Patent GB-1309551-A (GB)
Title: Measurement of optical density
IPC: G01N21/05
Filed: 1969-05-22
Abstract: 1309551 Measuring optical density photo-electrically NATIONAL RESEARCH DEVELOPMENT CORP 9 March 1970 [22 May 1969] 26287/69 Heading G1A The optical density of a fluid in a specific waveband is measured by deriving a first signal corresponding to the attenuation of light in that waveband, deriving a second signal corresponding to the concentration of solid matter suspended in the fluid, and utilizing the second signal to correct the first signal and hence remove any error caused by radiation scattering from suspended particles. Choppers 12, 12' are operated so that two light sources 3, 3' alternately irradiate a sample flow cell 1 containing two detector arrangements 7, 7'. The entire system is symmetrical about the axis 2. The arrangements 7, 7' each comprise three photo-cells 8-10, 8'-10', the photo-cells of each arrangement being located one above the other Fig. 2 (not shown). Each arrangement is provided with an identical pair of filters so that each detector of one arrangement is paired with a corresponding detector of the other arrangement as regards spectral response. For an instrument measuring the concentrations of organic carbon and NO 3 <SP>-</SP> in river water, the photo-cell pairs are:  a. One pair - 8, 8' - responds to a waveband in which both organic carbon and NO 3 <SP>-</SP> are strongly absorbing (2, 000-2, 800) b. One pair - 9, 9' - responds to a waveband in which organic carbon is strongly absorbing (2, 000 - 2, 400) c. The other pair - 10, 10' - has no filter and hence gives an indication of the amount of light scattered from solid particles in the liquid. Each arrangement 7, 7' receives radiation alternately through a long and a short path, and the outputs corresponding to these two paths are averaged to compensate for variations in both source output and detector sensitivity. This averaging is effected by operation of switches 32, 33 synchronously with rotation of the choppers 12, 12', so that when one photo-cell - 8 - of a pair is operative and acts as an input element to an operational amplifier - A1 -, the other photo-cell - 8' - acts as a feedback element and vice versa. The output from A3 represents the amount of particulate matter in the sample and a proportion of this is fed back to the inputs of amplifiers A1 and A2 as a correction. The amount of feedback necessary is determined by calibration on known liquid samples. Similarly, the output from A1, representing NO 3 <SP>-</SP> +organic carbon concentration, is fed to A2, whose photocells 9 and 9' responde to NO 3 <SP>-</SP> concentration, to give an output from A2 representative of organic carbon concentration alone. The output from each amplifier is converted to digital form at 34 and the pulses are counted for an appropriate time period to obtain a total output. The outputs from the three amplifiers A1-3 are gated so as to be printed out sequentially. The counter is then reset to zero and the cycle is resumed. In a further embodiment the light is provided by two alternately pulsed low pressure Hg lamps contained within a flow-cell Fig.4 (not shown). In this case the UV-detecting photo-cells are silicon-EMF cells coated with a UV-responsive phosphor. The two cells which monitor the suspended matter (P3, P6) may be located apart from the two photo-cell arrangements. A diaphragm is positioned (57) so that those two photo-cells receive alternately light direct from one source and light scattered via particles suspended in the sample from the other source. In another form of circuitry, Fig. 6 (not shown), for each detector pair 8, 8' (P1, P4) the outputs corresponding to the long path and those corresponding to the short path are separately combined (62) and summed (64). The two resulting outputs are then ratioed (65) to provide, for each detector pair, a signal (V7-9) corresponding to the NO 3 <SP>-</SP>, NO 3 <SP>-</SP>+ organic carbon and suspended matter concentrations respectively. The feedback correction process previously described is then carried out.