Thermophysical Properties Research Laboratory, Inc.
Phone : 765-463-1581
(ASTM B 193-02, ASTM C 835-06)
The multiproperty apparatus is designed to measure many thermophysical properties, including
thermal conductivity, thermal diffusivity,
specific heat, thermal expansion,
electrical resistivity, emissivity,
enthalpy, hemispherical total emittance, Wieddemann-Franz-Lorenz Ratio,
Thomson coefficient, Seebeck coefficient, Peltier coefficient, and Richardson coefficient.
The samples used in the multi-property apparatus must be rod-shaped materials which are reasonable
electrical conductors. Metals, alloys, and graphitic materials have been measured extensively using this device.
Measurements of most of these properties can be made from room temperature to about 1000°C using thermocouples
for temperature measurement. However, the apparatus is primarily a high temperature (>1000°C) device using optical
pyrometry for temperature determinations.
The temperature range of the multi-property apparatus is room temperature to an upper temperature
limitation imposed by the samples material characteristics, i.e., vapor pressure, melting temperature,
softening temperature, etc.
The multiproperty apparatus consists of a high vacuum system (10-7 torr), large bell jar equipped with
two long windows, interior piping and sample holders with provisions for sample expansion and contraction,
and regulated DC power supplies. An automatic optical pyrometer and elevating stand are required, and
twin telemicroscopes and stand are used for thermal expansion. Samples in the form of thin rods, tubes,
or wires are supported vertically between water-cooled movable electrodes. The electrode separation
distance is adjustable between 0 and 14 inches. Sample expansion and contraction is maintained stress-free
through a spring network mounted on a movable "c-cell" equipped with strain gauges. The bell jar which covers
the sample support system is raised and lowered by a hoist. The bell jar rests on a feed-through collar which
contains rotary feed-through's for protecting the window and for moving the c-cell, instrumentation leads,
electrical connections, and water lines. Usually instrumentation readout is accomplished using a minicomputer
based digital data acquisition system. The bank of regulated power supplies is equipped with remote controls,
reversing switches and calibrated current shunts. Temperature measurements are made using the automatic optical
pyrometer mounted on a positioning stand external to the vacuum system and viewing the sample through an optical
window. The system is shown schematically in Figure 1. Linear thermal expansion measurements use twin
telemicroscopes mounted on a second stand and viewing the sample through a second window.
Before the sample is inserted into the multiproperty apparatus, several preliminary steps should be taken.
The sample's diameter should be measured, the sample weighed, voltage probe wires or fiducial marks
(thermal expansion) attached, and the separation distances between voltage probes or fiducial marks determined.
The electrical resistivity determined along various portions of the sample also serves as a check of homogeneity
and is valuable in determining changes in the sample caused by high temperature heating.
Place the sample into the test cell, lower the bell jar, and create a vacuum. Using high-current, low-voltage
supplies, adjust the sample temperature to the desired level by adjusting the current. At the desired temperatures,
measure the voltage drop across the probes (V), the current (I), and the uniform central temperature (T). The
electrical resistivity (ρ) can be calculated with the following equation:
and the total hemispherical emittance (εH) is calculated:
where P is the circumference of the sample, σ is the Stefan-Boltzmann constant, and T0 is
temperature of the vacuum enclosure.