Readers letters
A motley variety of energy units are commonly used: kilowatt hours (kWh), kilojoules (kJ) and big calories (kcal). Joules should be preferred, named after the great Lancastrian brewer, James Prescott Joule, from the City of Salford. The joule is an SI unit. Incidentally, in Lancashire a gill is a half pint, not a quarter.
I may be wrong but believe that therms (100,000 Btu) are still in use.
Gas suppliers started to use kWh rather than MJ to indicate the illusory price advantage of gas over electricity by virtue of the ‘Second Law of Thermodynamics’.
As engineers know: 1 kWh = 3.6 MJ = 3600 kJ and a steam table calorie = 4.1868 J.
The joule and the watt are SI derived units; the watt hour and calorie are not. Energy suppliers’ bills should use megajoules. A thermochemical calorie = 4.184 J.
From a packet of brazil nuts, energy is quoted as 683 kcal or 2816 kJ per 100g. Perhaps it should be quoted as 0.782 or 0.794 kWh! I understand that ‘The European Food Information Regulation’ states that the energy content must be calculated using the conversion factors stated in ‘Annex XIV of the Official Journal of the European Union’. These are: for carbohydrate and protein: 17 kJ/g = 4 kcal/g, making 1kcal = 4.25 kJ; for fat: 37 kJ/g = 9 kcal/g, making 1kcal = 4.111 kJ. Why the difference? This seems absurd; to avoid confusion use joules only, an SI unit.
The first half of Drax Power Station was built using imperial units, the second half using metric units.
Blood pressure is measured in millimetres of mercury, not pascals nor bars.
A nautical mile (1 minute of arc) is defined as 1.852 kilometres. This can be derived from 40,000/(360 x 60) rounded to three decimal places. Are slugs still in use in aeronautics?
The oil barrel contains 35 imperial gallons or 42 US gallons.
Regarding engine output for road vehicles; imperial horsepower (hp) is not allowed to be quoted, it has to be metric horsepower (ps); however, torque in pounds feet (lbf.ft) is permitted. These are incompatible units. kW & Nm or ps & kgf.m or hp & lbf.ft are compatible pairings. It is probably best to use kilowatts and newton metres (SI Units) as used in IMechE’s ‘Automotive Engineer’.
‘Horses for courses’ seems to be appropriate.
Brian Cowell
Next letters: Windmills are a waste of energy