VISCOSITY OF FUEL OIL The viscosity of an oil is inversely proportional to its fluidity, and is a measure of the internal friction in the oil itself, that of its resistance to free flowing. Inasmuch as there are a number of different instruments for the purpose of measuring viscosity, and since there is no recognized standard instrument or method of measuring it, the term "viscosity" means nothing unless there are also stated the name of the instrument used, the temperature at which the viscosity was determined, and the amount of oil tested. The viscosity of an oil is generally stated as the time in seconds required for a given quantity of the oil in question to flow through a small orifice at the stated temperature. It can be stated as the ratio of the time of flow of the oil being tested to the time of flow of water or some oil chosen as a standard at a stated temperature. Common types of viscosimeters or instru•• ments for measuring the viscosity of oil are the Engler, Saybolt and l'agliabue. In stating viscosity the name of the instrument used should always be given. Figure 4 shows a Saybolt visco simeter. The tentative test for the viscosity of lubricants adopted by the American Society for Testing Materials" is as follows : ' 1. Viscosity shall be determined by means of the Saybolt Standard Universal Viscosimeter.
2. (a) The Saybolt Standard Universal Viscosimeter is made entirely of metal. The standard oil tube J is fitted at the top with an overflow cup E and the tube is surrounded by a bath L. At the bottom of the standard oil tube is a small outlet tube through which the oil to be tested flows into a receiving flask R, whose capacity to a mark on its neck is 60 (± 0.15) c.c. The lower end of the outlet tube is enclosed by a larger tube, which when stoppered by a cork, N, acts as a closed air chamber and prevents the flow of oil through the outlet tube until the cork is removed and the test started. A looped string is attached to the lower end of the cork as an aid to its rapid removal. The bath is provided with two stirring paddles, K, and operated by two turn table handles F. The temperatures in the standard oil tube and in the bath are shown by thermometers, A and B. The bath may be heated by a gas ring burner P, steam U-tube H, or electric heater, C. The standard oil tube is cleaned by means of a tube cleaning plunger V, and all oil the standard oil tube shall be strained through a 30-mesh brass wire strainer Q. A stop watch is used for taking the time of flow of the oil and a pipette, fitted with a rubber suction bulb, is used for draining the over flow cup of the standard oil tube. • (b) The standard oil tube should be standardized by the United States Bureau of Standards, Washington, and shall con form to the following dimensions : , 3. Viscosity shall be determined at 100° F. (37°.8 C.),
130° F. (54°.4 C.), or 210° F. (98°.9 C.). The bath shall be held constant within 0°.25 F. (0.14° C.) at such a temperature as will maintain the desired temperature in the standard oil tube. For viscosity determinations at 100 and 130° F., oil or water may be used as the bath liquid. For viscosity determinations at 210° F., oil shall be used as the bath liquid. The oil for the bath liquid should be a pale engine oil of at least 350° F. flash point (open cup). .Viscosity determinations shall be made in a room free from draughts, and from rapid changes in temperature. All oil introduced into the standard oil tube, either for cleaning or for test, shall first be passed through the strainer. To make the test, heat the oil to the necessary temperature and clean out the stand ard oil tube with the(plunger, using some of the oil to be tested. Place the cork stopper into the lower end of the air 'chamber at the bottom of the standard oil tube. The stopper should be suffi ciently inserted to prevent the escape of air, but should not touch the small outlet tube of the standard oil tube. Heat the oil to be tested, outside the viscosimeter, to slightly below the temperature at which the viscosity is to be determined and pour it into the standard oil tube until it ceases to overflow into the overflow cup. By means of the oil tube thermometer keep the oil in the standard oil tube well stirred and also stir well the oil in the bath. It is extremely important that the temperature of the oil in the oil bath be maintained constant during the entire time consumed in making the test. When the temperature of the oil in the bath and in the standard oil tube are constant and the oil in the standard oil tube is at the desired temperature, withdraw the oil tube ther mometer; quickly remove the surplus oil from the overflow cup by means of a pipette so that the level of the oil in the overflow cup is below the level of the oil in the tube proper ; place the 60 c.c. flask in position so that the oil from the outlet tube will flow into the flask without making bubbles ; snap the cork from its position, and at the same instant start the stop watch. Stir the liquid on the bath during the run and carefully maintain it at the previously determined proper temperature. Stop the watch when the bottom of the meniscus of the oil reaches the mark on the neck- of the receiving flask. The time in seconds for the 60 c.c. of oil is the Saybolt viscosity of the oil at the temperature at which the test was made.