Determine absolute viscosity of diesel fuel using u- tube viscometer

Determine absolute viscosity of diesel fuel using u- tube viscometer

Kinematic viscosity is absolute units. The determination is made in a U- Tube viscometer as illustrated in figure.

Five tubes, numbered from 0 to 4, are specified and their dimensions are given. The ranges covered are: No. 0-0.5 to 2.0, No. 1-1.5 to 6.0, No. 2-5.4 to 43, No. 3-32 to 260 and No. 4-190 to 1500 centistokes respectively.

The dimensions of the tubes specified by the institute of Petroleum are similar to but slightly modified from those given in British Standard Specification No. 188 in 1939.

The viscometer must be carefully cleaned before use and when ever the presence of dirt is suspected. This may be indicated by discordant duplicate reading. Any liquid remaining in the viscometer is first removed by means of a suitable volatile solvent, after which the tube is filled with a cleaning mixture of equal volumes of concentrated sulphuric acid and allowed to stand overnight. The instrument is then washed and thoroughly dried with precaution to prevent the entrance of dust.

When in use, the viscometer must be immersed in a bath of liquid to such a depth that the highest level of the liquid in the viscometer is at least 1cm below the level of the liquid in the bath. The liquid in the bath must be well stirred and its temperature, regulated for a sufficient period before and throughout the test, so that the extremes of temperature do not differ by an amount large enough to cause a change of 1 percent in the time of flow. For, viscometer No. 0, this preliminary period need not be much more than 10 Minutes.

While for No. 4 it must be increased to about 30 Minutes. To obtain the same percentage accuracy, regulation of temperature must be more accurate for the viscous liquids. Open skate thermometers must be known with the requisite degree of accuracy.

Redwood oil cup thermometers are suitable. The oil must be free from water and suspended matter. U-Tube viscometers, must be filled so that there are no bubbles of air. It should at the specified temperature for at least 10 Minutes (30 Minutes for the more viscous liquids).

The capillary of the instrument must not deviate from the vertical in any one place by more than 1°.

CALIBRATION:-

This consist in the determination of the instrument constant K from the relation K= Vs/_'. Where, _s is the time of flow in seconds. Observe during the calibration with a standard liquid of knower kinematic viscosity () in centistokes. The value of K is therefore expressed in centistokes per second. Suitable liquids are as below:

→ For Viscometer No. 0 and 1 Distilled water.

→ For Viscometer No. 2-40 percent solution of Su.

→ For Viscometer No. 3-60 percent solution of Su.

→ For Viscometer No. 4- No primary standard.

To Calibrate a No. 4 viscometer, a viscous liquid such as castor oil must be used its viscosity must first be determined by means of a No. 3 viscometer whose constant already known and the time of flow in the No. 4 instrument must then be determined. If the respective times of flow are 3 and 4 seconds since V = K3 × 3 = K4× 4 Where, V is the viscosity of the liquid.

This process of “Stepping up” may be employed for any pair of viscometers provided the time of flow of the liquid in either is not less than 100 seconds. Viscosities are normally reported as kinematic viscosities and expressed as centistokes. V=Kt. To obtain absolute viscosity*+,, the kinematic viscosity must be multiplied by the density of the liquid, Kg per ml at the temperature of the best then - . _p, where + is in centipoises, if ( is in centistokes. → 100 centistokes = 1 stokes → 1 cenGstokes = 10 ² stoke, 1 centistokes = 1/100 stokes)