Patent GB-1519842-A (GB)
Title: Measurement of heat transfer properties
IPC: G01N25/18
Filed: 1976-08-11
Abstract: 1519842 Measuring thermal diffusivity GENERAL ELECTRIC CO Ltd 11 Aug 1976 [6 June 1975] 24407/75 Heading G1N The invention relates to a method of determining the thermal diffusivity of a material and an apparatus for carrying out the method. The method includes the steps of placing a specimen of the material between first and second identical ribbon filaments 1, 2 of which the longitudinal axes are parallel and which in the region of the material are spaced a small distance, e.g. 1.5mm, apart, connecting each filament 1, 2 to an independent electric circuit arrangement for passing a heating current therethrough, heating both filaments initially to the same temperature by passing identical currents therethrough, varying the magnitude of the heating current through the first filament 1 to produce a temperature change(s) therein, detecting a corresponding temperature change(s) in the second filament as a result of heat transfer from the first filament, and measuring the time interval between the production of the temperature change in the first filament 1 and the detection of the corresponding temperature change in the second filament, and then determining the thermal diffusivity of the specimen from the time interval and the thickness of the  specimen between the filaments. The diffusivity may be determined from a calibration curve or from equations given in the Specification. As shown the filaments 1, 2 are provided with kinks to allow thermal expansion thereof without distortion. The independent electric circuit arrangements each comprise a battery 5, 6 and a series of parallel connected resistors R 1 -R 4 , R 5 -R 8  each resistor having a switch in series therewith. Thus the heating current can be increased stepwise. The circuit arrangement of the first filament 1 includes a modulating arrangement 7 for producing the aforesaid varying of the magnitude of the heating current through the first filament 1. The modulation may be a stepwise, sinusoidal or squarewave variation. Potential probes 8, 9, 10, 11 are spot welded to the filaments adjacent the specimen for monitoring the potential changes in the filament. Current changes may also be monitored. Alternatively the temperature may be taken directly using two photo-cells, one for each filament, or one photo-cell switched alternately between the two filaments 1, 2. The probes 8, 9, 10, 11 may be connected, in pairs through corresponding circuit arrangements to orthogonally disposed plates of an oscilloscope to produce, in use, on the oscilloscope screen a Lissajous figure Fig. 3 (not shown). Means are provided for introducing a variable and measurable phase lag into the signal from the first element such that the shape of the Lissajous figure may be varied until it becomes a straight line at which time the phase lag introduced into the probe circuit of the first filament 1 equals the aforesaid time interval. If the specimen is normally solid and the thermal diffusivity of the molten form of the specimen is required, the heating of the filaments is used to melt the specimen. The specimen and filaments may be housed in an argon filled chamber. The filaments are of a refractory material e.g. tungsten. The filaments may be generally a truncated V-shape, the free ends being bent over Fig. 2 (not shown).