# SI units

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## SI units

the units of measurement generally accepted for all scientific and technical uses; together they make up the International System of Units. (See also metric system.) The abbreviation SI, from the French Système International d'Unités, is used in all languages. There are seven base SI units, defined by specified physical measurements, and two supplementary units. Units are derived for any other physical quantities by multiplication and division of the base and supplementary units. The derived units with special names are shown in the accompanying table.

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.

## base u·nits

the fundamental units of length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity in the International System of Units (SI); the names and symbols of the units for these quantities are meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd).

## SI units

Etymology: Fr, système international
the international units of physical amounts. Examples of these units are the mass of a kilogram, the length of a meter, and the precise amount of time in a second.

## In·ter·na·tion·al Sys·tem of U·nits

(SI) (in'tĕr-nash'ŭn-ăl sis'tĕm yū'nits)
A system of measurements, based on the metric system, adopted at the 11th General Conference on Weights and Measures of the International Organization for Standardization (1960) to cover both the coherent units (basic, supplementary, and derived units) and the decimal multiples and submultiples of these units formed by use of prefixes proposed for general international scientific and technologic use. SI proposes seven basic units: meter (m), kilogram (kg), second (s), ampere (A), Kelvin (K), candela (cd), and mole (mol) for the basic quantities of length, mass, time, electric current, temperature, luminous intensity, and amount of substance; supplementary units proposed include the radian (rad) for plane angle and steradian (sr) for solid angle; derived units (e.g., force, power, frequency) are stated in terms of the basic units (e.g., velocity is in meters per second, m/sec-1). Multiples (prefixes) in descending order are: exa- (E, 1018), peta- (P, 1015), tera- (T, 1012), giga- (G, 109), mega- (M, 106), kilo- (k, 103), hecto- (h, 102), deca- (da, 101), deci- (d, 10-1), centi- (c, 10-2), milli- (m, 10-3), micro- (μ, 10-6), nano- (n, 10-9), pico- (p, 10-12), femto- (f, 10-15), and atto- (a, 10-18). The prefix zepto (z) has been proposed for 10-21.
[Fr. Système International d'Unités]

## SI units

SI 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 units

those 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 Metre m Length Kilogram kg Mass Second s Time Mole mol Amount of substance Ampere A Electric current Kelvin °K Thermodynamic temperature Candela cd Luminous intensity

 Name Symbol Quantity Joule J Work, energy, heat Pascal Pa Pressure Newton N Force Watt W Power Volt V Electrical potential, potential difference, electromotive force Hertz Hz Frequency Other values expressed in terms of base units Square metre m2 Area Cubic metre m3 Volume 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.

 Multiplication factor Prefix Symbol 1 000 000 000 000 1012 tera T 1 000 000 000 109 giga G 1 000 000 106 mega M 1000 103 kilo k 100 102 hecto h 10 101 deca da 0.1 10-1 deci d 0.01 10-2 centi c 0.001 10-3 milli m 0.000 001 10-6 micro μ 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).

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

 Linear measure 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) Conversions Metric Imperial 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) Volume 1 litre (L) = 1000 millilitres (mL) 1 millilitre (mL) = 1000 microlitres (mL) Conversions Metric Imperial 1 litre (L) = 1.76 pints (pt) 568.25 millilitres (mL) = 1 pint (pt) 28.4 millilitres (mL) = 1 fluid ounce (fl oz) Mass 1 kilogram (kg) = 1000 grams (g) 1 gram (g) = 1000 milligrams (mg) 1 milligram (mg) = 1000 micrograms (μg) 1 microgram (mg) = 1000 nanograms (ng) Conversions Metric Imperial 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)

## Conversions

• Fahrenheit (9/5× x °C) + 32

• Centigrade = °Celsius = ( x °F-32)×5/9

where x is the temperature to be converted.

## Conversion scales

Figure 1.

For Body Temperature: Figure 2.

## Conversion scales

Figure 1: Conversion scales.

Figure 2: Conversion scale for body temperature showing the normal and abnormal ranges.

## International System of Units

; Système International d'Unités; SI units system of weights and measures, e.g. metre (m; length); kilogram (kg; mass); litre (L; volume); second (s; time); ampere (A; electric current); kelvin (K; temperature); pascal (Pa; pressure); mole (mol; amount of substance); derived units (e.g. force, power, frequency) are stated in terms of the basic units; multiples of units are termed, in descending order: tera- (T; 1012), giga- (G; 109), mega- (M, 106), kilo- (K; 103), hecto- (h; 102), deca (da; 101), deci- (d; 10-1), centi- (c, 10-2), milli- (m, 10-3), micro- (μ, 10-6), nano- (n, 10-9)

## SI units

the units of measurement generally accepted for all scientific and technical uses. Together they make up the International System of Units. The abbreviation SI, from the French Système International d'Unités, is used in all languages. There are seven base SI units, defined by specified physical measurements and two supplementary units. Units are derived for any other physical quantities by multiplication and division of the base and supplementary units. The derived units with special names are shown in Table 3.
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) = 10−9m. The units of mass are specified in terms of the gram, e.g. microgram (μg) = 10−9kg.
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) = 10−9m3. 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 in typescript. For this reason, the capital letter L is also sometimes 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.
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