Q: A strange phenomenon caused much comment at a barbecue this summer. To keep the cans of beer cool they were placed in a large plastic dustbin that was filled with cold water. Strangely, some cans remained on the bottom of the bin while others floated on the surface of the water. There were more submerged cans than floating cans, but this behaviour was not confined to any particular brand. What could be the reason for this?
A: This owes a little to the laws of hydrostatics and a lot to consumer laws. The volume of beer that is printed on the side of a can is a legal minimum, and so beer cans are designed to accommodate a certain amount of extra liquid.
The exact amount of extra beer contained in the can depends on the brewer鈥檚 quality control procedures during can filling. If the beer is accurately metered during filling, then cans need to be only slightly larger than the nominal volume of beer they contain. Where the filling process is less accurate cans must be even larger, cost more to make and transport, and may be filled with more beer for which the brewer cannot charge.
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When the can is completely filled with more than the quantity of beer that is printed on the side it has a net density that is more than that of water (beer and aluminium being denser than water). When the can is filled with the minimum quantity of drink marked on the side, the rest of the can is filled with air and carbon dioxide, making the overall density less than when it is full.
Assuming that beer and air have densities of 1010 kilograms per cubic metre and 1 kilogram per cubic metre respectively, and that a nominal 440 millilitre aluminium can has a mass of 30 grams, a few rough calculations (based on the cans in my fridge) would suggest that around 8 per cent extra volume of air on top of the standard 440 millilitres of beer would be enough to make it float. This can be measured by the fill-level of the can: in this case it would be about 9 millimetres from the top.
The fact that there were more submerged than floating cans would confirm that brewers do overfill cans to comply with weights and measures regulations.
So, if your thirst needs plenty of quenching, or you just want more beer for your money, you should go for the cans that don鈥檛 float.