Patent GB-1103386-A (GB)
Title: Apparatus for measuring the concentration of a specific contaminant in a substance
IPC: E21F13/08
Filed: 1965-07-14
Abstract: 1,103,386. Infrared analyzers. ATOMIC ENERGY OF CANADA Ltd. July 14, 1965 [Sept. 1, 1964], No. 29885/65. Heading G1A. [Also in Division F1] In apparatus for measuring the heavy water concentration in natural water, infrared radiation of wavelength 3.7 Á is passed alternately through a reference sample and a test sample. At this wavelength the extinction length is the same for D 2 O, HDO and H 2 O. The concentration of other contaminants is thereby determined. The extinction lengths of reference and test samples are then compared at 4.0 Á, at which the extinction length for H 2 O exceeds that for HDO by a factor of 40. The concentration of heavy water in the test sample may thereby be determined. Adjustment is made for impurities other than heavy water which have different extinction lengths at 3À7Á and 4À0Á. Apparatus. The apparatus comprises a sample chamber 20, Figs.3, 4a with infrared transparent windows 19 and into which reference water and test water are fed alternately by means of a pump 25 (see Fig. 8, not shown). Light from an infrared source 27 passes through the sample chamber 20 and through composite Fabry-Perrot and band pass filters 30 or 31 to a PbSe detector 33. Filters 30, 31 are in a reciprocating mounting sector 32. The light may be collimated and focused. Variations in incident radiation at detector 33 vary the potential at the input of a pre-amplifier 48, the output of which is applied to an amplifier 55 and a band pass amplifier 60. The output of amplifier 60 is applied to an integrating circuit comprising an amplifier 71 (Fig.4b) and capacitors Ca, Cb. The output of amplifier 55 (see Fig. 4a) is also fed to a rectifier 80 which supplies a charging capacitor 83 which in turn feeds an integrating circuit comprising a capacitor 84 and an amplifier 86. The output from amplifier 86 is taken to a potentiometer 88 (Fig. 4b) whose lower end is earthed. The slider 91 of the potentiometer 88 and the slider 90 of a potentiometer 76 are taken as inputs to a difference amplifier 92. The output of amplifier 92, through a servomotor 93, drives a pen 95 connected to the slider 91. The slider 91 (and pen 95) indicates the ratio of the voltage on slider 90 to that at the upper end of potentiometer 88. Assume the 3.7 Á filter is in position. Radiation detected by the detector 33 varies as the reference and test waters are illuminated in accordance with other contaminants, excluding heavy water. A modulated signal is obtained. The modulation component is selected by filter amplifier 60 and this is rectified by a detector 67 which chooses contacts 68 or 69 alternately as one or other sample is in the chamber 20. Amplifier 71 is connected by contacts as a Miller integrator and the charge (V 1 ) that builds up at capacitor Ca is proportional to the modulation amplitude. Subsequently the 4À0 Á filter is in position the sector 32 carrying the filters 30, 31 being positioned by a motor 110, the direction of rotation of which is changed on operation of contacts by a cam driven from a timing motor (not shown). With the 4À0 Á filter in position the circuit of amplifier 71 is changed. The connections to capacitor Ca are reversed and the charge flowing in is added to (-V i ). The two charging periods are the same and the final charge of capacitor Ca represents the difference in absorption between the test and reference samples due to heavy water concentration differences and, in the absence of compensation, to other factors. Subsequently the voltage across capacitor Ca is fed to the potentiometer 76 of the recorder and finally capacitor Ca is discharged. Cam actuated relays ensure the correct sequence of operations and ensure that the pen 95 is only responsive after the two charging periods of capacitor Ca. A slider 69' of a potentiometer 73 is set so that the output of capacitor Ca is independent of the amount of certain wavelength dependent absorbents other than heavy water. Capacitor Cb is included for this purpose. Gating generator. A generator for switching contact 67 into connection with contact 68 or 69 comprises a motor driven shaft (144) Fig.11 (not shown) carrying a drum (145) having nine longi-  tudinal slots (146). The drum (145) is received within a coaxial cylindrical casing (147) Fig.10 (not shown) having two oppositely formed longitudinal slots (148). A filament bulb (157) is on the axis of the drum (145) and casing (147). Mounted outside the casing (147) are diametrically opposed lead sulphide or cadmium sulphide cells (149, 150). The cells (149, 150) are in a circuit Fig. 12 (not shown) which includes the winding (151) of a two state relay comprising contact (67). Cells (149, 150) are illuminated alternately by bulb (157) on rotation of drum (145), and contact (67) is moved alternately into connection with contacts (68) and (69).