The objective of this Bring Your Own Experiment session is to demonstrate use of strain gages to measure circumferential strain on a soda can and to infer the internal pressure of the soda can using the stress-strain relationships for thin-walled pressure vessels. Students record transient change of the strain as they open a soda can. As a technical setup, a soda can, a strain gage module, and an Arduino Uno board are used. This experiment is conducted as a part of a junior-level engineering experimentation course in a technological university. Typical student numbers in this course are 30.
In the classroom activity, the students are given the following excerpt:
“You are a product-line engineer for a leading soda company. Recent failures in the product line required you to measure the pressure inside an arbitrary unopened soda can (pressurized). Your team identified a suitable sensor for this purpose: a strain gage mounted on the soda can. Your role will be to set up the experiment so that you can show the change in the soda can’s pressure while it is being opened.”
In this hands-on lab exercise, students work in teams of two students and prepare their own experimental setups. Students start the experiment by attaching a unilateral strain gage on a soda can (ideally circumferentially or longitudinally). Once the strain gage is fixed on the soda can in an orientation that enables directional strain measurements, students adjust a potentiometer in a Wheatstone bridge to create zero balance. The whole process of establishing a zero-balance bridge is a good learning exercise. Once this balance is achieved, students open the soda can and record the resulting voltage deflection. Next, students calculate the strain using the strain gage constitutive equation with the gage factor of the strain gage, the diameter of the soda can and its wall thickness. The calculated strain is converted to the internal pressure of the soda can, using equations for thin-walled pressure vessels. The students typically find a pressure in the range of 30-60 psig depending upon the brand and type of soda selected.
The deliverables of this experiment are a lab report that contains the following,
• Description of the experimental setup,
• Theoretical background of strain measurements,
• A graph that shows the change in the electric potential as the soda can is opened.
• Calculated strain and pressure from the recorded electric potential as a function of time.
• Final soda can gage pressure,
• Uncertainty analysis based upon the multiple measurements and conclusions.
This experiment is one of most engaging experiments students conduct within the class. Students attach strain gages on soda cans, connect strain gages to a Wheatstone bridge, and conduct a transient experiment to infer pressure. While they are exposed to the theory of strain gages in class, in this lab exercise they have the opportunity to use them. Students initially go through a learning curve setting up their experiments, after which most deliver a report with accurate data. A common mistake in the reports is due to unit conversions when students calculate the interior pressure using the strain data.
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