GB710867A - Improvements in or relating to methods and means for testing a translucent fluid substance - Google Patents

Abstract

710,867. Spectroscopes. DALY, E. F., and UNICAN INSTRUMENTS (CAMBRIDGE), Ltd. April 2, 1952 [April 6, 1951], No. 8032/51. Class 97(1) [Also in Group XL(b)] In order to ascertain the component substances of a translucent fluid mixture its absorption spectrum is compared continuously with that of a reference fluid mixture of known composition which is adjusted to give exactly the same absorption spectrum, by passing light through each fluid and alternately through a mono-chromator and photocell, the outputs being alternately displayed on a cathode-ray tube, whereby any differences in the spectra may be determined and eliminated. Two beams of visible or infra-red radiation from lamp 1, Fig. 1, are focused by concave mirrors 4, 5 on two compartments. 6, 7 each comprising four cells 10 (described below), the cells of compartment 6 containing the test specimen, and those of compartment 7 containing the reference fluid. The beams are then refocused by mirrors 11, 12 on an oscillating mirror 14 serving as an optical switch, which directs the beams alternately via concave mirror 17 through a mono-chromator 21, having a scanning mirror 56, Fig. 4, attached to a steel bar 58 carrying two windings 59, 60 and attached by phosphor-bronze strips 58a to a rigid frame 58b. Coil 60 is fed with a 10 c/s sawtooth voltage wave from a generator 62, through a gain control 65 and cathode-follower 92 with negative feedback via coil 59 and amplifier 63. Thus by providing a magnetic field around coils 59, 60 the bar 58 and mirror 56 oscillate at 10 c/s, the amplitude of swing being controlled by gain control 65, and the mean angular position being controlled by a spring-loaded finger 66, attached to the bottom support of bar 58, pressed against an adjustable cam 68. The light emerging from the monochromator 21 is directed by mirror 72 to photo-electric device 74 or 75, whose output thus consists of a D.C. voltage with a ripple A.C. at the frequency of the optical switch (1500 c/s), which after amplifying and filtering at 80, 81, 82 has an amplitude equal to the difference between the absorption of the two paths, and is shown on a cathode-ray oscilloscope 85. The cells 10, Fig. 2, are mounted on slides 25 having a fused silica window 24, and which run in grooves 26 in the sides of the compartments. A threaded cup 28 is secured to the slide 25, and holds a ring 29 carrying a plate 30 supporting another fused silica window 23, the distance between the windows being adjustable by screwing ring 29, and being indicated. by an external ring 32 on the wall 33 of cup 28. The optical switch comprises the vibrating mirror 14 mounted on a soft iron armature 39, Fig. 3, in the centre of a torsion bar 15, between the tapered poles 43, 44 of an electromagnet fed with the resonant frequency of the system by an oscillator 45, which is maintained at that frequency by a pick-up coil 50 associated with the torsion bar 15 and feeding back into the oscillator 45.