Codd, J. M.

Southampton (e-Prints Soton)

INTERNAL DOCUMENT 1 8 2 INVESTIGATION INTO THE POSSIBLE APPLICATION OF THE INFRAMAX SUSPENDED SOLIDS METER FOR MEASURING SUSPENDED SEDIMENT IN THE FIELD by J M CODD [ This document should not be cited in a published bibliography, and is supplied for the use of the recipient only]. I N S T I T U T E O F O C E A I M O G R A P H I C S C I E N C E S % / • INSTITUTE OF OCEANOGRAPHIC SCIENCES Wormley, Godalming, Surrey GU8 BUB (042-879-4141) (Director: Dr. A. S. Laughton) Bidston Observatory, Birkenhead, IVIerseyside L43 7RA (051-653-8633) (Assistant Director: Dr. D. E. Cartwright) Crossway, Taunton, Somerset TA1 2DW (0823-86211) (Assistant Director: IVI. J. Tucker) INVESTIGATION INTO THE POSSIBLE APPLICATION OF THE INFRAMAX SUSPENDED SOLIDS METER FOR MEASURING SUSPENDED SEDIMENT IN THE FIELD by J M CODD Internal Document No 182 Institute of Oceanographic Sciences CrOSsway Taunton, Somerset May 1983 Investigation into the possible application of Che INFRAMAX SUSPENDED SOLIDS METER for measuring suspended sediment in the field OBJECTIVE A calibration test was carried out upon an Inframax suspended solids meter together with a pre-calibrated Partech siltmeter. The purpose of carrying out such a test was to investigate the possible application of the instrument for measuring suspended sediment concentration in the field. This involved looking closely at any practical difficulties in operation and at the range and sensitivity of the instrument. It was also necessary to look at the overall performance of the instrument in comparison with that of the Partech siltmeter currently used for field measurements. Both instruments operate on a photoelectric principle of single gap light absorption . This system consists of a single light source with one light beam focussed across a gap, operating a photoelectric cell. The photoelectric cell operates an electronic circuit to give a reading on the dial and a voltage or current output. METHOD The calibration test involved placing both instrument sensors in a tank containing a fixed volume of clear water and then increasing the concentration of suspended solids in steps by adding pre-weighed amounts of dried mud. After each addition, readings were taken from both instruments. This was repeated for solids content up to 20 g/1 and 10 g/1. Before the calibration could commence, however, the zero and maximum values for the Inframax had to be set. The sensor head was placed in clear water and the maximum control potentiometer turned fully on. Then the zero reading had to be adjusted by means of the minimum control potentiometer. This was very difficult as a very small turn of the potentiometer caused a full scale deflection. There was also a long delay before the reading dropped or increased to a constant value. Once the zero had been set the sensor had to be placed in a sample corresponding to the upper limit of solids concentration. As the directions manual quoted 20 g/1 as the upper limit of the span this was taken as the maximum value. Thus an accurately measured 20 g of dried mud was added to an exact litre of water and agitated continuously whilst the sensor was immersed. The maximum control potentiometer was then adjusted to give a deflection on the scale of 100%. After removal from the sample the sensor was washed and returned to the calibration tank. The calibration was carried out as described above. Three problems arose with the reading of the instrument during the calibration. 1. After switching on the instrument will stay on for about 45 seconds and then switch off automatically. 2. The scale was only marked in divisions of 5% and hence was difficult to read even to 1%. 3. Nowhere in the manual did it say whether the Z value given was % non-transmission of light or % suspended solids content. Hence it was not known if the relationship should be linear or otherwise. RESULTS The readings at various solids concentrations are shown in Table 1 and calibration graphs are shown in Figs 1 and 2. It was seen from the results of the first calibration that the saturation point seemed to be just over 10 g/1. (See Table la and Fig 1) Hence the instrument was re-set so that 10 g/1 was equivalent to 100% deflection and the calibration was repeated for the reduced range of values. The second set of results (Table lb and Fig 2) show a marked improvement. However, 10 g/1 is still close to the saturation point as can be seen. COMMENTS From the manual it seems that a linear law is assumed to relate suspended solids concentration to percentage non-transmittance of light. This is known not to be true for larger ranges of suspended sediment concentration; but as the range only extends as far as 10 g/1 the law could just be assumed to be linear. The range is too small to be of real use in the field as suspended solids concentrations in excess of 20 g/1 are common. The practical problems of setting up the instrument and its operating diffi-culties are generally worse than the Partech siltmeters. The output for 100% deflection was quoted as 20 mA which is difficult to read to a high accuracy. CONCLUSION The instirument is not recommended for use in the field due to its small range and its many operating problems. J.CODD 11.2.83. Table la Calibration results when 20 g/1 = 100% deflection g/1 % Deflection PARTECH Siltmeter 'D' F.S.D. 0.5 (mV) F.S.D. 1 (mV) 0 1 -30 -27 1 8 75 149 3 27 412 450 5 44 610 1250 7.5 67 735 1500 10 97 809 1650 15 off scale 882 1800 20 off scale 916 1860 0 1 -29 -25 Table lb Calibration results when 10 g/1 = 100% deflection PARTECH siltmeter 'D' g/l % Deflection F.S.D. 0.5 (mV) F.S. D. 1 (mV) 0 1 -29 -27 1 11 188 294 2 22 282 580 3 31 428 890 4 40 538 1100 5 50 617 1270 6 8 61 84 681 763 1390 1560 10 off scale 817 1665 0 1 -28 -25 (Setting Sample) 10 off scale 819 1670 INFRAIVIAX CALIBRATION 0-20 g/l 10 Fig la. 15 20 g/l % deflection (O O" 2000 m V 1500 1000 500 FSD 1 O - T 0 - j -10 « FSD 0.5 PARTECH CALIBRATION 0-20 g/l 15 g/l 20 Fig 2a. 2000 -1 1500 1000 FSD 1 FSD 0.5 PARTECH CALIBRATION 0-10 g/l g/l 1 10 Fig 2b. 

Investigation into the possible application of the Inframax suspended solids meter for measuring suspended sediment in the field

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