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The Observed Properties of Liquid Helium
at the Saturated Vapor Pressure

Chapter 18. Thermal Diffusivity of Helium I

The thermal diffusivity is defined as

The values of density have been given in Chapter 1 and of thermal conductivity have been given in Chapter 15. The specific heat at constant pressure needed in Bénard convection experiments in helium I is a somewhat unusual quantity. The heating of the cell is done with the cold end thermally anchored to the main bath at the saturated vapor pressure. The liquid on the warm end expands at constant pressure, and therefore the value of specific heat must be evaluated at each temperature at the vapor pressure. We denote this quantity C_ps as and defined as

where Vm is the molar volume of helium, is the first derivative of the saturated vapor pressure (Chapter 17), and the and thermal expansion coefficient and specific heat have been discussed in Chapters 1 and 7. Therefore we simply give a table of recommended values for reference purposes. To convert from m²/s to cm²/s multiply by 10⁴.

Experimental determinations of the diffusivity have been made by measuring relaxation times in a thermal conductivity apparatus and the associated relaxation time to reach steady state conditions. See M. Dingus, F. Zhong and H. Meyer “Thermal Transport Properties in Helium near the Superfluid Transition, I. 4He in the Normal Phase, J. Low Temp. Phys. 65, 185-212 (1986). The most recent and best, done with several cell spacings is shown in Fig. 19 in Daniel Murphy and Horst Meyer “Transport Properties of Dilute Superfluid Mixtures of 3He in 4He”, J. Low Temp. Phys. 99, 745-785 (1995) and again in Fig. 12 of Daniel Murphy and Horst Meyer “Heat Transport in Dilute Mixtures of 3He in Superfluid ⁴He” J. Low Temp. Phys. 107, 175-196 (1997).

Table 18.1. Table of recommended values of the thermal diffusivity of helium I at the saturated vapor pressure.

T90 (K)	Cps (J/mol·K)	DT(m2/s)
2.178	32.629	2.801E-8
2.180	27.100	2.374E-8
2.185	22.204	2.231E-8
2.190	19.399	2.326E-8
2.20	16.738	2.510E-8
2.25	12.301	3.150E-8
2.30	10.807	3.588E-8
2.35	10.041	3.912E-8
2.40	9.554	4.176E-8
2.45	9.272	4.380E-8
2.50	9.137	4.528E-8
2.55	9.106	4.630E-8
2.60	9.139	4.699E-8
2.65	9.200	4.751E-8
2.70	9.280	4.791E-8
2.75	9.378	4.819E-8
2.80	9.494	4.836E-8
2.85	9.627	4.843E-8
2.90	9.778	4.839E-8
2.95	9.947	4.826E-8
3.00	10.132	4.804E-8
3.05	10.334	4.774E-8
3.10	10.553	4.737E-8
3.15	10.789	4.693E-8
3.20	11.041	4.643E-8
3.25	11.309	4.589E-8
3.30	11.594	4.530E-8
3.35	11.894	4.467E-8
3.40	12.210	4.401E-8
3.45	12.542	4.333E-8
3.50	12.888	4.263E-8
3.55	13.250	4.192E-8
3.60	13.625	4.120E-8
3.65	14.015	4.048E-8
3.70	14.420	3.976E-8
3.75	14.841	3.903E-8
3.80	15.281	3.830E-8
3.85	15.743	3.756E-8
3.90	16.227	3.681E-8
3.95	16.737	3.606E-8
4.00	17.277	3.529E-8
4.05	17.848	3.452E-8
4.10	18.456	3.373E-8
4.15	19.106	3.293E-8
4.20	19.801	3.211E-8