UNITHERM ™ 6000 GUARDED HOT PLATE
THERMAL CONDUCTIVITY MEASURING SYSTEM

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Used to measure the thermal performance of insulations and other materials of high thermal resistance, the guarded hot plate is a primary measuring instrument that does not need to be calibrated with a known reference material. Due to the large specimen size, the instrument is usable for testing even specimens of inhomogeneous materials, including low density fibrous insulations, concrete, and composites. Single specimen size is 300mm square and up to 75mm thick. These guarded hot plate instruments are designed in accordance with ASTM C177 and ISO 8302 specifications and can be used to test materials from -170°C to 550°C, in various range increments, depending on the model. Guarded hot plate instruments are used mostly for R & D testing where having the flexibility to meet many different applications and ultimate in accuracy is often more important than the speed of testing. All models are fully computer-controlled using current model PC's and Windows™ operating platform. Each unit is supplied with extensive operating and data reporting software. Here the user needs only to select the desired specimen temperatures and the test proceeds unattended until completion. All testing parameters, such as equilibration criteria, heating rates, etc. are also user selectable within or outside of the applicable standards.

170°C to 550°C, in several ranges Absolute method, no reference needed ASTM C177 and ISO 8302 Fully automated operation Single specimen construction

GUARDED HOT PLATE TEST METHOD

The material to be tested is placed on a flat plate heater assembly consisting of an electrically heated inner plate (main heater) surrounded by a guard heater. The guard heater is carefully controlled to maintain the same temperature on both sides of the gap separating the main and the guard heaters. This prevents lateral heat flow from the main heater and ensures that all heat energy flows in the direction of the specimen. On the opposite side of the specimen are additional flat plate heaters (the "cold" plate) that are controlled at a fixed temperature selected by the operator. For a given heat input to the main heater, the hot plate assembly rises in temperature until the system reaches equilibrium. The final hot plate temperature depends on the electrical power input, the thermal resistance of the specimen, and the temperature of the cold plate. The average thermal conductivity, k, of the specimen is determined from the Fourier heat flow equation:

where W is the electrical power input to the main heater, A is the main heater surface area, dT is the temperature difference across the specimen, and d is the specimen thickness. The thermal resistance of the specimen is defined as:

During normal operation the heat sinks are cooled using city water. Since water temperatures can be as high as 30°C, the lowest attainable specimen mean temperature is 50°C. To go below that a chiller/circulator is used. To reach cryogenic temperatures, liquid nitrogen is injected into the heat sink.

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Specifications are subject to change without notice.