10th International 

Temperature Symposium

Disneyland Hotel, 

Anaheim, CA

April 4-7, 2023

A Closed-Cycle Immersion Cell Platform for Noble Triple Points

Near-term modifications. Available cooling power to zone 1 is limited by copper wire-rope strap 

conductance, an additional thermal strap will be fabricated and installed to augment the effective 

thermal conductance between the CT absorber and zone 1 plate.

Christopher Alvarez, Jay Nanninga, and Weston Tew

National Institute of Standards and Technology,

Sensor Science Division 

Gaithersburg, MD, 20899, USA 

email: westew@nist.gov

Abstract / Introduction

New technologies are needed for maintenance systems to support non-metal

fixed points below 200 K using immersion-type cells. In addition, we are unaware

of any immersion-type Xe triple point (TP) cell in the scientific literature. We have

developed a new closed-cycle refrigeration platform for the storage,

condensation, freezing and controlled melting of noble gas solids for direct

realization of their TP temperatures in an immersion-type cell. The system

accommodates long-stem standard platinum resistance thermometers (SPRTs)

through a single 7.75 mm inner diameter thermowell that is continuously purged

with helium gas and thermally coupled to four separate control zones and

strongly coupled to the noble-gas condensate. Three other capsule-type SPRT

thermowells are accessible from the interior vacuum space.

The system design incorporates several innovative features as well as some

derived from a legacy argon system designed by Furukawa [1] which was used at

NIST for more than 30 years prior to it’s decommissioning. This new patented

design [2] is based on two compact single-stage Stirling cryocoolers and does not

utilize liquid nitrogen for maintaining the melt conditions. A four-zone control

arrangement is implemented with active cooling applied to the first two outer

zones via the two cryocoolers while two other inner zones are passively cooled

and actively controlled via heating only. The Stirling coolers are soft-mounted

using bellows connections and thermally coupled to the two outermost zones via

flexible copper wire-rope straps which serve to decouple vibration transmission

from the Stirling’s linear drive motors operating at 60 Hz.

The cell holds approximately 210 mL of noble condensate, either argon,

krypton or xenon, and the integral expansion ballast volume of 21 L produces a

critical density ratio of 0.021, sufficient to keep pressures below 6 bar at ambient

conditions. The gas manifold provides both pressure and mass flow rate

regulation and is capable of controlling the rate of gas condensation and

monitoring the rate of gas expansion as the system changes temperature. A single

vacuum chamber encloses the four nested control zones and the cell. Testing has

been conducted using a set of diagnostic diode thermometers dispersed across

various points in all zones. Initial results indicate the system is suitable for Xe and

Kr TP realizations but that additional refinements in the thermal strap

configurations are needed to fully support Ar TP realizations.

Design and Construction Testing Results

Noble Gas Thermophysical Properties Summary / Future Work

Ttp ptp rl rs Dhf dpm/dT dT/dz

(K) (MPa) (kg/m3) (kg/m3) (J/mol) (MPa/K) mK/m

Ar[3] 83.806 0.06889 1416.8 1622 1190 3.98 3.48

Kr[4,5] 115.78 0.10675 2447 2824 1640 3.32{3.10} 7.2 {7.7}

Xe[4,5] 161.41 0.0818 2961 3545 2315 2.57{1.96} 11.3{14.8}

Fig. 1. (above) Key features of the platform. The top mounted highpurity gas manifold is constructed of orbital-welded 6.35mm 

stainless steel tubing and all-metal gasket fittings. The manifold (a 

schematic is shown in Fig. 7 below) is equipped with a mass flow 

controller (1), gas purifier (2), pressure regulator (3), mass flow 

meter (4), and two capacitance diaphragm gauges (CDGs) which 

are connected to two independent gas fill lines connecting the 

manifold to the cell. A series of 15 valves provide control and 

isolation, with check valves (8) for over-pressure relief. Continuous 

controlled cooling is provided via two Stirling-type cryocoolers, an 

80 W (5), and a 160 W (6) (input power), operating in tandem. A 

central thermowell (7) provides direct access to the TP cell.

RTD/Diode Location Temperature

MT MT Stirling Cold Tip 63.3 (0.7) K

CT CT Stirling Cold Tip 118.2 (0.5) K

1 Zone 1 Top Plate 153.3 (1.0) K

2 Zone 2 Base Plate 72.8 (1.0) K

3 Zone 3 anchor plate 83.9 (1.0) K

4 Zone 4 anchor plate 83.3 (1.0) K

5 TP Cell 83.7 (1.0) K

6 Gas fill line 133.4 (1.0) K

7 Thermowell Clamp 155.2 (1.0) K

8 Copper Plate 151.5 (1.0) K

Fig. 3. (above) Cross-sectional diagram of 

the vacuum chamber showing locations of 

the diagnostic silicon diodes (1-8) and 

Pt100 thermometers (MT & CT).

Fig. 3. Argon Triple Point System Schematic.

Disclaimer: Certain equipment, instruments, software, or materials, commercial or non-commercial, are 

identified in this paper in order to specify the experimental procedure adequately. Such identification is not 

intended to imply recommendation or endorsement of any product or service by NIST, nor is it intended to 

imply that the materials or equipment identified are necessarily the best available for the purpose.

1

2

3

4

5

6

7

Fig. 2. An unmounted Stirling cryocooler.

Fig. 4. (Left) The TP cell. Shown are the 

two gas fill lines, 3

rd zone and 4th zone 

anchor plates, thermowell, and burst disc. 

Fig. 5. (below) The complete assembled system as viewed from the front. 

Fig. 6. (below) The view showing the thermowell and a long stem 

SPRT fully inserted at a total insertion depth of 50 cm.

References

1. G.T. Furukawa, in Temperature: Its Measurement and Control in Science and Industry, vol. 6, ed. by J.F. Schooley 

(AIP, New York, 1992), pp. 265–269

2. Triple point immersion cell article, WL Tew, JR Nanninga - US Patent 11,204,288, 2021

3. Tegeler, Ch., Span, R., and Wagner, W., J. Phys. Chem. Ref. Data, 28(3):779-850, 1999.

4. Lemmon, E.W. and Span, R., J. Chem. Eng. Data, 51(3):785-850, 2006. doi: 10.1021/je050186n

5. Michels, A. and Prins, C., Physica, 28:101-116, 1962.

6. Pavese, F and Molinar, G, 1992, Springer, pp 471-475, https://doi.org/10.1007/978-1-4757-5869-6

Summary

Compact Stirling cryocoolers are suitable for implementing multi-zone refrigeration 

platforms for the realization of noble gas or other non-metal triple points in the range 70 K 

to 200 K, below that of conventional refrigeration cycles. The approach conveys certain 

key advantages over systems based on liquid nitrogen pool-boiling cryostats:

• The cooling is continuous and controllable in closed-loop operation

• Avoids drift and perturbations associated with liq N2 boil-off / refilling cycles

• Zones are cooled to any temperature as close to the TP temperatures as needed

• Zone settings are not constrained by liq N2 vapor pressure

• Avoids the high heating or high pressure requirements of liq N2 systems

Caveat: Improved melting line equations and/or data may be needed for Kr and Xe to 

enable accurate head corrections for those TPs via immersion cell realizations.

MT

CT

Cell

Zone 1

Zone 2

Phase Diagrams for Ar, Kr, and Xe. (above) Showing gas-phase isochors with critical density ratio CDR=0.021 

Calculated values (REFPROP, below) for thermophysical properties at the TPs. The melting line equation for Ar

from [3] predicts a value for the head correction coefficient that is 5.5 % larger than the ITS-90 value (3.3 mK/m). 

The melting line data from [5] are inconsistent with the Clapeyron Equation {dp/dT} for Kr (7 %) and Xe (~30 %) 

near their triple point temperatures. The molar enthalpy values are taken from [6].