Patent GB-1013171-A (GB)
Title: Method and apparatus for continuously indicating the amount of a substance in a basematerial
IPC: G01N21/31
Filed: 1962-08-08
Abstract: 1,013,171. Infra-red testing. GENERAL ELECTRIC CO. Aug. 8, 1962 [Oct. 9, 1961], No. 30390/62. Heading G1A. In an arrangement for determining the amount of a substance in a base material (e.g. the amount of water in a moving sheet of material utilized for paper manufacture) the material is exposed cyclically and alternately to two beams of infrared radiation in discrete wavelength bands which are selected such that the intensity of the radiation in one band which is reflected by the material is related to the amount of the substance present whilst the reflected intensity of the other band is independent of the presence of the substance, the reflected intensities of both bands, however, being equally affected by changes in the material itself. Fig. 1, shows the complete arrangement schematically whilst Figs. 2 and 3 show details of the "optics" of the arrangement. As shown, radiation from a lamp 2 is directed via the operative one of two filters 4, 5, rotated about axis 28, through infrared transmissive windows 8, 9 in a hollow "intergrating" sphere 7 on to a moving sheet 1 of wet material (e.g. in a paper making machine) reflected radiation from which enters the sphere via window 9 and is received by a photo-sensitive device 6, the sheet 1 being "canted" to prevent direct reflection back through window 8. The substance to be measured is the amount of water in the sheet and filter 4 is therefore selected to pass a frequency band at which infra-red radiation is strongly absorbed by water in paper. In the present arrangement filter 4 has a pass-band centered at 1.94 microns (which is one of several possible wavelengths) and filter 5, passing the reference radiation, has a pass-band centred at 1. 63 microns (which, again is one of several wavelengths). The photosensitive device 6 thus receives pulses of radiation the amplitudes of which depend, on the amount of water present and on factors independent of the water content, such as reflectance from the base material (filter 4 operative) and pulses of amplitude depending only on the latter factors (filter 5 operative). In operation the filters are rotated (about axis 28) at 600 r.p.m. by the belt, pulley and motor arrangement 30... 33 and the resulting electrical output of device 6 is fed via pre-amplifier 10 to an automatic variable gain amplifier 13. .. 15 which compensates for variations e.g. in line voltage, intensity of source 2 and in the bias on device 6 and supplies its output to a tuned amplifier 16 which amplifies the filter change frequency i.e. a 10 c.p.s. waveform. The latter is fed to a demodulator 19 which utilizing a reference signal generated photo-electrically by photosensitive device 18 and a 180 degrees slot 21 rotating with filters 4, 5, operates to integrate the input waveform and to supply an output D.C. current depending on the difference between the averaged electrical pulses representative of filters 4 and 5 to an indicating device 34 calibrated to indicate the percentage of moisture in the paper. The photosensitive device 6,which may consist of a plurality of single units in parallel, may comprise a lead sulphide photocell, a bolometer or a photovoltate cell. Instead of using a slot such as 21 for generating the 10 c.p.s. reference signal, a single hole at each end of the slot producing the pulses for triggering a multivibrator may be employed. Alternatively the reference signal may be generated by a magnet carried by the filter support and co-operating with a stationary coil.