Patent GB-1003335-A (GB)
Title: Thermal parameter indicator
Inventor: MALONE ERLE WILLIAM
IPC: G01K3/08
Filed: 1963-02-04
Abstract: 1,003,335. Infra-red detector, measuring temperature electrically. BOEING COMPANY. Feb. 4, 1963, No. 4527/63. Headings G1A and G1N. In an arrangement for measuring a thermal condition utilizing a thermal sensing means subject to an overall thermal transfer coefficient which introduces a sensing error factor into the measurement the latter is eliminated by combining differentially the output of such sensing means with the output of a second thermal sensing means (presented to the same thermal condition) having a thermal response characteristic bearing a known relationship to that of the first sensing means and an overall thermal transfer coefficient producing a sensing error factor differing from but bearing a known relationship to that produced by the first sensing means. Fig. 1 shows the general arrangement in which it is required to measure the temperature To of a location 10 subject to radiation to or from a surface S at temperature T 1 . The primary sensor 12 (e.g. a thermo-electric transducer) produces an output voltage e related to the temperature To together with an unknown error voltage #e due to the unknown overall transfer coefficient produced e.g. by intervening thermal impedance introducing a thermal gradient #T. The compensating sensor 14 is also associated with a different and unknown thermal gradient K#T and has a sensitivity assumed to be M times that of sensor 12 thus producing an output voltage Me + MK#e. The two output voltages are then subjected to respective known gains G 1 , G 2  and combined differentially in 16 which produces an output voltage e o . It is required that e o  = e and to achieve this the condition G 1  = G 2  KM must be attained, and G 1 -G 2 M must be known or be a constant. In practice M is known and K is then set by design. This is achieved in practice, as shown in Fig. 2, by making the thickness of the thermal impedance separating the surface whose temperature is to be measured from the sensors 12a, 14a in the ratio #T/K#T and in the simplest case K is made onehalf and the sensitivity of sensor 14a is made twice that of sensor 12a so that with equal gains G in the two channels the differentially combined outputs is equal to the voltage e of the primary sensor from which the temperature To is deduced. In order to remove initial transient variations due to differences in the thermal diffusivity of the two sensors the diffusion of heat to sensor 14a (having the thinner thermal impedance layer 16) may be delayed by loading the latter with a conductive layer e.g. a thin layer of copper (Figs. 3 and 4 not shown). Figs. 5 and 6 show a bridge arrangement for measuring the temperature of a surface S employing three temperature sensitive resistance elements 22, 24, 28 and a non-temperature sensitive resistance element 26 (also, Figs. 7 and 7a not shown) which may be physically romote from the former elements. The elements 24 and 28 are spaced from surface S by equal thermal impedance 32, 34 and element 22 is spaced by an impedance 36 of twice the value of the latter so that with the bridge initially balanced at some temperature, and meter 30 suitably calibrated the output voltage e 0  is proportional to the temperature of surface S since errors due to the thermal gradients are cancelled out. In the modification shown in Fig. 7b only two temperature responsive resistance elements R1, R2 are employed at the measuring position and in this case it is necessary for R1 to have a positive temperature coefficient and for R2 to have a negative coefficient, the latter e.g. comprising a suitable semi-conductor. Fig. 8, shows a further modification in which a second bridge circuit formed by the addition of resistors 40, 42 produces a voltage at terminals 46 proportional to the heating rate in addition to the temperature proportional voltage e 0  across terminals 44. Fig. 9, shows an arrangement for measuring the temperature at a point position P in which resistance elements Ra, Rc correspond to elements 24, 28 of Fig. 5, and resistance element Rb corresponds to element 22 of Fig. 5. Corresponding arrangements for measuring the temperatures on the exterior and interior of cylindrical objects are described with reference to respective Figs. 10 and 11 (not shown). In another arrangement three thermal junctions 56, 58, 60 (Figs. 14 and 15) in thermal contact with absorption surfaces S 1 , S 2 , S 3  are employed compensation being effected by appropriate choice of the transfer coefficients h and the thermo-electric sensitivities of the junctions. Other arrangements employing thermal junctions are shown in Figs. 13, 16a, 16b, 18 and 20.