One Good Test Is Worth A Thousand Expert Opinions.

“What happens if I shake this can?”

For more than 30 years we have been using typical carbonated drinks cans for simple experiments with strain gauges. The most common test is to bond a strain gauge around the can (hoop direction, for the highest value of strain). We then record the change of strain when we open the can, and this can enable the calculation of the pressure in the can, and also to discuss design optimisation.

Before opening the can we ask a simple question – “What would happen if we shake the can”. Most commonly the answer is simple, quick, and entirely wrong – “The pressure increases”. This is demonstrably not the case, but we start to examine the claim piece-by-piece:

Clarification – The strain will increase.

Assumption – The change of strain is a result of an increase in pressure.

Reason – The gas inside the can expands.

Viewpoint – The same happens with a bottle. Unscrew the cap and reseal it – the bottle is flexible, then hardens (pressure goes up) when shaken.

Implication – gas can change pressure purely by shaking.

Question – how is this possible in a closed system?

At this point people start to question their own assumptions. Gas pressure is described by Boyle’s Law – for a given volume of gas at constant temperature in a closed system, the Pressure times the Volume is a constant, or PV=k. Shaking the can does not change these constants  and easily demonstrated when the strain reading remains obstinately constant after vigorous shaking. (NOTE: in reality just holding the can may change the temperature slightly, and modern cans are so thin that slight distortions can cause small changes not related to pressure.)

Many more questions can be asked to find out why the liquid gets ejected from the can if shaken, and why the bottle analogy is not representative of the question. There are some interesting discussions online about this issue, but only the videos where this is tested actually prove the answer. The test of pressure change by dropping a Mentos into a closed bottle is extremely interesting in its disappointingly predictable result (it does not change). Other videos showing exploding bottles are most likely the result of pre-existing damage.

We often talk about “human intuition”, unfortunately it is an unreliable way to find answers – that’s why we have ways to measure quantitative values independent of someone’s opinion. VPG Micro-Measurements Advanced Sensors strain gauges are used all over the world for critical measurements in almost every conceivable application.

Thin-Walled Pressure Vessels – Demo. 

How to Convert A Structure Into A Load Cell? 

Thin Walled Pressure Vessel – Part 1.

Thin Walled Pressure Vessel – Part 2 (Data Acquisition).

Stress Analysis of Thin Walled Pressure Vessel. 

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Anton Chittey

United Kingdom