fluidmech

# Other units are engler degrees sae numbers and so on

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Unformatted text preview: OTHER UNITS Other units of viscosity have come about because of the way viscosity is measured. For example REDWOOD SECONDS comes from the name of the Redwood viscometer. Other units are Engler Degrees, SAE numbers and so on. Conversion charts and formulae are available to convert them into useable engineering or SI units. 1.2.4 VISCOMETERS The measurement of viscosity is a large and complicated subject. The principles rely on the resistance to flow or the resistance to motion through a fluid. Many of these are covered in British Standards 188. The following is a brief description of some types. © D.J.DUNN 5 U TUBE VISCOMETER The fluid is drawn up into a reservoir and allowed to run through a capillary tube to another reservoir in the other limb of the U tube. The time taken for the level to fall between the marks is converted into cSt by multiplying the time by the viscometer constant. ν = ct The constant c should be accurately obtained by calibrating the viscometer against a master viscometer from a standards laboratory. Fig.1.3 REDWOOD VISCOMETER This works on the principle of allowing the fluid to run through an orifice of very accurate size in an agate block. 50 ml of fluid are allowed to fall from the level indicator into a measuring flask. The time taken is the viscosity in Redwood seconds. There are two sizes giving Redwood No.1 or No.2 seconds. These units are converted into engineering units with tables. Fig.1.4 © D.J.DUNN 6 FALLING SPHERE VISCOMETER This viscometer is covered in BS188 and is based on measuring the time for a small sphere to fall in a viscous fluid from one level to another. The buoyant weight of the sphere is balanced by the fluid resistance and the sphere falls with a constant velocity. The theory is based on Stokes’ Law and is only valid for very slow velocities. The theory is covered later in the section on laminar flow where it is shown that the terminal velocity (u) of the sphere is related to the dynamic viscosity (µ) and the density of the fluid and sphere (ρf and ρs) by the formula Fig.1.5 µ = F gd2(ρs -ρf)/18u F is a correction factor called the Faxen correction factor, which takes into account a reduction in the velocity due to the effect of the fluid being constrained to flow between the wall of the tube and the sphere. ROTATIONAL TYPES There are many types of viscometers, which use the principle that it requires a torque to rotate or oscillate a disc or cylinder in a fluid. The torque is related to the viscosity. Modern instruments consist of a small electric motor, which spins a disc or cylinder in the fluid. The torsion of the connecting shaft is measured and processed into a digital readout of the viscosity in engineering units. You should now find out more details about viscometers by reading BS188, suitable textbooks or literature from oil companies. ASSIGNMENT No. 1 1. Describe the principle of operation of the following types of viscometers. a. Redwood Viscometers. b. British Standard 188 glass U tube...
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