Units of Measurement

What the curriculum thinks you need to know:

PC_BK_07 SI Units: fundamental units and derived units
PC_BK_08 Other non SI units relevant to anaesthesia: including mmHg, bar, atmospheres, cm H2O, psi

What you need to know (The theory):

This is classical Primary MCQ exam fodder. Make sure you know these well.

The Systéme International (S.I.) is the most commonly used system for measurement. It consists of seven ‘base’ units and a huge number of derived units. All units of measurement can be derived down to these seven base units.

You’ll hear the acronym SMMACKK banded around, just remember it has two M’s and two K’s if you’re going to use it or you’ll get royally confused. I’ve ordered them in this order for ease.

General points –

  • Dont worry about the BIG numbers in these, use the term ‘a certain number’ instead. I’ve included the actual numbers as a guide. You need to know the smaller numbers in definitions however as these are frequently asked.

Second (a measure of time – s)

One second = 9,192,631,770 oscillations of a Caesium 133 atomic clock.

The second is one you’ll all be familiar with, but not the actual definition!

Metre (a measure of length – m)

One metre = The distance travelled by light in vacuum in 1/299,792,458 second

Mole (a measure of amount of substance – mol)

One Mole = The amount of substance containing the same number of atoms/molecules as there are atoms in 12g of Carbon-12

Technically this should read 0.012kg of carbon-12 but 12g of carbon-12 is easier to remember.

Ampere (a measure of current – A)

Right, remember with this one that current flow through a conductor will induce an magnetic field. If two conductors are close to each other they will attract/repel each other…

One Ampere = The current applied to two parallel conductors of negligible cross section and infinite length, one metre apart in a vacuum which would produce a force between them of 2.0 x 10-7 Newtons per metre.

Pfffffft! So the current needed to produce a certain electromagnetic force between two conductors one metre apart. simplified.

Candela (a measure of light – cd)

Technically this should be called a measure of ‘luminous intensity’, but ‘light’ will do.

One Candella = The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 5.4×1014 hertz and that has a radiant intensity in that direction of 1/683 watt per square radian.

Again… Ughhh… So a amount of light that supplies a certain intensity of light energy.

Kilogram (a measure of mass – kg)

This is the odd one out, the only unit which has a prefix (just to confuse you). Remember kg is the S.I. Unit, NOT g.

One Kilogram = The mass of the international kilogram prototype in Pavillon de Breteuil, Sévres, France.

Out of interest, this is a big hunk of platinum-Iridium Alloy…

Kilogram prototype

Kelvin (a measure of temperature – K)

Although a kelvin and a degree Centigrade have the same magnitude of change, the Kelvin is the S.I. unit!

One Kelvin = 1/273.16 of the thermal energy of the triple point of water.

Remember the triple point of water is 0.01oC, and absolute zero is -273.15oC.

What you need to know (How it works in practice):

Given these standard ‘Base’ S.I. Units, you can work out any other unit and derive it into the base S.I. units. Given, these can be a bit long winded, but its possible! Below are some examples:

Newton – Unit of Force – kg.m.s-2

Pascal – Unit of pressure – kg.m-1.s-2

Joule – unit of Energy – kg.m-2.s-2

Watt – Unit of Power – kg.m-2.s-3

The List goes on and on…

Random Exam factoids (i.e. the things the college like asking):

  1. The college occasionally asks what derived SI units are in Base SI units… Look here for a big list…

© Sam Beckett and Physics4FRCA, 2018. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited.

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