Patent GB-674541-A (GB)
Title: Method of and apparatus for measuring flame temperatures
IPC: G01J3/26
Filed: 1950-05-04
Abstract: 674,541. Measurement of temperature. BRITISH THOMSON-HOUSTON CO., Ltd. May 4, 1950 [May 18, 1949], No. 11052/50. Class 97 iii). The temperature of a flame or of a zone in a complex flame is determined by resolving light from the flame by a spectroscope of high resolving power, e.g. a Fabry - Perot interferometer, so as to produce fringes whose brightness is graded at various points in accordance with the distance in wavelength units of the points from the centre-point of the fringes, resolving similarly the light from a substantially monochromatic reference source and comparing the brightness of corresponding points of the two sets of fringes. Such determinations are necessary in order to improve the design and operation of rocket and jet motors. The resolving power #/## of the spectroscope is preferably at least 100,000. An arrangement of apparatus for carrying out the invention, Fig. 1, comprises a light-source 7 which may be a sodium vapour electric lamp or an oxy-acetylene flame containing sodium vapour, an optical wedge 8 varying in opacity along its length, and a lens 9 which forms an image of the wedge within the flame 10 whose temperature is to be measured. The rays from the image are collimated by lens 11 to form a parallel beam for each point of the image. The light passes through a filter 12 passing a band of about 100 Š (Angstrom unit) wide, centred at 5893 Š and through a Fabry-Perot interferometer 14 to a camera 15 whereby an image of the wedge and flame is.formed on screen 17. A mask 13 may be provided before or after the filter 12 for limiting the area from which light rays are admitted to the interferometer and camera. Owing to the action of the interferometer the image exhibits bands of varying brightness. From the photographic record a brightness graph may be prepared. Fig. 6 shows such a graph for the light from a uniform flame in the absence of lamp 7. The spacing between the peaks and troughs is determined by the spacing of the interferometer plates and in an example has the value in wavelength units ## = Œ 0.42 Š. Fig. 11 shows the corresponding graph for the light from lamp 7 and wedge 9 with the flame absent. Fig. 14 shows the graph for the case when the light from lamp 7 and wedge 9 passes through a flame 10 comprising an intensely hot inner zone 10<SP>1</SP> surrounded by a cooler-zone 10<SP>11</SP>, the middle part of the graph corresponding to light passing through both hot and cooler zones, and the outer parts to light passing through the cooler zones only. The small depressions in the humps of the middle part of the graph indicate resonance absorption. Fig. 15 shows the graph of Fig. 14 superimposed on that of Fig. 11. Consider the intersection 351 which occurs in the region for which the light passes through the cooler zone of the flame only. By noting (i) the corresponding distance along the wedge (ii) the horizontal deviation in Š of the intersection from the adjacent maximum of curve 24, the temperature of the flame can be calculated if the temperature or brightness of the source 7 is known. If an intersection 35 be chosen for which the light passes through both zones of the flame, the temperature calculated will be intermediate that of the hotter and cooler zones. To obtain that of the hotter zone a correction taking account of the distance travelled by the light through the two zones respectively has to be applied. The correction necessary is less if the intersection chosen be one such as 35<SP>11</SP> occurring at a wing of the spectrum. Similar methods are applicable where the flame comprises a relatively cool inner zone surrounded by a hotter zone as in a Bunsen flame. An optical wedge may be dispensed with if the comparison light source be such that its interference graph 39, Fig. 17, in the absence of the flame, gives a sufficient number of intersections with the graph 40 obtained when the flame is interposed. In a modified apparatus the light source 7, wedge 8 and lens 9 are dispensed with and a mirror is placed behind one half of the flame 10. The interference pattern recorded in the camera is shown in Fig. 22, the left-hand side corresponding to the mirrored half of the flame. Graphs of brightness taken along the lines 57, 58 are shown at 59, 60, Fig. 23. The temperature is derived by comparison of these with graphs from a calibrated light source and wedge. As a light-graduating device in place of a wedge there may be employed a rotating sector shaped to obscure the light during varying fractions of a rotation at different radii. The interferometer may be adjusted to bring the fringe systems resulting from the two D-lines of sodium into coincidence.