SEDL / STP / STP217-EB / STP43655S

Unbalance Errors in Guarded Hot Plate Measurements

Woodside, W.
Research Officer and Head respectivey, Building Services Section, Division of Building Research, National Research Council of Canada, Ottawa,

Wilson, A. G.
Research Officer and Head respectivey, Building Services Section, Division of Building Research, National Research Council of Canada, Ottawa,

Pages: 17    Published: Jan 1957

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Abstract

The errors in thermal conductivity measurement in three different designs of guarded hot plate caused by small temperature differences (unbalances) between test area and guard ring are described. The dependence of these errors upon: (a) the magnitude and direction of the unbalance, (b) the size and design of heater plate, (c) the conductivity and thickness of the specimens tested, and (d) the temperature difference between hot and cold plates is investigated. The percentage error in measured conductivity is shown to increase as the conductivity of the specimens tested decreases. Errors as high as 6 per cent were obtained even when the ASTM balance requirement was satisfied.

There are two constants for any heater plate which determine its error sensitivity to unbalance. If these constants are known, the maximum tolerable unbalance required to achieve any desired accuracy in measured conductivity may be calculated. The experimental determination of these constants, involving a minimum of two tests on each of two sets of specimens of different conductivity, is described.