Tonne

The tonne (unit symbol t) or metric ton (U.S.), often written tautologically as metric tonne, is a unit of mass equal to (10³ kilograms) which is almost exactly the mass of one cubic metre of water at four degrees Celsius. It is sometimes abbreviated to mt in the United States, although this conflicts with other SI symbols. The tonne is not a unit in the International System of Units (SI), but is accepted for use with the SI. In SI units and prefixes, the tonne is a megagram (Mg). The spelling tonne pre-dates the introduction of the SI in 1960; it has been used with this meaning in France since 1842 (when there were no metric prefixes for multiples of 10⁶ and above), and is now used as the standard spelling for the metric mass measurement in most English-speaking countries. In the United States, the unit was originally referred to using the French words millier or tonneau, but these terms are now obsolete. A full tun, standing about a metre high, could easily weigh a tonne. The old English wine cask volume measurement known as a tun is close to a metric tonne in weight as it defines about 954 litres which for many commonly used liquids (aqueous solutions) approximates to as many kilograms.

Conversions

One tonne is equivalent to:

One megagram (exactly);

*megagram is the official SI term, but generally not used in industry or shipping, nor colloquially

pounds (exactly by definition), giving approximately

*2205 lb (to four significant digits)

98.42% of a long ton

*One long ton (2,240 lb) is 101.605% of a tonne

110.23% of a short ton

*One short ton (2,000 lb) is 90.72% of a tonne

Explanation

The unit symbol for the tonne is t. T and mT and mt (especially in the combination mmt for million metric tons compare to Mt for megatonne) are also occasionally used, but all of these are deprecated since they conflict with internationally agreed SI symbols. (T is the SI symbol for the tesla and m is SI prefix 'milli', meaning 0.001.) Te is also sometimes used, particularly in the offshore and nuclear industries.

In France and the English-speaking countries that are predominantly metric, the spelling tonne is widespread. This is generally true in Britain; however, the ton used prior to metrication was the long ton of and this is so close to the tonne that some people draw little distinction and continue to use the old spelling. For example, even the Guinness Book of World Records accepts metrication without marking this by changing the spelling. For the United States, metric ton is the name for this unit used and recommended by NIST. In the U.S. an unqualified mention of a ton almost invariably refers to a short ton of .

Like the gram and the kilogram, the tonne gave rise to a (now obsolete) force unit of the same name, the tonne-force, equivalent to about 9.8 kilonewtons: a unit also often called simply "tonne" or "metric ton" without identifying it as a unit of force. Note that it is only the tonne as a unit of mass (an exact decimal multiple of the SI unit of mass, the kilogram) which is accepted for use with SI: the tonne-force or metric ton-force is not acceptable for use with SI, partly because it is not an exact multiple of the SI unit of force, the newton.

==Use of mass as proxy for energy== The tonne of trinitrotoluene (TNT) is used as a proxy for energy, usually of explosions (TNT is a common high explosive). Prefixes are used: kiloton(ne), megaton(ne), gigaton(ne), especially for expressing nuclear weapon yield, based on a specific combustion energy of TNT of about 4.2 MJ/kg (or one thermochemical calorie per milligram). Hence, 1 kt TNT = 4.2 TJ, 1 Mt TNT = 4.2 PJ.

The SI unit of energy is the joule. Assuming that a TNT explosion releases 1,000 small (thermochemical) calories per gram (4.2 kJ/g), one tonne of TNT is equivalent to 4.2 gigajoules.

Alternate usage

A metric ton unit (MTU) can mean within metal (e.g. tungsten, manganese) trading, particularly within the USA. It traditionally referred to a metric ton of ore containing 1% (i.e. 10 kg) of metal.

In the case of uranium, the acronym MTU is sometimes considered to be metric ton of uranium, meaning 1,000 kg.

See also

Ton

* Short ton

* Long ton

* Tonnage

* Ton (volume)

Metre-tonne-second system of units

Tonne of oil equivalent

Orders of magnitude (mass)

References

NIST Special Publication 811, Guide for the Use of the International System of Units (SI)

Category:Non-SI metric units Category:Units of mass