ISO 3734 Petroleum products - Determination of water and sediment in residual fuel oils - Centrifuge method
4 Reagents and materials
4.1 Toluene, conforming with the requirements for grade 1 toluene as specified in ISO 5272.
The solvent shall be water-saturated at the test temperature, 50°C +/- 1°C or 60°C +/- 1°C, as appropriate, but shall be free of suspended water (see annex B for the solvent/water saturation procedure).
4.2 Demulsifier, to promote the separation of water from the test portion and to prevent its clinging to the walls of the centrifuge tube. The type of demulsifier, and the concentration and quantity used, shall be agreed between the parties. The recommended stock solution is 25 % (V/V) demulsifier to 75 % (V/V) toluene. For some fuel oils a different ratio of demulsifier may be required.
Demulsiliers used in the concentration and quantity recommended will not add to the water and sediment volume determined. Store the solution in a tightly-stoppered brown glass bottle.
NOTE - For the purposes of this International Standard, the expression % (V/v) is used to represent volume fractions.
5 Apparatus
5.1 Centrifuge
The centrifuge shall be capable of spinning two or more filled, cone-shaped 200 mm centrifuge tubes (5.2) at a speed that can be controlled to give a relative centrifugal force (rcf) of between 500 and 850 at the tip of the tubes.
The revolving head, trurmion rings and tnmnion cups, including the cushions, shall be soundly constructed to withstand the maximum centrifugal force capable of being delivered by the power source. The trunnion cups and cushions shall firmly support the tubes when the centrifuge is in motion. The centrifuge shall be enclosed by a metal shield or case strong enough to eliminate danger if any breakage occurs. The centrifuge shall be controlled thermostatically to avoid unsafe conditions. It shall be capable of maintaining the sample temperature during the entire run at 50°C +/- 3°C.
The speed of the rotating head in revolutions per minute (r/min) is calculated as follows:
r/min = 1335√rcf/d
where
rcf is the relative centrifugal force;
d is the diameter of swing measured between the tips of opposite tubes when in the rotating position, in millimetres.
5.2 Centrifuge tubes
Each centrifuge tube shall be a cone-shaped tube, nominally 200 mm in height, with a maximum height of 203 mm, conforming to the dimensions given in figure 1 and made of thoroughly annealed glass with a straight sides taper. The graduations, numbered as shown in figure 1, shall be clear and distinct, and the mouth shall be constricted in shape for closure with a cork. Scale-error tolerances and the smallest graduations between various calibration marks are given in table 1 and apply to calibrations made with air-free water at 20°C when reading the bottom of the shaded meniscus. The accuracy of the graduations on the centrifuge tube shall be volumetrically verified, in accordance with ISO 4787, before use of the tube.
The verification shall include calibration at each mark up through the 0.5 ml mark and at the 1.0 ml, 1.5 ml, 2.0 ml, 50.0 ml and 100 ml marks. The tube shall not be used if the scale error at any mark exceeds the applicable tolerance from table 1.
5.3 Heating bath
The bath shall be either a solid metal block bath or a liquid bath of sufficient depth for immersing the centrifuge tube in the vertical position to the 100 ml mark. Means shall be provided for maintaining the temperature at 50°C +/- 1°C or 60°C +/- 1°C.
5.4 Thermometer, capable of measuring the temperature of the sample to the nearest 1°C.
6 Sampling (see annex A)
6.1 General
Sampling is defined as all steps required to obtain a representative sample of the contents of any pipe, tank or other system and to place the sample in the laboratory test container.
6.2 Laboratory sample
Only representative samples obtained as specified in ISO 3170 and ISO 3171 shall be used in this International Standard. Before taking a test portion from the sample received by the laboratory, homogenize the sample using the procedure described in annex A.
