Non-SI units
A number of non-SI units are commonly used, even though the SI system of units allows for complete coverage of all scientific measurements. The reasons for this are historical and political as well as for everyday convenience.
Dr Chris Sutton
Throughout his distinguished career, Dr Chris Sutton was a leading figure in the world of metrology. In 1999, as the Director of MSL, Chris signed a global arrangement for the mutual recognition of calibration and measurement certificates issued by national metrology institutes. Before he passed away in 2018, he also designed a unique Kibble balance that could measure the kilogram in terms of Planck’s constant.
Image: Dr Chris Sutton, Measurement Standards Laboratory of New Zealand
Almost every country in the world, except the United States, Liberia and Myanmar, has adopted the SI system of units for everyday commercial and scientific use. In 1988, the American Congress adopted SI as the preferred US system but failed to pass effective laws enforcing it. As a result, the units of measurement in common usage in the US are from the imperial system of weights and measures. For example, distance is measured in inches, feet, yards and miles, rather than millimetres, metres and kilometres.
Some imperial measures (UK)
Length
Mass and weights
Liquid capacity
12 inches = 1 foot
437.5 grains = 1 ounce
20 fluid ounces = 1 pint
3 feet = 1 yard
16 ounces = 1 pound
4 gills = 1 pint
220 yards = 1 furlong
14 pounds = 1 stone
2 pints = 1 quart
8 furlongs = 1 mile
8 stone = 1 hundredweight (cwt)
4 quarts = 1 gallon
5280 feet = 1 mile
20 cwt = 1 ton
8 pints = 1 gallon
1760 yards = 1mile
Some commonly used non-SI units
Physical quantity
Non-SI unit
Symbol
Conversion Factor
Energy
calorie
cal
1 cal = 4.184 J
Length
Ångstrom
1 Å = 10-10 m
Mass
tonne
t
1 t = 103 kg
Pressure
atmosphere
atm
1 atm = 1.013 x 105 Pa
Temperature
degree Celsius
°C
1 °C = 1 K
Time
minute
min
1 min = 60 s
hour
h or hr
1 h = 3600 s
day
d
1 d = 86 400 s
Volume
litre
L
1 L = 1 dm3 = 10-3 m3
Energy
It has been difficult to replace the non-SI measure for energy since the term ‘calorie’ has become very much part of the food/diet domain. Diet-conscious people often measure or assess their food intake and energy output in calories rather than the SI unit of the joule.
A calorie is defined as the amount of energy needed to raise the temperature of 1 g of water by 1 °C. This 1:1:1 correspondence is perhaps easier to understand than the joule definition: 4.18 joules is the amount of energy needed to raise the temperature of 1 g of water by 1 °C.
Food labels in New Zealand are required to carry energy content information in kilojoules, or kilojoules and calories.
Time
In the original metric system devised in France in the 1790s, an attempt was made to decimalise time measurement. For example, the day was to be divided into 10 hours with 100 minutes per hour and 100 seconds per minute. This idea failed to catch on since it required a remake of all time-measuring devices like clocks. As a result, the use of hours, minutes and seconds with its awkward numerical relationships is still in place today.
The second became the universal base unit of time:
1 hour = 3600 s
1 day = 86 400 s
1 year ~ 31.6 mega seconds (Ms)
Pressure
The SI unit of pressure is the pascal (Pa) where 1Pa = 1 Nm-2.
Still in common usage is the atmosphere, which relates to the pressure exerted by the weight of the atmosphere over the surface of the Earth. On an average day at sea level, this pressure is known as 1 atmosphere, which equates to 101.325 kPa.
When inflating pneumatic tyres such as on cars and bicycles, the non-SI unit pounds per square inch (psi) is frequently used:
1 atmosphere = 14.7 psi
Volume
Although the litre is commonly used as a volume measure, it is not the SI base unit of volume, which is the cubic metre (m3). In the original French metric system, the base unit of volume was the litre (L) defined as the volume of 1 kilogram of water at 4 °C. This volume is equivalent to 1000 cm3 and is also known as 1 decimetre cubed (dm3
1 m3 = 1000 dm3 = 1 000 000 cm3
1 m3 = 1000 L = 1 000 000 mL
Litre (L) and millilitre (mL) are non-SI units, but because they are easier to write and pronounce, they are in common use.
Confusion over units
In everyday activities, we often see units of measure either incorrectly written or applied:
Car sales yards often advertise as a vehicle as having ‘low Ks’, but if they mean low kilometres, then the unit is ‘km’. The unit ‘Ks’ is kelvin second which has no physical meaning.
When people talk about their ‘power bill’, what they really mean is their ‘electrical energy’ bill. Power is measured in watts whereas energy is measured in joules. We buy ‘electrical energy’ not electrical ‘power’ from the ‘power’ station.
Some people go to the gym to ‘burn calories’ in an effort to keep their ‘weight’ down. From the scientific viewpoint, they are expending energy, measured in kilojoules, in an attempt to lose some body mass, measured in kilograms.
Most often, no harm is done as a consequence of this incorrect use. However, using the incorrect units had an unfortunate and costly effect for the Mars Climate Orbiter – a spacecraft designed to enter orbit around the planet Mars and collect data about the Martian atmosphere and climate.
Mars Climate Orbiter
On 23 September 1999, the Mars Climate Orbiter, a spacecraft designed to enter orbit around the planet Mars, was lost when incorrect units were used to programme an orbit path.
On 23 September 1999, the Mars Climate Orbiter was lost when the spacecraft entered a lower orbit path than planned. This lower orbit caused the spacecraft to heat up and disintegrate before crashing onto the surface of the planet.
The main cause of this mishap was traced to a thruster calibration table where British imperial units instead of SI units were used. The software for navigation at the Jet Propulsion Laboratory expected thruster impulse data to be expressed in newton seconds but Lockheed Martin Astronautics, which built the orbiter, provided the values in pound-force seconds. This caused the impulse to be interpreted as a quarter of its actual value. As a result, the orbiter dropped into a lower orbit for which it was not structurally designed.
The cost of this mishap was estimated to be $125 million.
Nature of science
One of the key aspects of the nature of science is that it produces, demands and relies on careful measurements. The rigorously developed International System of Units provides a reliable way of achieving this. If scientists and engineers are communicating with one another using different measurement systems, then costly misunderstandings can occur.
Useful link
Find out about the consequences of small measurement errors in the Mars Climate Orbiter case and other examples in the SimScale website.
