measure the cell 

(A) and bath (B) temperatures; (C) borosilicate 

glass adapter tubes; (D) nylon gas-tight fittings; 

(E) 50 l ethanol bath volume; (F) condensed 

(shown in green) portion of SF6 (G) pressure 

wall of cell volume; (H) SF6 fill line; (J) central 

thermowell; (K) bellows valve; (L) pressure 

gauge; (M) vacuum chamber; and (N) ISO-63 

vacuum flange with aluminum gasket. 

216.589

216.59

216.591

216.592

216.593

216.594

216.595

216.596

216.597

1975 1985 1995 2005 2015 2025

T90 / K

Year

CO2 TP Realizations

Bedford,

et al. 1996 NIST 2019

810 (F=0.5)

Preliminary

216.5908

216.5913

216.5918

216.5923

216.5928

0 12 24 36 48 60 72 84 96 108 120 132 144 156 168 180

T90 /K

Time /h

CO2 Cell 810 3rd Melt

0.71

0.66

0.56 0.61

0.31 0.41 0.46 0.51

0.26

0.21

0.11 0.16

0.06

216.582

216.583

216.584

216.585

216.586

216.587

216.588

216.589

216.59

216.591

216.592

0 10 20 30 40 50 60 70 80

T90 /K

Time /h

CO2cell 810 3rd Freeze

CO2 Cell 810 , 2019

Direct Heating 

Pulse Melting

5 % DF / heat pulse

Stable Plateau F < 0.56

223.55

223.551

223.552

223.553

223.554

223.555

223.556

223.557

223.558

223.559

223.56

1992 1997 2002 2007 2012 2017

T90, K

Year

Blanke, 1993

Rourke, 

2015

3021202

Funke, 2002

801

401

803

SF6 TP Realizations

CO2Source Cylinder

NIST Gas Metrology Group

Measured:

Ar: 0.30 ± 0.03 ppm 

N2: 1.05 ± 0.03 ppm

Specification:

CO < 0.2 ppm *

H2 < 1 ppm*

N2 < 5 ppm 

O2 < 2 ppm*

H2O < 2 ppm*

THC (as CH4)* < 4 ppm

*Removed by in-line Getter

SF6Source Cylinder

Concorde Specialty Gas†

Report of Analysis:

CO2- ND*

SO2- ND*

N2 + O2* = 3.3 ppm 

H2O < 1 ppm*

CF4 = 0.3 ppm

*Removed by in-line Getter

† Certain commercial equipment, instruments, or materials 

are identified in this poster in order to specify the 

experimental procedure adequately. Such identification is 

not intended to imply recommendation or endorsement by the 

National Institute of Standards and Technology, nor is it 

intended to imply that the materials or equipment identified 

are necessarily the best available for the purpose.

Immersion Profiles, CO2 Cell 810 , 2019

• Some distortions observed

• 10 mm steps downward

• Predicted from Clapeyron Equation :

• 0.0247 mK/cm

• assumes melting line: 4.69 MPa/K

d

d d

d

tp

tp

T T

T T

T g

z p

T

r

=

= =

Recent History of SF6TP realizations. History of CO2TP realizations since 1979.