Mechanical Engineering Technology Fluid Mechanics Laboratory

Measurement of Viscosity

Purpose:

1. Understand the effects of temperature on viscosity.
2. Understand the saybolt method of viscosity testing.
3. Understand the causes of viscosity difference.

Objectives:

1. Establish variation in viscosity of a liquid with temperature.
2. Compare the viscosities of two lubricating oils and discuss/explain the results of the viscosity tests. Explain any differences.
3. Present all viscosity test results in tabular form.
4. Plot kinematic viscosity of the oils versus temperature.
5. Discuss all sources of error.

Theory:

There are various methods employed for the measurements of viscosity of liquids. In the Saybolt method (See ASTM Standard D-88), the sample to be tested is placed inside of a carefully machined container. The container itself is placed inside an oil bath where the temperature can be accurately controlled. An orifice nozzle is mounted to the bottom of the container and, at a given instant, the flow of the liquid is started. The measurement of viscosity itself is related to the measurement of time. More particularly, the time required for 60 mL of the sample to pass through a 0.1795” diameter orifice is known as the Saybolt Universal Viscosity (unit) of the liquid.

The experiments will disclose that the viscosity of liquids is sensitive to temperature changes. More particularly, the viscosity of oils decreases drastically at higher temperatures (200 F and higher). Generally, viscosity decreases with increasing temperature of liquids while it increases with increasing temperature of gases.

Both equations below are used to calculate kinematic viscosity using the Saybolt method.

NOTE: “SSU” Standard Saybolt Universal test measured time in seconds.

1. For time greater than 100 seconds =

2. For time less than 100 seconds =

where ν is the kinematic viscosity

Description of Apparatus:

A variety of liquids may be tested in succession by placing containers in the temperature controlled oil bath. The oil bath is electrically heated and an electric motor driven stirrer ensures even temperature distribution. A small glass flask is placed under each container and cork stoppers are placed above them into the outlet of the containers. A thermometer is placed into the container. Care must be taken that a thermometer with the correct immersion should be employed. The controls are situated under the platform on which the glass containers are placed. A thermostat enables the setting of the temperature of the oil bath. Figure 1 shows the apparatus and Figure 2 shows the cross section of a container. Use the electric clock for measuring time.

Experimental Procedure:

1. Fill the container with the liquid to be tested. Siphon off excess with a pipette so that the liquid level is fixed precisely at the overflow rim (See figure 2).
2. Set the thermostat to the desired temperature and allow adequate time to pass for establishing steady conditions. If necessary, use the thermometer as a stirrer in the container.
3. Remove stopper with an abrupt movement and simultaneously start the timing clock. Stop the timer at the instant when the liquid meniscus reaches the 60-mL mark on the catch vessel.
4. Repeat the experiment at temperature intervals designated by the instructor.
5. Then, from the equations provided, determine the kinematic viscosity of the liquid and plot viscosity against temperature in a graph.

NOTE: HEATER RHEOSTAT SHOULD BE OPERATED ON LOW (L) DURING MOST OF THE EXPERIMENT.

Results:

1. Present in tabular form the results of the experiment. (Viscosity versus temperature)

2. Calculate the kinematic viscosity in.
3. Generate a graph of viscosity in (ordinate) versus temperature in F (abscissa), for both oils on the same graph.

4. Record the room temperature.

5. Discuss the curves including a comparison of the viscosities of the two different lubricating oils.

6. Discuss all sources of error.