Wednesday, 23 October 2019

A Perverse Choice of Units

Earlier today a news broadcast included the claim that Britain has reserves of shale gas amounting to about thirteen billion cubic feet. That measurement is in several respects perverse.

The foot is an obsolete measure of length, and volume is an inconvenient measure for a quantity of gas.The volume of a gas depends on temperature and pressure, varying greatly with even quite modest changes in either, so any specification of volume should include a statement of temperature and pressure. The default values are 105 pascals regarded as standard atmospheric pressure, and 0 degrees C, though it is unlikely that natural gas would be stored under those conditions.

Mass would be a more illuminating measure. A foot is about 0.3 metres so a cubic foot is about 27 litres, The molecular weight of methane is about 16 and gram molecular volume at standard temperature and pressure is about 22.4 litres, so a cubic foot of gas would contain about:
30*16/22.4 grams = about 21.4 grams.

For 13 billion cubic feet, multiply by 13 billion, and divide by a million to convert grams to tonnes, giving: 21.4*13000 = roughly 279 000 tonnes, not such a lot!

1 comment :

Frank Little said...

One might think that the unit was chosen by the shale gas industry to make their contribution to energy supply appear more significant than it is, justifying its special treatment by Conservative chancellors going back to George Osborne.