|MadSci Network: Physics|
Hi Katy, Viscosity is a property that is commonly measured in chemistry labs, so this might be considered a chemistry project. All fluids resist changes in shape or form. The measurement of the resistance to change in shape is known as viscosity. The more resistant a liquid is to change, the higher is its viscosity. The unit of measure is the poise. The poise has units of dyne-seconds per cm2. A dyne is the amount of force needed to accelerate one gram of mass at the rate of one centimeter per second. The poise defines the distance that a unit volume of liquid will move when a known amount of force is applied to it. So how do you get from the definition to a measurement of the viscosity of ketchup using household equipment? A viscometer is a device used to measure viscosity. One type of viscometer uses the force of gravity (9.8 meters/second/second) to move a liquid through an orifice. The viscosity is found by measuring the time required for a known volume to pass through the opening. A simple viscometer can be constructed from a disposable clear plastic cup. Put a hole in the bottom of the cup using a drill or a punch. A ¼ inch diameter hole would be a good starting point for measuring ketchup. With a marking pen, mark a line on the outside of the cup near the bottom. Mark a second line somewhere below the top of the cup. Your viscometer needs to be calibrated. Some readily available liquids (try a pharmacy) of known viscosity are: Glycerin – 1,490 centipoise, caster oil – 986 centipoise, and olive oil – 84 centipoise. Cover the hole in the cup with tape to seal it. Use a long piece of tape so that it can be easily removed. Fill the cup above the top line with the liquid to be measured. Remove the tape and let the liquid drain into another container (you might want to do this over a sink in case it spills). Using a stopwatch or the second hand on a clock, measure the time it takes for the liquid level to change, starting at the top mark and ending at the bottom mark. Clean and dry the viscometer. Repeat the test with a second liquid with a known viscosity. Clean and dry the container and repeat the test with the ketchup. Plot the viscosity vs time for the two or three known liquids on graph paper or using a spreadsheet. Using the graph, you can estimate the viscosity of the ketchup by plotting the time for the ketchup to drain. You could use the two or three standards to calculate a slope and intercept of a calibration curve for your viscometer. The viscosity of any unknown liquid can then be estimated from the calibration curve or the graph. Chemistry and physics can be much easier to understand through experiments. For most of us it’s more meaningful when you can see the results of our experiment. There are very few people that can accomplish experiments in their heads. And you don’t need fancy or expensive equipment to do interesting experiments. Try measuring the viscosity of the ketchup at different temperatures and see what happens. Have Fun, Bob Novak Specialist, Process R&D Carpenter Technology
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