It’s a jungle out there!
There is a lot of different pressure units in use around the world and sometimes this can be very confusing and may cause dangerous misunderstandings. In this blog post I will discuss the basics of different pressure units and different pressure unit families.
What is pressure?
When I talk about pressure in this post, it does not refer to the stress you may be suffering in your work, but to the physical quantity. It is good to first take a quick look at the definition of pressure, this will also help to better understand some of
the pressure units.
If you remember the studies of physics in school … as most of us don’t remember… a short reminder is in order: pressure is defined as force per area perpendicular to the surface. That is often presented as formula p = F/A. Pressure being indicated with the letter “p”, although capital letter “P” can also be seen being used in some occasions.
So what does this force per area mean in practice? It means that there is certain force effecting to a specified area. When we look at force, it is specified being Mass x Gravity. As there are so many different engineering units used for both mass and area, the number of combinations of these is huge. Plus there are also a lot of pressure units that do not directly have the mass and area in their names, although it often is in their definition.
It is good to notice that in practice the “force” is not always included in the pressure unit names. For example pressure unit kilogram force per square centimeter should be indicated as kgf/cm², but often it is indicated just as kg/cm² without the “f ”. Similarly, pound force per square inch (pfsi) is normally indicated as pounds per square inch (psi).
International System of Units (SI system) / Metric
Let’s start to look at the pressure units by looking at the SI system, which is the International System of Units, derived from metric system. Now that I mentioned the metric system, I can already see some of you taking a step back… but please stay with me!
SI system is the world’s most widely used system of measurement. It was published in 1960, but has a very long history even before that.
For pressure, the SI system’s basic unit is Pascal (Pa), which is N/m² (Newton per square meter, while Newton is kgm/s²).
To say that in a formula:
Pascal is a very small pressure unit and for example the standard atmospheric pressure is 101325 Pa absolute.
Out of Pascal’s definition, the kg force can be replaced with different units like g (gram) force, and meter can be replaced with centimeter or millimeter. By doing that, we get many other combinations or pressure units, such as kgf/m², gf/m², kgf/cm², gf/cm², kgf/mm², gf/mm², just to list a few.
The unit “bar” is still often used in some areas. It is based on metric system, but is not part of SI system. Bar being 100000 times Pascal (100 times kPa) it is anyhow easy to convert. In some areas (like NIST in USA) the bar is not recommended
to be used widely.
And like for all pressure units, SI or not SI, we can use the common prefixes/coefficients in front of them, most commonly used are milli (1/100), centi (1/10), hecto (100), kilo (1000) and mega (1000000). To list a few examples, that already gives us different Pa versions, all being commonly used: Pa,
kPa, hPa, MPa. The unit bar is most commonly used without prefix or with prefix milli: bar, mbar.
But taking all mass units and combining those with all area units from SI system, we get many combinations.
Read the full blog article here http://blog.beamex.com/pressure-units-and-pressure-unit-conversion?utm_campaign=Bltd%20-%20Instrumentation%20newsletter&utm_source=ppc
Or to use Beamex free pressure convertor click here https://www.beamex.com/resources/pressure-unit-converter/?utm_campaign=Bltd%20-%20Instrumentation%20newsletter&utm_source=ppc
