You got pascal, bar, psi, atm, torr, and all kinds of column heights of mercury, water and whatnot.
Why do we need all these different units to express pressure?
In: Physics
Imagine you have a bunch of balloons, and you want to measure how much air is inside them. Different people in different places came up with their own ways to measure it, just like we have different languages to say the same thing.
1. **PSI (Pounds per Square Inch):** This is like saying, “How many pounds of air are pushing on every little square inch of the balloon?” People in the United States often use this.
2. **Bar:** This is another way to measure pressure, used in many places around the world. It’s like using meters instead of feet to measure height.
3. **Atmosphere (atm):** This measures pressure based on the air pressure at sea level on Earth. It’s like saying, “How much air is in the balloon compared to the air around us?”
4. **Torr:** Named after a scientist named Torricelli, this measures pressure in a way that’s useful for scientific experiments, like how tall a column of mercury would be pushed up by the air pressure.
Each of these units measures the same thing—how much air is pushing on something—but they just use different systems or ways to describe it. It’s like how you can count fingers in English, Spanish, or French; the number of fingers is the same, but the words are different.
Different units of pressure exist because they evolved from various fields and historical contexts. Scientists, engineers, and industries developed their own units based on what was practical for their work. For example, Pascals are common in science, psi is used in engineering and tires, and atmospheres are handy for comparing to Earth’s air pressure. These units persist because they are deeply ingrained in each field’s standards and practices, making them convenient for specific applications
There is only one SI unit of pressure, Pascal. The rest are obsolete nonsense and there is no need to use them.
1. PSI/Pa: These are imperial/metric units expressed as a force per area (yes, all pressure is a force per area, in this case though the definition is a 1:1 between a force unit and an area unit). These are commonly used in engineering as you will frequently have two of: Force, Area and Pressure/Stress and when you need to find the target value it’s a simple multiplication/division calculation. There’s also things like pounds per square foot, kips per square inch, kPa and so on depending on use case.
2. Bar: This is 100,000Pa, it’s also fairly close to air pressure at sea level – generally off by about 1-2% depending on weather. Sometimes more in severe weather. Has similar use as PSI/Pa, but commonly applied for things being pressurized by gasses.
3. ATM: This is a weird one. It’s a measurement of air pressure at a very specific temperature and altitude, but has an awkward conversion to force – 1 ATM equals 101325 Pa, or just over 1 Bar. It’s kinda a relic, I’m not sure of any formal use for it as any conversion to force requires scaling the value after multiplying by area.
4. Mercury: One of the instruments used to measure pressure is a U shaped tube with mercury in it. One side of the U can be pressurized to a different pressure than the other – forcing the mercury to one side of the U. By measuring the height difference you can measure the pressure. This unit is useful when all you care about is knowing the pressure and you don’t need to convert to a force, so you can just read the length off the device using a readily available ruler – the most common use the average person will see for this is the blood pressure measurement device used in medicine. Similar devices exist using water, but result in much larger measurements because water is so much less dense – a blood pressure measurement tool using water would need to be something like 8-10 feet tall, so somewhat unwieldy.
We have very few fields that have singular, consistent units. Those fields have one thing in common – they originated in the modern time. Radioactivity for example is measured the same everywhere, because it was pioneered by a narrow group and they made their mark on science.
Steam power was developed at similar times in the US, Prussia, Great Britain, France, Austria etc. and while they all copied each other, everyone has found their own special way to do things. Standardization was a national affair rather then an international one. If you were to buy a British made boiler or a Prussian one, you’d find different units to describe internal pressure, and different instrumentation to measure it. Those with sufficient access to mercury would use it in pressure gages, in places where it’s hard to find, you’d go with a water column.
Historical hick-hack is the short term answer.
It’s a wonder we only ended up with 3 temperature scales (I guess because Kelvin got it pretty much right immediately).
They’re all just conventions based on how the industry evolved. They can (mostly) all be directly correlated back to pascals. You’re probably accustomed to the concept of the notation of mass-force, even if you aren’t consciously aware of it: kg or lb being used as a unit of force, despite being defined as a unit of mass. This is just another convention which makes it easier to understand the situation at hand.
Though some units incorporate correction factors intrinsically in non-obvious ways. Specifically column heights – if they’re uncorrected (not referencing a standard temperature, e.g. 20 C), they can float as a result of temperature at the point of measurement, which can be useful in certain HVAC and weather applications.
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