Home  |  Software Products  |  Engineering Services  |  About Us  |  Contact Us

HEPAK Specifications...


HEPAK ( copyright Cryodata Inc.) is a computer program for calculating the thermophysical properties of helium-4 (4He) from fundamental state equations.

The state equations are valid from 0.8 Kelvin or the melting line to 1,500 Kelvin, including the superfluid range, the lambda line, and liquid vapor mixtures.  As a function of pressure, calculations are valid up to 1,000 bars, except between 80 and 300 Kelvin where they are valid to 20,000 bars.

HEPAK development continues, based on new reference data combined with minor corrections of the existing code around the critical point and at high pressure in the liquid. Also, new development will extend down to 0 K, rather than being terminated at 0.8 K.

Calculated state properties include:


Enthalpy, Entropy, Gibbs and Internal Energy

Specific Heats

Sound Velocity, including 2nd and 4th sound in the superfluid


Joule-Thomson coefficient


Gruneisen parameter

Latent Heat

dP/dT of the saturation line

several other PVT derivatives

Lambda line location as a function of P,V,or T

Normal fluid density fraction in Helium II

Mutual friction and conductivity function in He II

State properties (as listed above) within microdegrees of the Lambda line.

Other calculated properties include:

Viscosity down to 1.2 K

Thermal Conductivity down to 3 K

Dielectric Constant

Refractive Index

Fluid Surface Tension

A total of 41 different pairs of input parameters (i.e., pressure and temperature, density and internal energy, saturation line and entropy) are available for specifying the fluid state. Included in the input parameter list are both isochoric and isobaric delta-T to the lambda line, valid for delta-T as small as 50 microkelvin.

HEPAK is written by Vincent Arp, Bob McCarty, and Jeff Fox, using code from Brian Hands of the Oxford University Cryogenics Laboratory. HEPAK is referenced to NIST Technical Note 1334 (1989), by Arp and McCarty, then employed at NIST, and an update of this Technical Note (1998) by D.G. Friend, V. Arp, and R.D. McCarty.

HEPAK is fully consistent with the updated NIST standard reference data for helium. It is also consistent with the International Temperature scale of 1990, defined by helium vapor pressure from 1.2 to 5.0 K.


Currently, HEPAK is offered:

As an Excel Add-In (32 bit) function access to property calculations extensively tested on version of Excel from Excel97 through Excel 2010 (including Excel 2010 running on 64 bit OS).  The Excel Add-In has also been tested on the MAC Excel 2011 version with property calculations working correctly and occasional messaging glitches that are currently being debugged. 

As an ActiveX DLL (32 bit) for interfacing to a variety of Windows programs. This version is delivered with sample Visual Basic and C++ code, illustrating how to interface your own application to the HEPAK properties.

Fortran77 Source Code.

Copyright 2003-2013