Patent GB-711924-A (GB)
Title: Method and apparatus for measuring the temperatures of thermally radiating gaseous fluids and in particular flames
IPC: G01J5/0805
Filed: 1952-02-05
Abstract: 711,924. Photo-electric measurement of flame temperature. OFFICE NATIONAL D'ETUDES ET DE RECHERCHES AERONAUTIQUES (O.N.E.R.A.). Feb. 5, 1952 [Feb. 5, 1951 ; April 3, 1951], No. 3036/52. Class 40 (3) In apparatus for measuring the temperature of a flame, a beam of radiation energy modulated according to a known law is passed through the flame and the energy of the emergent radiation is compared with the energy of reference radiation corresponding to that incident on the flame to determine the value of energy of the reference radiation for which the energies of the incident and emergent radiations are equal and from which the flame temperature may be calculated as set forth below ; the incident radiation being obtained from a known intensity source of wavelength 0.2 to 15 microns and the energy of the incident radiation being modulated by interruption means interposed between the source and the flame to vary the ratio of the energy of the source radiation to the energy of the incident radiation. Fig. 1 shows a device wherein a tungsten filament lamp of known brightness for a given band of wavelengths emits equal beams A and B which are respectively intensity modulated by a disc 3 (Fig. 2, not shown) rotating e.g., at 100 revolutions per second, having two concentrie rings of apertures corresponding to beams A and B carrying light filters of uniformly graduated transmission coefficients. The filters are staggered so that the light beams are alternately obscured and exposed ; the light intensity of each beam being modulated linearly with time over each revolution of the disc. Beam A is optically directed through its appropriate filter ring and a chemically resistant annular chamber 9 containing a flame whose temperature is to be determined, to a pair of diaphragms 13 and a lens 12a focused upon a disc 10 (Fig. 3 not shown), driven synchronously with disc 3 and having two concentric rings of staggered apertures aligned with the fi'ter apertures in disc 3, to respectively interrupt beams A and B ; the latter of which is directly focused on disc 3 by lens 12b. The beams alternately emergent from the apertures of disc 10 are projected by mirrors 13a, 14a, 13b, 14b through a wave'ength filter 16 on to a common linear photocell 15 whose output is displayed by a recording galvanometer or cathode ray oscillograph. The resultant trace (Fig. 4) wherein output energy is displayed as abscissa and time corresponding to one disc revolution as ordinate shows a series of linearly increasing peaks alternately corresponding to the radiation energies of the beams A and B as modulated by the uniformly graduated filters. The envelope line I of the peaks corresponding to filters a, b, c, d, e, f represents the variation of energy of beam A incident on the photocell, and the envelope line II of the peaks corresponding to filters a1, b1, c1, d1, e1, f1 represents the variation of energy of the incident beam B, so that the two lines may be superimposed for comparison by shifting line II to the right through a distance corresponding to the time interval between two successive peaks, into position IIa, where it intersects line I at point X, denoting a value at which the emergent light energy of beam A is equal to the directly transmitted light energy of beam B, for each successive revolution of the filters so that where E c =value of incident radiation energy such that energy absorbed by the flame = energy given off by the flame to the emergent radiation e = radiation energy supplied by flame to emergent radiation A α = coefficient of absorption of the gaseous fluid constituting the flame. The temperature of the flame is then that of a black body radiating energy Ec, so that where C, and C 2  are constants, X is the radiation wavelength, T is the flame temperature required, e is the base of natural logarithms. Plural sets of graduated filters may be provided on a single disc so that several successive modulation cycles are obtainable per revolution. A single disc may modulate the source radiation, which is thereafter optically split into a measurement and comparison beam. Fig. 5 shows a modification wherein a radiation beam from source 101 is modulated by a rotary filter disc 104 as above, and passes through a flame F to a photocell 107, whose output peaks correspond to line I in Fig. 4. A rotary potentiometer 109 driven synchronously with disc 104 produces a cyclically varying voltage corresponding to line II in Fig. 4. This voltage and the amplified photocell output are alternately connected through an automatic mechanical or electric changeover switch to display on an oscil'ograph screen 112 the traces of the lines I and II above whose intersection point enables the flame temperature to be determined. The rotary potentiometer 109 is energized from a preset potentiometer 113 which is initially adjusted so that the above oscillograph lines are superimposed when no flame is interposed in the modulated beam. The automatic switch may be dispensed with if the graduated filter is disposed over a semicircu'ar arc of the rotary disc, Fig. 7 (not shown), the rotary potentiometer resistance is disposed along the complementary semicircle of rotation, and the potentiometer and photocell outputs are connected in common to the oscillograph input. The amplified photocell and potentiometer outputs may be automatically and successively switched to a cathode ray oscilloscope in parallel with an automatic recorder which marks the point at which the two outputs are equal, during each revolution of the modulating disc. The region of the flame spectrum in which the temperature is to be measured may be selected by an adjustable monochrometer in the radiation beam falling on the photocell, and if the flame is non- illuminating a substance (e.g. sodium) giving substantially monochromatic light may be added thereto. The radiation beam may be modulated by a rotary disc having a circular converging slot defined by two intersecting spirals (Fig. 11 not shown) and the cyclically variable comparison voltage is obtainable from a thyratron sawtooth generator or from a photocell illuminated by a modulated auxiliary light beam. Fig. 12 shows a modification in which an objective 205 projects parallel light from a source 201 on a rotary disc 202 having a ring of uniformly spaced apertures (Fig. 13 not shown). After passing through a flame F the modulated beam is focused by an objective 207 through an interference filter or monochrometer 208 on to a multiplier photocell 209 whose amplified output deflects a cathode ray oscillograph or recording galvanometer. Initially the system is operated without the interposition of the flame, and the oscillograph or recorder displays the waveform on the left of Figs. 14 and 15, whose peak ordinate y2 corresponds to E. The flame is then interposed, and the trace displayed has the form shown on the right of Figs. 14 and 15, whose peak ordinate yl corresponds to E (1- α) + e and whole base ordinate y3 corresponds to e. Then it is shown that where y corresponds to the value of E for which E = E (1 - α) + e and enables the flame temperature to be calculated. The oscillograms may be photographed for interpretation.