Sistemas de unidades comunes
Un sistema de unidades, o sistema de medida, es un sistema compuesto por unidades de medida interrelacionadas. A lo largo de la historia han existido varios sistemas de unidades, y su importancia sigue siendo evidente hoy en día, como se ve por su uso generalizado dentro de la sociedad.
En el pasado, los sistemas de unidades se definían localmente y, a menudo, de forma muy arbitraria. Como tal, la longitud de una unidad podría variar significativamente de una región a otra. Por ejemplo, dado que algunas unidades en el pasado a menudo se basaban en partes del cuerpo, la unidad del "pie" podría tener una definición diferente según el tamaño del pie del rey o señor feudal de una región determinada. Dado que los viajes eran más limitados en el pasado, la definición local y el uso de unidades podrían verse como más prácticos. Sin embargo, con el advenimiento de la globalización, particularmente el crecimiento del comercio y la ciencia, la discutible necesidad de un sistema universal de medición se hizo más evidente.
Las unidades de medida estandarizadas facilitan la comunicación entre diferentes culturas y países que, de lo contrario, podrían optar por utilizar sistemas locales, lo que podría generar confusión y falta de comunicación. El Sistema Internacional de Unidades (SI), el sistema de medida más utilizado en la actualidad, se desarrolló en un esfuerzo por proporcionar un sistema estandarizado y más racional que pudiera usarse en todo el mundo. A pesar de los esfuerzos para implementar SI a nivel mundial, todavía hay algunos sistemas de unidades de uso común, incluidas las unidades tradicionales de los Estados Unidos y el sistema imperial de medición (aunque la mayoría de los países que no han adoptado oficialmente SI todavía usan SI hasta cierto punto).
Si bien es importante luchar por la estandarización, ya que es difícil eliminar por completo el uso local de los sistemas de unidades históricas, es igualmente importante aceptar que existen otros sistemas de medida y poder usarlos, o al menos relacionarlos y convertirlos al sistema de unidades preferido.
Los tres sistemas de unidades comunes que están en uso hoy en día son el Sistema Internacional de Unidades, las unidades consuetudinarias de los Estados Unidos y el sistema imperial de unidades.
Sistema Internacional de Unidades
El Sistema Internacional de Unidades (SI) es la forma moderna del sistema métrico y se compone de siete unidades básicas que utilizan veinte prefijos métricos para denotar múltiplos o submúltiplos decimales de la unidad básica. SI pretende ser un sistema de medición coherente y racional. Es un sistema estrictamente concebido y definido en base a constantes invariantes de la naturaleza, incluida la velocidad de la luz, el punto triple del agua y un prototipo físico. Para mantener la confiabilidad y la precisión, SI requiere la definición precisa de las constantes además de la precisión de los estándares de medición. Como tal, es un sistema en evolución que cambia cuando se descubren constantes más estables u otras constantes se pueden medir con mayor precisión. SI es el sistema de medición más utilizado, y la evolución del sistema aún continúa en la actualidad.
The SI base units as well as a table of metric prefixes (courtesy of Wikipedia.org) are listed below:
SI base units:
- ampere (symbol: A) – unit of electric current
- kelvin (symbol: K) – unit of temperature
- second (symbol: s) – unit of time
- meter (symbol: m) – unit of length
- kilogram (symbol: kg) – unit of mass
- candela (symbol: cd) – unit of luminous intensity
- mole (symbol: mol) – unit reflecting amount of a substance
Metric prefixes:
| Metric prefixes in everyday use | |||
| Text | Symbol | Factor | Power |
| exa | E | 1000000000000000000 | 1018 |
| peta | P | 1000000000000000 | 1015 |
| tera | T | 1000000000000 | 1012 |
| giga | G | 1000000000 | 109 |
| mega | M | 1000000 | 106 |
| kilo | k | 1000 | 103 |
| hecto | h | 100 | 102 |
| deca | da | 10 | 101 |
| (none) | (none) | 1 | 100 |
| deci | d | 0.1 | 10-1 |
| centi | c | 0.01 | 10-2 |
| milli | m | 0.001 | 10-3 |
| micro | μ | 0.000001 | 10-6 |
| nano | n | 0.000000001 | 10-9 |
| pico | p | 0.000000000001 | 10-12 |
| femto | f | 0.000000000000001 | 10-15 |
| atto | a | 0.000000000000000001 | 10-18 |
SI-derived units:
In addition, SI also includes 22 units of measurement that are derived from the 7 SI base units. These units are either dimensionless or are expressed as a product of one or more of the SI base units. Some of the more common examples of these include:
- radian (symbol: rad) – unit of angle
- newton (symbol: N) – unit of force or weight
- watt (symbol: W) – unit of power
- volt (symbol: V) – unit of voltage, electrical potential difference, and electromotive force
- degree Celsius (symbol: °C) – unit of temperature
Non-SI units accepted for use with SI:
There are also numerous units that are accepted for use with SI that are not considered SI units or SI derived units. Some of the more common examples include:
- minute, hour, day (symbol: min, h, d respectively) – units of time
- degree – (symbol: °C) – unit of temperature
- liter – (symbol: L) – unit of volume
- bar – (symbol: bar) – unit of pressure
- millimeter of mercury (symbol: mmHg) – unit of pressure
History of SI:
The International System of Units (SI) is the most widely used unit system in the world. It is the modern version of the metric system which was developed over time while borrowing units or ideas from other systems, in large part by a committee of the French Academy of Sciences, beginning in 1791.
The process of metrication, including the eventual definition and adoption of the International System of Units was a slow process. France officially introduced the metric system in 1799, and the system spread throughout Europe during the 19th century. By the 1970s, metrication in the form of SI was complete in almost all countries around the world. Notable exceptions to this include the United Kingdom, the United States, Liberia, and Myanmar. Excluding the United Kingdom, these countries are the only countries that have not officially adopted SI.
The United States:
- 1866 – the metric system was legalized as a system of measurement, though not widely used.
- 1975 – the metric system was officially adopted for government and military use as well as for trade and commerce.
- 1992 – an amendment to the Fair Packaging and Labeling Act required that food labels on federally regulated consumer commodities include both metric and US customary units. This was amended again in 2010 to allow manufacturers to voluntarily use only metric units for labeling.
- 2012 – a petition was created to "Make the Metric system the standard in the United States, instead of the Imperial system." The white house responded by stating that United States customary units were defined within the metric system, and that the choice to use the metric system should be made by individuals, implying that there is no federal intent to mandate the nationwide use of the metric system.
The United Kingdom:
- 1862 – preparations for conversion to the metric system began, and metric units could be legally used in the UK for nearly a century before efforts to fully convert to the metric system actually began.
- 1965 – the government set a 10-year plan for full metrication.
- 1969 – the Metrication Board was created to promote and coordinate country-wide metrication. No plans were made to mandate the use of the metric system, and the Metrication Board was abolished in 1980 after a change in government.
- 1989 – the UK again chose not to make the use of the metric system compulsory by avoiding the European Units of Measurement Directive.
- Following the UKs exit from the European Union, there has been a movement from retailers to shift back to the use of imperial units.
Although there are other countries in which units other than SI are primarily used, the US and the UK are the largest contributors to two of the other unit systems commonly used today. As such, the other countries, many of which use unit systems influenced by either the UK or the US, will not be discussed here.
United States customary units
United States customary units (UCS) are a system of measurements used in the United States (US). UCS originated from English units (not to be confused with imperial units) which were used by the British Empire starting as early as 1495 and continued to be used in the UK up until replacement by the imperial system in 1824. While UCS is primarily used for commercial, social, and personal applications, the US uses the International System of Units (SI) in many other areas including science, medicine, industry, the government, and the military.
History:
- 1824 – creation of the imperial system, marking the split of the unit systems used in the UK and the US. Although both systems are based on English units and have many similarities, this development also resulted in significant differences between the two systems.
- 1893 – the Mendenhall Order officially redefined most US customary units in terms of meters and kilograms.
- 1959 – the international yard and pound agreement of 1959 further refined the definitions of UCS in terms of metric units.
- 1975 – the Metric Conversion Act of 1975 was passed, making the metric system "the preferred system of weights and measures for U.S. trade and commerce."
Units of length:
The four US customary units that are in everyday use are listed below, along with their SI equivalents based on the definition of 1 yard as 0.9144 meters:
- inch (symbol: in) – 0.0254 meters
- foot – 0.3048 meters
- yard – 0.9144 meters
- mile – 1609.344 meters
Units of area:
Square feet, square inches, square yards, etc., are units of area commonly used in UCS, but the only area measurement in the system that is not related to one of its lengths is the acre, which is equal to 4,046.873 m2.
Units of volume:
UCS uses many different measurements of volume. It uses some general units of volume, as well as volume measurements specifically for dry or liquid volumes. Below are some, but not all of these volumes, and their SI equivalents.
General approximate units of volume:
- cubic inch (in3) – 0.0000164 meters3
- cubic foot (ft3) – 0.0283 meters3
- cubic yard (yd3) – 0.765 meters3
Approximate liquid volumes:
These volumes have similar names as their counterparts in the imperial system, but the actual measurements differ slightly. Also, for most of these measurements, the US simply adds the term "dry" before the unit to distinguish a unit from its liquid definition. This distinction does not exist in the imperial system, which does not have separate dry or liquid volumes.
- teaspoon (symbol: tsp) – 4.929 milliliters
- tablespoon (symbol: tbsp) – 14.787 milliliters
- fluid ounce (symbol: fl oz) – 29.574 milliliters
- cup (symbol: cp) – 236.588 milliliters
- pint (symbol: pt) – 473.176 milliliters
- quart (symbol: qt) – 946.353 milliliters
- gallon (symbol: gal) – 3785.41 milliliters
Approximate dry volumes:
- dry pint (symbol: pt) – 550.610 milliliters
- dry quart (symbol: qt) – 1,101.221 milliliters
- dry gallon (symbol: gal) – 4,404.884 milliliters
Units of weight and mass:
The most commonly used system of mass in the United States is avoirdupois weight. Troy weight is sometimes used, but not widely. UCS is based on the avoirdupois pound, which was defined in 1959 as exactly 453.59237 grams. The pound is sometimes used as a force, using the term "pound-force." This should not be confused as a unit of mass. Below are listed some of the USC units of mass, along with their approximate SI equivalents:
- ounce (symbol: oz) – 28.350 grams
- pound (symbol: lb) – 453.592 grams
- ton (symbol: ton) – 907.185 kilograms
- long ton (symbol: long ton) – 1,016.047 kilograms
Units of temperature:
UCS uses the unit degrees Fahrenheit to measure temperatures for everyday purposes. Like most of the rest of the world however, UCS uses degrees Celsius and kelvins within scientific contexts.
Imperial system
The imperial system of measurements, also known as British Imperial, was defined in 1824, replacing the previous English units known as the Winchester Standards, which were in effect from 1588 to 1825. Although the United Kingdom has technically adopted the International System of Units (SI), in practice, the UK is much like the United States in that it uses SI in many areas such as industry, healthcare, government, science, and others, but still commonly uses a system derived from English units (the imperial system) in everyday life. The imperial system is also still in use in Canada as well as some other countries previous under the control of the British Empire.
- 1818 – discussion of adoption of the metric system occurred in Parliament.
- 1824 – creation of the imperial system as part of the Weights and Measures Act of 1824. This act allowed the use of English units as long as imperial equivalents were marked.
- 1960 – certain industries and government agencies underwent, or were in the process of undergoing metrication by this point in time.
- 1965 – the UK officially agreed to a policy to support metrication. Specifically, the policy was intended to support voluntary metrication with some government subsidization.
- 1969 – the Metrication Board was founded to promote and coordinate metrication.
- 1978 – the government began mandating metrication in certain sectors in response to carpet retailers reverting to imperial measurements of area for pricing.
- 1989 : la política del gobierno cambió nuevamente a la preferencia por la métrica voluntaria.
- 1995 : el Reino Unido completa su transición parcial oficial al sistema métrico. Esto no se aplica a la cerveza de barril, las señales de tráfico y los velocímetros, todos los cuales todavía usan unidades imperiales exclusivamente o incluyen medidas métricas junto con las unidades imperiales.
A continuación se muestran algunas de las diversas unidades del sistema imperial y sus equivalentes métricos aproximados. Muchos de estos son similares a las unidades del sistema tradicional estadounidense.
Unidades de longitud:
- pulgada (símbolo: in) – 0,0254 metros
- pie (símbolo: ft) – 0,3048 metros
- yarda (símbolo: yd) – 0,9144 metros
- cadena (símbolo: ch) – 20,1168 metros
- estadio (símbolo: piel) – 201,168 metros
- milla (símbolo: mi) – 1.609,344 metros
- legua (símbolo: lea) – 4.828,032 metros (3 millas)
Unidades de área:
- percha – 25.293 metros cuadrados
- roed – 1011.714 metros cuadrados
- acre – 4046.856 metros cuadrados
Unidades de volumen:
Si bien estas unidades tienen los mismos nombres en las unidades tradicionales de EE. UU., sus valores son diferentes y el sistema imperial no tiene volúmenes secos o líquidos separados.
- onza líquida (símbolo: fl oz) – 28.413 mililitros
- gill (símbolo: gi) – 142.065 mililitros
- pinta (símbolo: pt) – 568,261 mililitros
- cuarto (símbolo: qt) – 1.136,523 mililitros
- galón (símbolo: gal) – 4.546,09 mililitros
Unidades de peso y masa:
Estas unidades son similares a sus contrapartes UCS con la excepción de la tonelada. La tonelada imperial (2240 libras), comúnmente conocida como tonelada larga en los EE. UU., está mucho más cerca de la tonelada métrica (2204,6 libras) y es más grande que la tonelada corta estadounidense (2000 libras).
Si bien la piedra está relacionada con la onza y la libra, y tendría la misma medida en la USC, la piedra no se usa en los Estados Unidos y se usa principalmente como medida del peso corporal en el Reino Unido.
- onza (símbolo: oz) – 28.350 gramos
- libra (símbolo: lb) – 453,592 gramos
- piedra (símbolo: st) – 6.350 kilogramos
- tonelada (símbolo: t) – 1.016,047 kilogramos