UNITHERM

TM

MODEL 2022

GUARDED HEAT FLOW METER THERMAL CONDUCTIVITY MEASURING SYSTEM The Unitherm

Model 2022 is used to measure thermal conductivity of a variety of materials. These include poly-mers, ceramics, composites, glasses, rubbers, some met-als, and other materials of low to medium thermal conduc- tivity. Only a relatively small test sample is required. Non-solids, such as pastes or liquids, can be tested using spe-cial containers. Thin films can also be tested accurately using a multi-layer technique. The tests are in accordance with the ASTM E1530 Standard .

GUARDED HEAT FLOW METER TEST METHOD

A sample of the material is held under a uniform compres-sive load between two polished surfaces, each controlled at a different temperature. The lower surface is part of a calibrated heat flow transducer. The heat flows from the upper surface, through the sample, to the lower surface, establishing an axial temperature gradient in the stack. Af-ter reaching thermal equilibrium, the temperature differ-ence across the sample is measured along with the output from the heat flow transducer. These values and the sample thickness are then used to calculate the thermal conductivity. The temperature drop through the sample is measured with temperature sensors in the highly conductive metal surface layers on either side of the sample. Each instrument is factory calibrated using samples of known thermal resistance, spanning the particular range. The contact resistance is kept small by applying a reproducible, pneumatic load to the test stack, and if needed, a thermally conductive interface compound. A guard furnace surrounds the test stack to reduce the effect of heat transfer across the edges of the sample. For testing at temperatures below ambient, the device is supplied with an airtight compartment, where the atmosphere can be kept relatively moisture free with dry air purge. Polymers can be tested through the melt using special containment cells. Also, special containment cells are available for fluids, pastes, and powders. It typically takes from 45 to 60 minutes to complete a test at a temperature. It is unavoidable to have a substantial temperature difference between the cold face of the sam-ple and the heat sink. For this reason, a city water cooled heat sink allows operation with a lowest sample temperature of about 50

C. To fully utilize the range of the instrument, the optional chiller circulator is used, that can provide heat sink temperature to –10

C. The instrument is provided with one of three operating range modules. Each module covers a different thermal resistance re-gion. The various modules are field exchangeable

.