how do I do my experiment for measuring the viscosity of oil

[pepifeo]

I have been attempting to figure out a way to measure the viscosity of oil for the past week, but to no avail. I've learned that one can use a viscosity meter to measure one's findings but the cost is too high (170 dollars). Then I found out that I could fill up a graduated cylinder with oil, drop a steel bearing into the cylinder, and time how long it takes for it to fall. The only question that still confuses me is how long does the graduated cylinder have to be to measure the results accurately. Also, what kind of steel bearing should I use?

[ChrisG]

Hi pepifeo,
Welcome to the ask an expert forums. I am going to copy your question over to the physical sciences section, where it will get more attention from our experts in fluid dynamics and physical experiments.

For the sake of the experts who will try to help, here is the general information and procedure for this experiment:
http://www.sciencebuddies.com/science-f ... p019.shtml

Glycerine would be a good low-toxicity alternative to motor oil for this experiment. There is not a single value of cylinder length or ball bearing dimensions that will work. The column of oil must be long enough that the bearing will reach terminal velocity, and wide enough that there are no "boundary effects" of fluid movement along the solid walls of the cylinder. If you run a web search for "viscosity cylinder ball bearing" or similar search terms, you will find other web sites that give specific recommendations for cylinder and bearing size. One recommendation is a 1000mL cylinder and 1.5 to 3 mm steel ball bearings. For lower viscosity liquids, you will want to use smaller and less dense spheres.

With all that in mind, the only way you will find out for sure what will work for your experiment is to give it a try. Even after the most careful planning, you might have to make changes to your procedure and try again (for example, with a different type of viscous fluid, different spheres, etc). That is a normal occurrence for a good, challenging experiment.

Looking forward to hearing more about your experiment,
Chris

[Craig_Bridge]

how long does the graduated cylinder have to be

The taller the cylinder, the greater the time to traverse it will be which will reduce the effect of small timing errors. The inside diameter of the cylinder and the diameter of your test sphere will also affect your results. My recommendation would be to use a cylinder that is twice the diameter (or more) of your test sphere. Of course you will want to limit both the height and diameter of the cylinder to reduce the amount of test liquid required. If you are using a glass graduated cylinder, be sure to put something like a piece of rubber in the bottom to prevent breaking the cylinder expecially when you test something that isn't very viscous.

what kind of steel bearing should I use?

It really doesn't matter and it doesn't have to be steel. The two things that matter are that it is a sphere and that it is denser (heavier than the test liquid it displaces) than the liquids you are testing.