Time: 0 second
Question No. 2
Q: What are SI units? Why are they relevant to anaesthesia?
Answer No. 2
- SI stands for the ‘Système lnternational (SI) D'Unités’, which translates from the French as the ‘International System of Units’
- Represent a set of fundamental base units, derived units and prefixes used throughout the world:
- Based on metric units with the decimal system
- Definitions are not static and undergo frequent evolution
- The latest change in 2019 saw all definitions become based upon “constants” of physical sciences
- The need to create highly accurate and precise definitions has led to very complex definitions
- 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
Question No. 3
Q: What are the base SI units?
Answer No. 3
Unit
Unit Symbol
Base Measure
Second
s
Time
Metre
m
Length
Mole
mol
Amount of Substance
Ampere
A
Electric Current
Candela
cd
Luminous Intensity
Kilogram
kg
Mass
Kelvin
K
Temperature
MEMORY TIP
Can be remembered by the acronym 'SMMACKK'
S econd
M etre
M ole
A mpere
C andela
K ilogram
K elvin
Question No. 4
Q: How do you define a second?
Answer No. 4
Unit
Unit Symbol
Base Measure
Physical Constant
Definition
Second
s
Time
Hyperfine transition frequency of caesium (ΔνCs)
The time taken by 9,192,631,770 oscillations of the light emitted by a caesium-133 atom
Question No. 5
Q: How do you define a kilogram?
Answer No. 5
Unit
Unit Symbol
Base Measure
Physical Constant
Definition
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 m2⋅kg⋅s–1
Question No. 6
Q: How was a kilogram previously defined?
Answer No. 6
- 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
Question No. 7
Q: Is a mole an SI unit? How is it defined?
Answer No. 7
Yes, a mole is the SI unit of 'amount of substance':
Unit
Unit Symbol
Base Measure
Physical Constant
Definition
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 is equal to Avagadro's number
Question No. 8
Q: What is Avogadro’s constant?
Answer No. 8
- Avagadro's constant is the physical constant used to define the mole
- It is equal to a value of 6.02214076 × 1023
Question No. 9
Q: What is the volume occupied by 1 mole of gas at standard temperature and pressure?
Answer No. 9
One mole of any gas at STP occupies a volume of 22.4L
Question No. 10
Q: What is standard temperature and pressure?
Answer No. 10
- 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)
Question No. 12
Q: What is capacitance and what are the SI units?
Answer No. 12
- Capacitance is defined as:
The ability of a system to store electric charge.
- The unit of capacitance is the Farad
Question No. 13
Q: How is a Farad defined?
Answer No. 13
Unit
Unit Symbol
Base Measure
Base Units
Definition
Farad
F
Capacitance
kg−1⋅m−2⋅s4⋅A2
The capacitance if a potential difference of 1 volt is present across its plates, when a charge of 1 coulomb is held by them
Question No. 15
Q: What is force?
Answer No. 15
- Force is defined as:
An interaction that when unopposed causes a change in an object's motion
Question No. 16
Q: How can the newton be defined?
Answer No. 16
Unit
Unit Symbol
Base Measure
Base Units
Definition
Newton
N
Force
kg.m.s−2
The force required to give 1 kilogram an acceleration of 1 metre per second per second
Question No. 17
Q: What is pressure?
Answer No. 17
- Pressure is defined as:
The force applied perpendicular to the surface of an object per unit area over which that force is distributed
Question No. 18
Q: What is the SI unit of pressure and how can it be defined?
Answer No. 18
Base Measure
Unit
Unit Symbol
Base Units
Definition
Pressure
Pascal
Pa
N.m−2
The pressure of 1 Newton per square metre
Question No. 19
Q: Which non-SI units of pressure are commonly used in anaesthesia? How do you convert them to kPa?
Answer No. 19
Unit
Unit Symbol
Base Measure
SI Unit
Conversion to SI Unit
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
Question No. 20
Q: What are the SI prefixes?
Answer No. 20
- 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
Question No. 21
Q: Which SI prefixes are commonly used?
Answer No. 21
Prefix Name
Prefix Symbol
Base 10
English Word
yotta
Y
1024
septillion
zetta
Z
1021
sextillion
exa
E
1018
quintillion
peta
P
1015
quadrillion
tera
T
1012
trillion
giga
G
109
billion
mega
M
106
million
kilo
k
103
thousand
hecto
h
102
hundred
deca
da
101
ten
100
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