Patent GB-960618-A (GB)
Title: Apparatus for determining contamination in a substance by means of radiation
IPC: G01N21/31
Filed: 1961-05-26
Abstract: 960,618. Infra-red absorption analysis. ATOMIC ENERGY OF CANADA Ltd. May 26,1961 [June 1, 1960], No. 19126/61. Heading G1A. [Also in Division H5] The proportion of a contaminant in a sample of a substance (wherein the substance shows transmission, and the contaminant absorption, in at least part of the ultra-violet, visible, and infra-red spectrum) is determined by successive measurement of the absorption of different wavelengths of a beam of radiation directed by a source through the sample on to a detector. The apparatus comprises at least two filters sequentially intercepting the radiation incident upon the detector and passing thereto different wavebands of radiation, with means for presenting to an integrator an output from the detector occasioned by one filter in opposite polarity to that occasioned by another filter, and means responsive to the integrator to vary the amplitude of at least one of the outputs in a sense to balance the outputs. In an embodiment to determine the quantity of ordinary water in a sample of heavy water infra-red radiation in the range of wavelength 2 Á to 3. 7 Á (which includes at 3 Á an absorption wavelength for H2O but excludes any for D2O or HDO) is directed by a source 10, Fig. 2, through a cell 11 containing the sample on to a photo-conductive cell 19. Between the sample and the photocell 19 is rotated about an axis parallel to the radiation path a disc 16, which sequentially interposes in the path four interference filters 12,13, 14 and 15 having narrow pass-bands centred respectively at wavelengths of 3.0Á, 2. 5Á, 3. 0Á, and 3. 5Á, so that the transmission by the sample of radiation of an absorption wavelength of H2O is measured twice per disc revolution and of radiation of wavelengths greater and less than this once per revolution. Two further photocells 20 and 21 are alternately energized twice per disc revolution by the passage, between each cell and one of lamps 22 and 24 proximate to it, of diametrally spaced holes 23 and 25 in a rim flange 28 of the disc, the two lamp and cell pairs being spaced at 90 degrees round the flange. As each of the filters 12 to 15 passes the photocell 19, Fig. 3, opposite polarity voltage pulses are produced across the halves of the secondary winding of a transformer 44. These are fed via sliders on potentiometers 62 and 63 to the contacts of a changeover relay 53 actuated four times per disc revolution by the photocells 20 and 21. Such switching of the relay results in the feeding to an integrator, comprising a resistor 54, an amplifier 55, and a capacitor 56, of voltage pulses corresponding to the passage of filters 12 and 14 past photocell 19 in opposite polarity to those occasioned by filters 13 and 15. Thus the integrator sums the difference between voltage pulses whose amplitude is dependent on the transmission by the sample of radiation of 3. 0Á wavelength and the average amplitude of pulses dependent on the transmission at wavelengths of 2. 5Á and 3. 5Á. The concentration of H2O, absorbing the 3. 0Á radiation, is given by the device in terms of the extent of movement of the sliders of potentiometers 62 and 63 necessary to balance these pulses. The summation output of the integrator, at 61, controls an impulse motor 64 so that when the summed voltage at 61 reaches sufficient magnitude the motor rotates through a discrete small angle, in a sense governed by the polarity of the summed output. The motor moves either one or both of the potentiometer sliders to balance the opposite polarity pulses supplied to the integrator. Thus the angular position of the motor shaft provides an indication of the degree of contamination and the shaft may drive a digital display output device. When such shaft rotation occurs the integrating capacitor 56 is momentarily short circuited by closure of contacts 65 so that integration can re-commence. The device may be used with suitable filters 12 to 15 for absorption measurement on solid, gaseous, or liquid samples. By adjustment of apertures containing the filters, together with a suitable choice of filter pass-bands, a procedure is outlined for obviating the effects of bubbles in a liquid sample and, by the use of further filters, obviation of sample temperative variation effects. A suitable infra-red source employing an electrical heating coil is described, Figs. 6 and 7 (not shown). Specification 960,619 is referred to.