ME 335
Lab 1: Viscosity
Dan Harter
Wednesday, 10am Section 10
January 29, 2014
I. Abstract
Viscosity is a measure of a fluid’s resistance to flow. In this lab, our team measured the viscosity
of the oil 5W 20 using both a rotating-cylinder viscometer and a capillary tube viscometer. The purpose
of this study is to familiarize ourselves with the two apparatus and to gain insight into the methods of
determining viscosity. We will see how the two methods differ and what effect this has on the precision
and accuracy of the two devices. This lab also serves to facilitate a short study in error and variance in
data collection.
II. Results
a. Rotating-cylinder viscometer
ω=28.006 rad/s
μ=.002276 lbf*s/ft2
b. Capillary Tube Viscometer
ρoil=1.366 slug/ft3
ν=.0009546 ft2/s
μ=.001304 lbf*s/ft2
Error: 74.54%
(Calculations attached)
III. Analysis
By using two different methods, we are able to approximate the viscosity of an oil, 5W 20. The
first method uses a rotation-cylinder viscometer. The rotation is powered by the downward movement
of a weight. By equating the rate of energy of the weight to the energy dissipation of the rotating
cylinder, the torque on the cylinder can be found. From the torque, rotational speed, and size
parameters of the viscometer, the viscosity of the oil can be calculated.
The second method employs a capillary tube viscometer. By measuring the time the fluid
requires to travel through the viscometer, and understanding the calibration parameters of the
viscometer, the kinematic viscosity can be obtained. Next, the density of the oil must found in order to
convert the kinematic viscosity to dynamic viscosity. By finding the Specific Gravity of the oil, density is
quickly derived and dynamic viscosity shortly after.
There is some discrepancy between the two values of viscosity found. There are several
possible causes that could attribute to the difference between the values. Firstly, the properties of the
fluid and capillary tube viscometer did not appear to be compatible. From the lab manual, the
viscometer flow is laminar if the flow time is greater than 200 seconds. The oil under observation
consistently took 186 seconds to flow through the viscometer, so laminar flow cannot be assumed.
Next, the rotation-cylinder viscometer is known to be less precise than the capillary tube. There are a
number of factors playing into the rotation-cylinder’s imprecision. Firstly, there are significant friction
forces in the pulley and gearbox mechanisms. Under perfect conditions, this could be ignored. However,
under real-world circumstances, there is some power loss to the gearbox. Next, because the cylinder
rotates, there are probably some phantom turbulent forces in the fluid. Because our calculations are for
laminar flow, they will not be perfectly accurate. The next source of error comes from our timing
method. Using a stopwatch and marking off revolutions is a reasonably imprecise way of measuring
time. Adding the human factor almost necessarily means that our values will be off in at least a small
way. Other sources of error could include temperature fluctuations or differences between the oils
used in each viscometer. They are supposed to be the same, but if they come from different bottles,
there may be slight differences in their properties. All of these things contribute to the discrepancy
between our found values.
IV. Conclusion
In this lab, we found the viscosity of a liquid using two different methods. The two methods had
different characteristics that led to discrepancies between their findings. In general, it appears that a
method which employs fewer moving parts will be more reliable. Furthermore, in a lab setting in which
multiple fluids must be examined, it would be important to use the same method to examine each fluid.
Even if your method of determining viscosity is inaccurate, retaining precision by using the same
techniques will at least give you a qualitative comparison between multiple fluids.