TEST METHOD: VISCOSITY
Revised 26 February 2003 (formating)
Salt crystals formed from high viscosity brine are fragile with a low density and retain a large amount of the mother liquor within the crystal. This method is used to determine the viscosity of brines, from which an assessment can be made about the biological performance of a saltfield or saline lake, and the impact of nutrients.
Assess the safety aspects of each step. Use safety glasses, and a pipetting bulb
|SPECIFIC JOB STEPS||HAZARDS WITHIN THIS STEP||HAZARD CONTROL|
|Viscosity of brines can be determined by measuring the time a fixed volume takes to flow between two known points under controlled conditions.||Particularly sensitive to temperature.||Monitor temperature closely.|
|The following method should be read in conjunction with the figure attached below.|
|Determine specific gravity of sample using the method described in Specific Gravity|
|Place the 5 litre beaker filled with water to the top so as to cover the viscometer to above level 'E' on the magnetic stirrer and stir the solution slowly.|
|Maintain the temperature at 20.0 ± 0.2°C.||The temperature varies.||Adjust using ice, iced water or hot water.|
|Fill the syringe with sample and filter the brine as the viscometer is filled via tube L. The viscometer is filled to or just above level 'G'.||The viscometer is wet from previous brine.
|The tube needs to be rinsed with filtered brine twice.
|Place the viscometer in the beaker filled with water (water bath) and suck the brine through the capillary tube several times (ie from 'G' to 'E') so as to quickly adjust the temperature of the brine to 20°C.
The resting level of the brine at 20°C must be at level 'G' exactly.
|The volume of the brine will change with the temperature so it will need adjusting after the brine has reached 20°C.||The quickest way to achieve this is to suck (with the pipetting bulb) the brine to about point 'F' and then remove a small amount of brine with a pasteur pipette, allow the brine to reach its resting level, then add the removed brine drop by drop.|
|The viscometer tube must remain in a vertical position for the rest of the procedure.|
|The brine is sucked up the capillary tube past point 'E' and allowed to fall with gravity. As the brine level passes point 'E' the stop watch is started and stopped at point 'F'.|
|The time recorded in seconds is multiplied by a calibrated constant referenced from the calibration sheets held in the laboratory. The value gives the viscosity in centistokes. This is then divided by the theoretical viscosity for a brine at the same specific gravity to give the relative viscosity.||Loss of information||Record data in the laboratory daybook|
|(Note: The ISO standard 3105 and BS 188:1977 state that significant error can occur if the time for the brine to flow through the tube is less than 200 seconds. The brines normally take between 40 and 70 seconds using the above method. The wider bore viscometer was chosen to reduce the chance of small crystals affecting the reading. Using the formula on page 12 BS 188:1977, the factor 'B' was estimated as being about 2 units. Substituting into the formula v = Ct - B/t (BS 188:1977) given t = 40 sec and C = .03 means that v or viscosity = 1.2 - .05 or 1.15. That is the error due to using the faster viscometer tube is about four percent. However the final viscosity is expressed as a relative figure, that is the viscosity of the sample brine divided by the viscosity of normal brine at the same SG. This means that the magnitude of the error is much less since both the sample and reference brine are out by the same amount. In summary, the faster viscometer gives inaccurate results but these error are tolerable within the described method. If the readings are to be used as absolute values then they would be in excess by about four percent.)|
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