Converting Metric Pressure Units to PSI using the Bar-O-Meter

Professional Mariner Forum
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A few years back we got a new fire main pressure gauge in MPa. What the? Looking it up; 1 MPa = 10 bar.

That had me scratching my head, till I went to log the barometric pressure which is measured in millibars (mb). Atmospheric pressure varies of course but is about 1000 millibars or 1 bar. I know atmospheric pressure at sea level is about 14.5 psi so that gives a number to use for rough conversions.

1000 mb = 1 bar. (about atmospheric pressure)

1000 mb is about 14.5 psi.

So 1 bar is about 14.5 psi

Bar is not an official SI unit but it is based on the metric system so bar can be converted exactly to the pascal which is the official SI unit of pressure. Pascal (Pa) is a whimpy European unit so it takes a lot to amount to anything, 100,000 Pa just to get to 1 bar, so 10 bar is 1,000,000 pascals or 1 MPa, which is the units the new gauge uses.

So;

Atmospheric pressure is about 1000 millibars or 1 bar.(from the barometer)

Atmospheric pressure is about 14.5 psi. (from grade school science class).

10 bar=1 MPa (had to look it up)

So if 1 bar (0.1 MPa) is about atmospheric pressure or 14.5 psi than:

[U]1 MPa is about 145 psi.[/U]

So on the fire main if the gauge reads 0.5 MPa that’s about 70 psi, 100 psi is going to be about 0.7 MPa etc.

This gauge is in kPa, 10,000 kPa (10 MPa) should be about 1450 psi.

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Been awhile since I’ve worked conversions, but make sure you lay down the correct unit prefixes. If I’m not mistaken, it should be MPa (1x10e6 Pascals), not mPa (1x10e-3 Pascals).

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[QUOTE=PineappleOranges;187623]Been awhile since I’ve worked conversions, but make sure you lay down the correct unit prefixes. If I’m not mistaken, it should be MPa (1x10e6 Pascals), not mPa (1x10e-3 Pascals).[/QUOTE]

Yes, you’re right. I changed it. Thanks

Also standard atmospheric pressure pressure is 1013.25 mb or 1.01325 bar which is about 14.7 psi which is the number some people may remember as atmospheric pressure.

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True.

That had me scratching my head, *till I went to log the barometric pressure which is measured in millibars (mb). Atmospheric pressure varies of course *but is about 1000 millibars or 1 bar. I know atmospheric pressure at sea level is about 14.5 psi so that gives a number to use for rough conversions.

True. 1 atm=14.696 psi, 1 bar=14.504 psi

1000 mb = 1 bar. (about atmospheric pressure)

*1000 mb is about 14.5 psi.

So 1 bar is about 14.5 psi

Yes, almost exactly 14.5 psi

Bar it is is not an official SI unit but it is *based on the metric system so *bar can be converted exactly to the *pascal which is the official SI unit of pressure. Pascal ¶ is a whimpy European unit so it takes a lot to amount to anything, 100,000 Pa just to get to 1 bar, so 10 bar is *1,000,000 pascals or 1 MPa, which is the units the new gauge uses.

All true.

So;
*
Atmospheric pressure is about 1000 millibars or 1 bar.(from the barometer)

Atmospheric pressure is about 14.5 psi. (from grade school science class).*

10 bar=1 MPa (had to look it up)*

So if 1 bar (0.1 MPa) *is about atmospheric pressure *or 14.5 psi than:

*1 MPa is about 145 psi.

Yes. 1 MPa is 145.038 psi

So on the fire main if the gauge reads *0.5 MPa that’s about 70 psi, *100 psi is going to be about 0.7 MPa etc.

True: 0.5 MPa=72.519 psi, 100 psi=0.689 MPa

This gauge is in kPa, 10,000 kPa (10 mPa) *should be about 1450 psi.

True: 10’000 kPa=1450.377 psi

So your math is all good. It’s important to know that pressure is measured in different ways, which make certain readings not directly comparable to others. “Gauge pressure” is pressure relative to the current atmospheric pressure. If I have a pressurized tank, and a low pressure weather system moves in, the gauge pressure inside the tank will be higher, even though nothing inside the tank has changed. If my gauge pressure is ‘zero’ it means that my tank is equal to the current atmosphere, not that it has a vacuum.

The second type of reading is “Absolute pressure.” When this reading is zero, my tank is experiencing hard vacuum. When my tank is 14.5 psi, then it’s about the same as atmosphere, and if it reads more than that, it is under pressure.

The third type of reading is “Differential pressure.” If you have a gauge in parallel with a pump or an orifice, it is measuring the difference between the inlet and outlet pressure. If the inlet pressure on my pump is Absolute 14.5 psi, and the outlet pressure is absolute 24.5 psi, the differential gauge will show 10 psi.

The fourth type of reading is called “Sealed pressure.” I’ve never seen this one used. Apparently it reads the pressure relative to some chosen fixed value. Let’s imagine that we are making a CO2 extinguisher to use in a hyperbaric chamber. We might want the gauge to read relative to some number higher than atmospheric
 But why wouldn’t you just use a differential pressure gauge? I don’t know why we have this, but we do.

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Metric prefixes: http://astro.unl.edu/classaction/tables/intro/si_prefixes.html

Metric units: http://astro.unl.edu/classaction/tables/intro/si_units.html

What can be simpler??? (If you don’t try to convert to some archaic system that went out the window years ago in the rests of the world)

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[QUOTE=ombugge;187628]
What can be simpler??? (If you don’t try to convert to some archaic system that went out the window years ago in the rests of the world)[/QUOTE]

But I thought being multi-lingual was an important European value. :stuck_out_tongue:

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Love to work with American oil rigs and get volume in barrels. And the first thing we have to ask is which type of barrel. It’s a reason the rest of the world has SI units.

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[QUOTE=Kraken;187632]Love to work with American oil rigs and get volume in barrels. And the first thing we have to ask is which type of barrel.[/QUOTE]

Check your Bar(rel)-O-Meter

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I saw The Barrel Itself. Its in the Norsk Oljemuseum in Stavanger. Maybe they’d send it out to you if you needed to callibrate

some archaic system that went out the window years ago in the rests of the world.

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[QUOTE=Emrobu;187630]But I thought being multi-lingual was an important European value. :P[/QUOTE]

Multi-lingual?? We are talking NUMBERS here!!!

What do you call anybody that speaks three or more Languages??
Don’t know??? What about two languages???
OK, let us make it simple, what about somebody that know only ONE language???

Answers to follow tomorrow. Have a good night.

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[QUOTE=Emrobu;187634]I saw The Barrel Itself. Its in the Norsk Oljemuseum in Stavanger. Maybe they’d send it out to you if you needed to callibrate[/QUOTE]

Barrels?? what about KIPS, FOOT/LBS, FAHRENHEIT and all the other totally irrelevant measures???

When will you guys learn how to use a simple system like the metric?? Just because Ronald Reagan didn’t doesn’t mean it is rocket science.

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Great technical stuff mates. Kennebec Captain what type of firemain system was on the ship that got the new gauge?

Did you know? Myanmar, Liberia, and the United States of America. Those are the only three countries left in the world that don’t use the metric system.

//youtu.be/7bUVjJWA6Vw

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[QUOTE=ombugge;187637]When will you guys learn how to use a simple system like the metric?? Just because Ronald Reagan didn’t doesn’t mean it is rocket science.[/QUOTE]

I use the metric system. It’s my primary. Kennebec Captain uses the metric system. Everybody uses the metric system. But if I didn’t know exactly what 9/16" looks like, I’d be a crappy mechanic. Both I use both because I can interface with the world more effectively that way. The good Captain from Maine, seemed happier with psi, so I used psi for my examples. The ability to use both and convert between them is super useful. All my old-timey mentor-type people prefer the old units. There’s lots of perfectly good boats and other technology out there that require you to speak their language.

And by the way, numbers /are/ a cultural-linguistic thing. Some things are easier to think about as fractions, some work better as decimals. Some number systems are base-5 or base-20, or base-2, or base-16. It’s useful to be able to think in ways other than base-10. “Metric is simple, because base-10 is simple, and therefore it’s better for everything,” is the doctrine I was taught. It’s true, on the surface, but just because something is simple doesn’t mean that it’s good for you. Having to use more than one system is both directly useful in and of itself, and helps people develop intellectual complexity, creativity, and logic. It also helps, by analogy, understand how complexity is built up in computers.

1 Like
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Multi-lingual

Don’t know??? What about two languages???

Bilingual

OK, let us make it simple, what about somebody that know only ONE language???

American.

It’s a good joke, but I think I just proved it wrong. After all, computers and many other useful technologies were invented and advanced by people who knew both systems.

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[QUOTE=Emrobu;187640]I use the metric system. It’s my primary. Kennebec Captain uses the metric system. Everybody uses the metric system. But if I didn’t know exactly what 9/16" looks like, I’d be a crappy mechanic. Both I use both because I can interface with the world more effectively that way. The good Captain from Maine, seemed happier with psi, so I used psi for my examples. The ability to use both and convert between them is super useful. All my old-timey mentor-type people prefer the old units. There’s lots of perfectly good boats and other technology out there that require you to speak their language.[/QUOTE]
Old is the magic word here.

[QUOTE=Emrobu;187640]And by the way, numbers /are/ a cultural-linguistic thing. Some things are easier to think about as fractions, some work better as decimals. Some number systems are base-5 or base-20, or base-2, or base-16. It’s useful to be able to think in ways other than base-10. “Metric is simple, because base-10 is simple, and therefore it’s better for everything,” is the doctrine I was taught. It’s true, on the surface, but just because something is simple doesn’t mean that it’s good for you. Having to use more than one system is both directly useful in and of itself, and helps people develop intellectual complexity, creativity, and logic. It also helps, by analogy, understand how complexity is built up in computers.[/QUOTE]
And that’s why you have history lesons, to learn about arcaic masuring systems. Metric is not used by the majority of the world population because it’s easy, if that was the case I would measure with my foot and weigh myself in stone. It’s used because it’s a [B][U]International[/U] System of Units[/B].

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[QUOTE=Kraken;187646]Old is the magic word here.

And that’s why you have history lesons, to learn about arcaic masuring systems. Metric is not used by the majority of the world population because it’s easy, if that was the case I would measure with my foot and weigh myself in stone. It’s used because it’s a [B][U]International[/U] System of Units[/B].[/QUOTE]

I don’t think anyone is trying to claim that we shouldn’t be using the SI system. I use it almost exclusively at work. But try explaining to the USCG why the pressure gauge is showing your fire pump is putting out zero point something pressure. Best to know the answer before they ask.

Emrobo’s point is that we might gain insights when we expand our world a bit. In this case that barometer holds the key for pressure unit conversion because many people know atmospheric pressure in both bar and psi. Along the way this thread has hit MPa, Pa, Bar, mb (also mbar) atm, Standard Atmospheric pressure, gauge pressure and now whatever Emrobu’s point is about creativity (which I think is interesting).

Should add hPa, hectopascals is used on some weather charts (and barometers) which is equal to millibars.

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[QUOTE=Kennebec Captain;187648]I don’t think anyone is trying to claim that we shouldn’t be using the SI system. I use it almost exclusively at work. Emrobo’s point is that we might gain insights when we expand our world a bit. In this case that barometer holds the key for pressure unit conversion because many people know atmospheric pressure in both bar and psi. Along the way this thread has hit MPa, Pa, Bar, mb (also mbar) atm, Standard Atmospheric pressure, gauge pressure and now whatever Emrobu’s point is about creativity (which I think is interesting).[/QUOTE]

And my point is that Units of measurement is not something you should be “creative” with. Way too many misunderstanding and accidents happen because of miscommunication between transmitter and receiver of messages. And every time I encounter United States customary units I want to murder someone.

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[QUOTE=Kraken;187649]And my point is that Units of measurement is not something you should be “creative” with. Way too many misunderstanding and accidents happen because of miscommunication between transmitter and receiver of messages. And every time I encounter United States customary units I want to murder someone.[/QUOTE]

Well, hopefully this thread will help you a bit with pressure units.

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[QUOTE=Kraken;187646]And that’s why you have history lesons, to learn about arcaic masuring systems. Metric is not used by the majority of the world population because it’s easy, if that was the case I would measure with my foot and weigh myself in stone. It’s used because it’s a [B][U]International[/U] System of Units[/B].[/QUOTE]

Learning about something isn’t good enough. If it was, we wouldn’t send cadets to sea at all. We’d just give the a pile of books and a few homework assignments and after a few years of that, we’d call them Captain. If you don’t do something it doesn’t get integrated into who you are. I know my compass points, a little bit of geodesy, I can kinda read a chart, I know how that theodolite-type thing works. But I’m not a navigator.

You’re completely 100% correct about the need for an international system, though. It’s deffinatly safer if everyone is on the same page units-wise. It’s vital.

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If you are going to use SI units why does it matter that Imperial units are not

integrated into who you are

Sextant?