An overview of Metric vs. Imperial Systems
For many years, two systems were widely used for measuring distances, temperatures, volumes, pressures and other important quantities.
One is defined as the Metric system, while the other one is defined as the Imperial system.
Why the name Imperial?
Because it dates from the time of the British Empire.
This means that for decades (not to say centuries), the old British colonies used the Imperial system, while almost the rest of the world was using the metric system.
Even when the US fought for its liberties and declared itself independent from Britain, the colonies still decided to keep the Imperial system.
Nowadays, most countries are progressively moving to the metric system, including Canada and the United Kingdom, with often the use of both systems in parallel, at least for technical purposes (they are still using miles for distances).
Note that there is also a difference between the metric system, as it is used in many countries, and the International System.
The International System of units, using the international abbreviation of SI, is based on the metric system and is the only system of measurement with an official status.
But this system defines only one type of unit, meter, for example for distances, with 7 base units, among which:
You can define all other units as a combination of just these units and there is an unlimited number of additional units, called derived units, such as Pascal for the Pressure, or Joule for Energy.
To make it a bit more complex, not all countries within the metric system are using the same units for a given dimension. This ends up with coherent SI units (based on the 7 base units) and non-coherent SI units.
Many non-coherent SI units are using metric prefixes, which are decimal multiples of each unit.
Common metric prefixes include kilo-, for thousand, centi-, for one one-hundredth, and milli-, for one one-thousandth.
As an example, one kilometer is 1000 meters, and one millimeter is 1/1000 meter.
Take the example of a flow measurement, expressed in GALLONS PER MINUTE (GPM) in the US.
It is sometimes expressed in liters per second (one US gallon is 3.785 liters), while liter is not a pure SI unit for a volume, or in cubic meter per hour, while hour is not a pure SI unit for time. These are non-coherent SI units.
You may also find derivation from these values, with expressions in liters per minute, which is the conversion generally used in NFPA standards, or even in KILOGRAM PER SECOND (kg/s).
This last one could be considered as a coherent SI unit, as kg is a base unit as well as second. But this is based on the fact that the density of water is one kilogram per liter, which is under normal temperature and pressure conditions.
One liter (volume) of water has a mass of one kilogram.
The metric system uses units such as degrees Celsius for temperature, or cubic meter per hours for a flow, where the SI system would require degrees Kelvin and cubic meter per seconds for a coherent use of the base units.
Also, perhaps important to note here is that the US gallon is different from the Imperial gallon that is used in the United Kingdom or Canada.
One US gallon is 3.785 liters, while one Imperial gallon is 4.546 liters.
So be careful, when using the terminology gallon outside of the US, to be sure what you are measuring. A UL listed fire pump installed in Canada will be defined in gpm with US gallons, but the volume of the water supply may be expressed in Imperial gallons.
This means that even in the US, with the use of the Imperial system, some units, such as volume, is not purely based on this system.
In summary, the metric system is used worldwide with the notable exception of the US (and some countries from the old British colonies that are using a mix or both systems at the same time).
The Metric system is sometimes referred as the International System, which by definition is not identical, as practices of measurement in some countries may use derived units from the International system, resulting in non-coherent SI units.
We’ll dig into some examples and how this practically works here in the coming videos.
For Franck Orset, I’m Jeff Kelm, this is MeyerFire University.
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