# OBJECTIVES & QUESTIONS

Introduction

What are SI units?

- SI stands for the ‘Système lnternational (SI) D’Unités’, which translates from the French as the
**‘International System of Units’** - There are three types of SI units:
- Base SI units
- Based on metric units with the decimal system
- Definitions are based upon “constants” of physical sciences
- Definitions not static and undergo frequent evolution – the latest change was in 2019
- The need to create highly accurate and precise definitions has led to very complex definitions

- Supplementary SI units:
- Used along with base units to form the derived SI units
- Contains only two, purely geometric units, the unit of plane angle (the radian) and the unit of solid angle (the steradian)

- Derived SI units:
- Obtained through equations containing the seven base units and supplementary units

- Base SI units

Why are SI units relevant to anaesthesia?

- Adopted to overcome the use of diverse and inconsistently applied units and quantities that had developed in medicine
- Allows increased ease of communication and avoidance of errors and misunderstandings

Base SI Units

What are the base SI units and how are they defined?

Unit

Unit Symbol

Base Measure

Physical Constant

Definition

Second

s

Time

Hyperfine transition frequency of caesium (Δ

_{νCs})The time taken by 9,192,631,770 oscillations of the light emitted by a caesium-133 atom

Metre

m

Length

Speed of light (c)

The length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second

Mole

mol

Amount of Substance

Avagadro's constant - a value of 6.02214076 × 1023

The amount of substance that contains the same number of particles as there are atoms in 0.012 kg (12 g) of carbon-12, which isequal to Avagadro's dumber

Ampere

A

Electric Current

Elementary charge (e) - the charge of a single proton

Defined by taking the elementary charge e to be a fixed numerical value of 1.602176634×10 A⋅s

Candela

cd

Luminous Intensity

Luminous efficacy of monochromatic radiation of frequency 540×1012 Hz (K

_{cd})Defined by taking the fixed numerical value of Kcd, to be 683 expressed in the units cd⋅sr⋅kg

^{–1}⋅m^{–2}⋅s^{3}Kilogram

kg

Mass

Plancks Constant (h) - relationship between a photons energy and frequency

Defined by taking Plancks constant (h) to be a fixed numerical value of 6.62607015×10−34 m

^{2}⋅kg⋅s^{–1}Kelvin

K

Temperature

Boltzmann's Constant (k) - relationship between a particles kinetic energy and temperature

Defined by taking the Boltzmann constant k to be a fixed numerical value of 1.380649×10−23 kg⋅m

^{2}⋅s^{–2}⋅K^{–1})#### Memory Tip

Can be remembered by the acronym ‘SMMACKK’

S econd

M etre

M ole

A mpere

C andela

K ilogram

K elvin

How has the definition of an kilogram changed?

- Until 2019 the kilogram was defined as
*“the mass equal to the International Prototype of the Kilogram (IPK), a platinum-iridium rod housed at the International Bureau of Weights and Measures in Sevres, France”* - This was changed to create a definition based on a physical constant in line with the other base SI units

How has the definition of an ampere changed?

- Until 2019 the ampere was defined as “
*the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 newtons per metre of length*“ - This was changed to create a definition based on a physical constant in line with the other base SI units

What volume does one mole of a substance occupy at standard temperature and pressure?

- One mole of any gas at STP occupies a
**volume of 22.4L**

What is standard temperature and pressure?

- As the volume of gases are greatly affected by changes in temperature and pressure it is important to specify these at which any measurement of volume is made
- It is often useful to correct results to standard temperature and pressure:

Standard Temperature = 273.15 K (0⁰C)Standard Pressure = 100 kPa (1 Bar)

Derived SI Units

What are the derived SI units?

- Derived SI units are units obtained through equations containing the seven base units
- Thus, they allow any physical quantity to be expressed in terms of SI units

What are the commonly used derived SI units and how are they defined?

Unit

Unit Symbol

Base Measure

Base Units

Definition

Hertz

Hz

Frequency

s

^{−1}The frequency of one cycle per second

Newton

N

Force

kg.m.s

^{−2}The force required to give 1 kilogram an acceleration of 1 metre per second per second

Pascal

Pa

Pressure

N.m

^{−2}The pressure of 1 Newton per square metre

Joule

J

Energy or Work

N.m

The energy expended when the point of application of a force of 1 Newton moves 1 metre in the direction of the force

Watt

W

Power

J.s

^{−1}1 joule per second

Volt

V

Electrical Potential

kg⋅m

^{2}⋅s-3⋅A^{−1}The difference of electrical potential between two points of a conductor carrying a constant current of 1 ampere, when the power dissipated between these points is 1 watt

Ohm

Ω

Electrical Resistance

kg⋅m

^{2}⋅s^{−3}⋅A^{−2}The resistance when one volt is applied across a conductor and produces a current of 1 ampere

Coulomb

Q

Charge

s⋅A

The quantity of electricity transported in one second by a current of 1 ampere

Farad

F

Capacitance

kg

^{−1}⋅m^{−2}⋅s^{4}⋅A^{2}The capacitance if a potential difference of 1 volt is present across its plates, when a charge of 1 coulomb is held by them.

Henry

H

Inductance

kg⋅m

^{2}⋅s^{−2}⋅A^{−2}The inductance in a circuit when an electric current that is changing at 1 ampere per second results in an electromotive force of 1 volt across the inductor:

Weber

Wb

Magnetic Flux

kg⋅s

^{−2}⋅A^{−1}The magnetic flux that, linking a circuit of one turn, would produce in it an electromotive force of 1 volt if it were reduced to zero at a uniform rate in 1 second

Tesla

T

Magnetic Flux Density

kg⋅m

^{2}⋅s^{−2}⋅A^{−1}The magnetic flux density equal to 1 weber per square metre

Prefixes

What are the SI metric prefixes?

- Used to indicate
**multiples**or**fractions**of a**unit** - Based on the
**decimal system**– represent multiples of 10 - Standardised for use in the international system of units
- Units
**still**considered to be**SI units when given a prefix**

Which SI prefixes are used in modern practice?

Prefix Name

Prefix Symbol

Base 10

English Word

yotta

Y

10

^{24}septillion

zetta

Z

10

^{21}sextillion

exa

E

10

^{18}quintillion

peta

P

10

^{15}quadrillion

tera

T

10

^{12}trillion

giga

G

10

^{9}billion

mega

M

10

^{6}million

kilo

k

10

^{3}thousand

hecto

h

10

^{2}hundred

deca

da

10

^{1}ten

10

^{0}one

deci

d

10

^{−1}tenth

centi

c

10

^{−2}hundredth

milli

m

10

^{−3}thousandth

micro

μ

10

^{−6}millionth

nano

n

10

^{−9}billionth

pico

p

10

^{−12}trillionth

femto

f

10

^{−15}quadrillionth

atto

a

10

^{−18}quintillionth

zepto

z

10

^{−21}sextillionth

yocto

y

10

^{−24}septillionth

Non-SI Units

Which non-SI units are recognised by the System International (SI)?

- A number of units are recognised by the SI despite not being true SI units
- Often found in anaesthesia due to ease of use
- Important to understand how they relate to true SI units

Unit

Unit Symbol

Base Measure

SI Unit

Conversion to SI Unit

minute

min

Time

s

x 60

hour

h

Time

s

x 3600

litre

l

Volume

m

^{3}x 10

^{–3}bar

bar

Pressure

kPa

x 100

pounds per square inch (PSI)

lb.inch

^{−2}Pressure

kPa

x 6.895

atmosphere

atm

Pressure

kPa

x 101.325

centigrade

°C

Temperature

K

+273.15