Thursday, 16 June 2011

Can't you just use metric?

I mean, what kind of unit is this?

Watts are SI; feet and farenheit are imperial, also known as US customary units in the country which is ironically about the last bastion of these units based on the size of royal body parts and the temperature of sheep's blood, over three hundred years after the idea of a unified, decimal system was proposed. I've heard the metric has yet to be adopted in Burma or Myanmar (depending on what you want to call the country), and Liberia is the only other country that has not officially adopted the system.

There is another website here that has different pipe diameters in inches, surrounded by recommended insulation in millimetres:

I suppose motorists in Britain can only relate their fuel efficiency in miles per litre, as they drive along the roads in the former, but put the latter in their tanks. It almost seems as if someone is trying to stop them from thinking about fuel efficiency.

Anyway, there are several different units for measuring energy.

The joule gets its name from the 19th century Manchester brewer who started to use very accurate thermometers in his vats of mash, and discovered that the work of the paddles stirring his proto-beer resulted in a rise in temperature. This brought on the first law of thermodynamics. Although Joule's beer was served in good old pints, one joule is the energy required to work against a force of one Newton by one metre. One newton is roughly the gravitational force on a 100 gramme object, such as an apple. In electrical terms, a joule is the work done to get one amp of current through one ohm of resistance. 

Joules are pretty tiny. Apparently the human body gives off 60 joules in heat every second. The kilowatt hour is much more tangible. Watts measure power (Watt was the name of the person who invented the steam engine, as I like to ask people). James Watt invented the idea of horsepower, but the eponymous SI unit was named after him. It is customary among units to spell them out in lower case, to avoid confusion between the power of Mr. Watt and a watt of power, but capitalise them when initialed, so in Nm or W/mK, the initials of newtons, watts and kelvins are capitalised in honour of Newton, Watt and Kelvin, but the m is small as the metre is not named after anyone.

One watt is equivalent to one joule per second. One watt hour is therefore 3,600 joules, and a kilowatt hour is 3.6 million joules. You'll find the kilowatt hour on your electricity bill. Switch on a 100W bulb for ten hours, and you'll use one. Ovens run at around a kilowatt, so leaving an oven on for an hour will use one kWh. One litre of kerosene contains about 10 kWh. This may be a practical conversion in Japan where kerosene is the heating fuel of choice for those who don't have any choice.

The calorie is another unit of energy. It is not an SI measurement, but it is metric: based on the amount of energy needed to raise one gramme of water by one degree centigrade. According to wikipedia, its use in most fields is archaic, although in my experience, heating engineers in Japan still seem to like it. It is still used for representing energy in food, where a food calorie is actually a kilocalorie, the amount of energy needed to raise one kilogramme of water by one degree centigrade. Water can carry a lot of heat for its weight, with a specific heat capacity of 4.2 kJ/kgoC, which makes it very useful for heating and cooling. That's why it flows around radiators in cars and houses. One calorie is around 4.2 joules, although the amount of energy required to raise water temperature varies with temperature, so a literal calorie is not an exact unit,and some averaging and standardising has gone on, and in fact it is defined in joules, which go back to the precision of metres, kilogrammes and seconds. 

The British thermal unit is the amount of energy required to raise one pound of water by one degree farenheit. This is the same idea as the calorie, although in imperial units. I don't wish to waste any of my energy discussing this unit, or giving a translation to the other units. Nor can I be bothered to talk about tonnes of TNT, or barrels of oil.

I will add that the sun hits the earth at roughly one watt per square metre, and photovoltaic solar panels produce about 190 watts of electricity per square metre.

To add some hot water to the hot air, if you have 860 litres of water it'll take one kWh to heat it by one degree, or looking at it the other way around, it will give out a kWh of heat if it drops by one degree.

My favourite unit must be the beard-second. In contrast to the light-year--the distance light travels in a year, used for very large distances--the beard-second indicates how much a beard grows in one second, and corresponds to roughly 5 nanometres, or 5 millionths of a millimetre.