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SI is a coherent system. This means that units are always combined without conversion factors. The derived unit of velocity is the meter per second (m/s); the derived unit of volume is the cubic meter (m3). If you know that pressure is force per unit area, then you know that the SI unit of pressure (the pascal) is the unit of force divided by the unit of area and is therefore equal to 1 newton per square meter.
The metric prefixes can be attached to any unit in order to make a unit of a more convenient size. The symbol for the prefix is attached to the symbol for the unit, e.g., nanometer (nm) = 10−9 m. The units of mass are specified in terms of the gram, e.g., microgram (μg) = 10−9 kg.
Only one prefix is used with a unit; the use of units such as the millimicrometer is no longer acceptable. When a unit is raised to a power, the power applies to the prefix as well, e.g., a cubic millimeter (mm3) = 10−9 m3. When a prefix is used with a ratio unit, it should be in the numerator rather than in the denominator, e.g., kilometers/second (km/s) rather than meters/millisecond (m/ms). Only prefixes denoting powers of 103 are normally used. Hecto-, deka-, deci-, and centi- are usually attached only to the metric system units gram, meter, and liter.
Owing to the force of tradition, one noncoherent unit, the liter, equal to 10−3 m3, or 1 dm3, is generally accepted for use with SI. The internationally accepted abbreviation for liter is the letter l; however, this can be confused with the numeral 1, especially in typescript. For this reason, the capital letter L is also used as a symbol for liter. The lower case letter is generally used with prefixes, e.g., dl, ml, fl. The symbols for all other SI units begin with a capital letter if the unit is named after a person and with a lower case letter otherwise. The name of a unit is never capitalized.
See also: International System of Units.
In·ter·na·tion·al Sys·tem of U·nits(SI) (in'tĕr-nash'ŭn-ăl sis'tĕm yū'nits)
SI unitsSI is abbrev. for Systeme Internationale. SI units are now almost universally used in medicine. They include the metre for length, the kilogram for weight, the mole for amount of substance in a solution, the joule for energy and the pascal for pressure. These units are qualified by decimal multipliers or divisors such as mega- (a million), kilo- (a thousand), deci- (a tenth), centi- (a hundredth), milli- (a thousandth), micro- (a millionth), nano- (a thousand millionth), pico- (a million millionth), and femto- (a thousand million millionth). SI units were initially adopted by US doctors and then abandoned.
SI unitsthose units of measurement forming the Système International, consisting of the metre, kilogramme, second, ampere, kelvin, candela and mole. See Appendix B.
SI units; the metric system
Système International (SI) units
At an international convention in 1960, the General Conference of Weights and Measures agreed to an International System of Units: SI or Système International, the name for the current version of the metric system, first introduced in France at the end of the 18th century.
In any system of measurement, the magnitude of some physical quantities must be arbitrarily selected and declared to have unit value. These magnitudes form a set of standards and are called base units. All other units are derived units.
The SI measurement system is used for medical, scientific and technical purposes in most countries and comprises seven base units with several derived units. Each unit has its own symbol and is expressed as a decimal multiple or submultiple of the base unit by use of the appropriate prefix; for example, a millimetre is one thousandth of a metre.
|Name of unit||Symbol||Quantity|
|Mole||mol||Amount of substance|
|Joule||J||Work, energy, heat|
|Volt||V||Electrical potential, potential difference, electromotive force|
|Other values expressed in terms of base units|
|Metres per seond||m/s m.s-1||Speed, velocity|
|Metres per second per second||m/s2 m.s-2||Acceleration|
|Kilogram per cubic metre||kg/m3 kg.m-3||Density|
|Mole per cubic metre||mol/m3 mol.m-3||Concentration|
Decimal multiples and submultiples
The metric system uses multiples of 10 to express number. Multiples and submultiples of the base unit are expressed as decimals, with appropriate prefix to the name of the unit. The most widely used prefixes are kilo, milli and micro: e.g. 0.000 001 g = 10-6 g = 1 microgram.
Rules for using units and numbers
The symbol for a unit is unaltered in the plural and should not be followed by a full stop except at the end of a sentence: 5 cm not 5 cm. or 5 cms.
The decimal point between digits is indicated by a full stop. Commas are not used to divide large numbers into groups of three: a space is left after every third digit. If the numerical value of the number is less than 1, a zero should precede the decimal point: 0.123 456 not .123,456.
The SI symbol for 'day' (i.e. 24 hours) is 'd' but excretion of substances, fluid intake or output, for example, should preferably be expressed as amount or mass or volume 'per 24 hours'.
'Squared' and 'cubed' are expressed as numerical powers and not by abbreviation: square centimetre is cm2 not sq cm.
|1 000 000 000 000||1012||tera||T|
|1 000 000 000||109||giga||G|
|1 000 000||106||mega||M|
|0.000 000 001||10-9||nano||n|
|0.000 000 000 001||10-12||pico||p|
|0.000 000 000 000 001||10-15||femto||f|
|0.000 000 000 000 000 001||10-18||atto||a|
Units used for common measurements
Temperature The SI base unit is the kelvin but temperature is normally expressed in degrees Celsius (°C).
1° Celsius = 1° Centigrade
Energy The SI unit joule replaces the calorie.
1 calorie = 4.2 J
1 kilocalorie (calorie) = 4.2 kilojoules
Energy requirement, expenditure and content of food are expressed in kilojoules (kJ), but the kilocalorie (kcal) remains in common use.
Amount of substance in SI units is expressed in moles (mol), and concentration (amount per unit volume) in moles per litre (mol/L) or millimoles per litre (mmol/L). This replaces milliequivalents per litre (mEq/L). There are some exceptions: grams per litre (g/L), e.g. for haemoglobin and plasma proteins; international units (IU, U or iu) for enzyme activity.
Pressure The SI unit is the pascal (Pa); the kilopascal (kPa) replaces millimetres of mercury pressure (mmHg) for arterial blood pressure and partial pressure (or tension) of oxygen and carbon dioxide (PO2 and PCO2) in gas or in blood.
1 mmHg = 133.32 Pa
1 kPa = 7.5006 mmHg
Arterial blood pressure is, however, still widely measured in mmHg, cerebrospinal fluid pressure in mmH2O and central venous pressure in cmH2O.
Volume is calculated as length × width × depth. The metre(m) is the SI unit for length but a cubic metre is not a practical unit of volume for most purposes. The litre (L or l - the volume of a 10 cm cube) and the millilitre (mL, ml) are therefore used (1 L = 10-3 m3 = 1 dm3).
Weights and measures
|1 kilometre (km)||= 1000 metres (m)|
|1 metre (m)||= 100 centimetres (cm) or 1000 millmetres (mm)|
|1 centimetre (cm)||= 10 millimetres (mm)|
|1 millimetre (mm)||= 1000 micrometres (μm)|
|1 micrometre (mm)||= 1000 nanometres (nm)|
|1 metre (m)||= 39.370 inches (in)|
|1 centimetre (cm)||= 0.3937 inches (in)|
|30.48 centimetres (cm)||= 1 foot (ft)|
|2.54 centimetres (cm)||= 1 inch (in)|
|1 litre (L)||= 1000 millilitres (mL)|
|1 millilitre (mL)||= 1000 microlitres (mL)|
|1 litre (L)||= 1.76 pints (pt)|
|568.25 millilitres (mL)||= 1 pint (pt)|
|28.4 millilitres (mL)||= 1 fluid ounce (fl oz)|
|1 kilogram (kg)||= 1000 grams (g)|
|1 gram (g)||= 1000 milligrams (mg)|
|1 milligram (mg)||= 1000 micrograms (μg)|
|1 microgram (mg)||= 1000 nanograms (ng)|
|1 kilogram (kg)||= 2.204 pounds (lb)|
|1 gram (g)||= 0.0353 ounce (oz)|
|453.59 grams (g)||= 1 pound (lb)|
|28.34 grams (g)||= 1 ounce (oz)|
Fahrenheit (9/5× x °C) + 32
Centigrade = °Celsius = ( x °F-32)×5/9
For Body Temperature: Figure 2.