3. VISCOMETRY
Introduction
Some of the background for this experiment is found in your fluids text in §1.6, pp. 16-20. You will be looking at the viscosity of a glycerol-water mixture as a function of temperature, and, as it says on p. 19 "Viscosity is very sensitive to temperature". The text gives a model equation for viscosity as a function of temperature: Andrade's equation (1.11 in the text)
where m denotes the viscosity, T the temperature and D and B denote empirical coefficients that can be determined from experiment you are going to do.
The viscosity of glycerol-water mixtures is also very sensitive to the water content. The figure shows an example of this variation.
This link will lead you to a pdf file of viscosity of glycerol and water solutions as a function of composition and temperature that I retrieved from the DowCorning web site. The results of the first two groups' work are shown graphically (pdf) here.
The book discusses capillary viscometers. These are easy to use, and we have several, but they take a long time to make a measurement. We have used them in the past for this experiment, but most student groups found it very difficult to complete an appropriate set of measurements in two hours. You will be using a cup and cone viscometer, which is a (nominally) constant shear device. This device is fairly delicate, and we will provide some assistance to avoid damaging the device. I give instructions in the next section.
In the lab
Measure the viscosity of your sample at five to seven different temperatures, starting at 5°C above the room temperature and going up in increments of 5°C. Most of the remainder of this section deals with the use of the Brookfield cup and cone viscometer. Experimental details and tips follow.
The following instructions for the use of the Brookfield viscometer are adapted from the instruction manual. Follow these with care. Do not adjust the zero offset. If there is an apparent problem, see one of the staff. Do not attempt to fix it yourself. (There were some problems during the first experimental session.) A brief description of what to do follows this excerpt. Look at it first.
1. After the Viscometer is calibrated [done by the staff], it should be zeroed before running sample tests. The following procedure can be performed with the spindle installed and the sample cup on or off. [It will work better with the cup on.]
[1a. Connect the hoses from the water bath to the fittings on the Viscometer.]
[1b. Choose the lowest temperature to be used during the experiment and start the flow of water. Make sure the hoses are not kinked and that the water is flowing freely.]
2. Turn power switch "on".
3. Check bubble level to be sure Viscometer is level. Turn motor switch "on" and set speed selector knob to 12 rpm.
4. Allow Viscometer to run until display reading stabilizes (or fluctuates by no more than 0.1). [This may take one or two minutes.] Turn zero adjustment knob until the display reads 00.0.
5. Turn motor switch "off", placing Viscometer in standby mode.
Operation
1. {Turn off the water flow.] Remove the sample cup.
Place sample fluid in cup according to the table below, being sure that
the sample is bubble-free and spread evenly over the surface of the cup.
Sample volume must be sufficient to wet the entire face of the spindle
and approximately 1.0 mm up the spindle's outside edge. [For the CP-42
used in this experiment, add 1 ml of liquid.]
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Volume (ml) |
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3. [Restart the water flow and] allow sufficient time for the sample fluid to reach the desire temperature.
4. To make a viscosity measurement, turn on the power switch, which energizes the Viscometer drive motor. Allow time for the dial/display reading to stabilize. The time required for stabilization will depend on the speed at which the Viscometer is running and the characteristics of the sample fluid.
The Digital Viscometer displays 00.0-99.9. When making a viscosity measurement, the reading should be noted and multiplied by a factor that is obtained by consulting the range table for the Viscometer model and spindle in use. The factor is calculated by dividing the viscosity range [maximum viscosity] for the speed in use by 100. For maximum accuracy, readings below 10 should be avoided. [I give range tables for the CP-42 and CP-51 cones below.]
5. Turn off the Viscometer motor switch when changing or cleaning a spindle, changing samples, etc.
It is recommended, when operating the Viscometer for a lengthy period, that zero be checked occasionally as described previously.
Accuracy [according to Brookfield]
All models of the Wells-Brookfield Cone/Plate Viscometer are guaranteed to be accurate to within 1% of whatever full-scale range is employed when used in the specified manner. Readings should be reproducible to within 0.2% of full scale subject to variations in fluid temperature, etc.
Fault Diagnosis [consult staff if any
of this happens]
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Maximum Measurable Viscosity (cp = 103 x Pa-sec) for the Brookfield Viscometer (from Appendix)
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1 ml SAMPLE |
0.5 ml SAMPLE |
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Experimental details and tips
Fit your data to a curve of the Andrade form. How good is the fit? Does it make a difference if you use absolute or Celsius degrees? Which is better? Can you provide a rationale to explain why one is better than the other?
Be sure to provide an estimate of your experimental error. This is not the same as errors associated with the fit to the Andrade formula! Which errors are bigger? What does this tell you about the validity of the Andrade law?