On 2004 May 17 , at 8:20 PM, john mercer wrote:
... we got are gas bill last week for the month of april.� It was calibrated in kj.�
I hope it was in "kJ, not "kj".
"kJ" is the SI symbol for kilojoules, which is a unit of energy.
Burnable gasses can be measured in many different ways (as can most things). Just as we usually buy eggs by the dozen (or by the egg), we could buy them by the kilogram. Potatoes are bought by the kilogram (mass) but can also be bought by the bushel (volume) and could be bought by the dozen, I suppose. In the US, farmers sell milk to dairies by the pound (mass) but the dairies package and sell it by the quart and gallon etc. (volume). Which way it is done depends as much on historical precedent and perhaps ease of measurement than it does on which method logically tells us what we need to know about the product.
Measuring gas in kilojoules is to measure it in terms of the heat that can be produced by burning it. Measuring it in cubic metres (the alternative that John asked about) is to measure it by the volume* that it occupies. Either way is OK. Why the gas company decided to change it at this particular time is something you would have to ask them. But since the value of a burnable fuel is in how much heat energy it can produce rather than how big a space it occupies, it seems sensible to pay for it by the kilojoule rather than by the cubic metre.
The same could be said of solid fuels like coal and liquid fuels like oil. It would make more sense to buy it and sell it by the kilojoule of heat it can produce rather than how much it weighs (coal by the tonne) or how much volume it occupies (oil by the barrel). If the same thing were done for all fuels, we could more easily understand the relative costs of energy in the various fuels we use.
Regards, Bill Hooper
* The volume of a gas depends on the temperature and the pressure of the gas, as is well known. When volumes are used for measuring gasses, it is necessary to specify what temperature and pressure those are measured at (or calculated for). Typically, the value used for the pressure is standard atmospheric pressure and the value often used for the temperature is zero degrees Celsius.
