Gage, vacuum, and absolute pressure explained 

Glad to hear that my terrible drawing skills made the cut!!

Thanks for the compliment :)

@@Engineer4Free i agree

Even After 3 years, this comment is valid. This 5min saved me. Thank You @Engineer4Free

You're welcome!! Glad I could help. And make sure to check out the rest of the Fluids videos here: ru-vid.com/group/PLOAuB8dR35oeOIPMOBH6hjwobuIJHPKSN =)

Hmm. Sorry if that diagram on the bottom mislead you, it was not intended to represent altitude, but more resemble a bar chart demonstrating absolute value, with longer bars representing a larger quantity. Atmospheric pressure certainly decreases when altitude increases.

The diagram in the bottom left is just to illustrate relative magnitude. Don’t take it as a literal height measurement. Bigger magnitude = bigger pressure, that’s all.

Yeahhhhhh glad I could help! Da boom

Glad it helped! There are some more related videos here too: ru-vid.com/group/PLOAuB8dR35oeOIPMOBH6hjwobuIJHPKSN =)

Jah Welcome

101 kPa Absolute is often sued as the reference 0 kPa Gauge. 110 kPa Absolute would then be 10 kPa Gauge, and 91 kPa Absolute would then be -10 kPa Gauge (or more commonly, 10 kPa Vacuum).

Hey Don thanks for the question. The Atmospheric pressure is 101 kPA abs. The pressure in the tire is 321 kPa abs. 321 kPa abs is 220 kPa higher than 101 kPa abs, so another way we can describe the pressure in the tire is in gauge (instead of absolute), and it is 220 kPa gauge. Chances are if you go look at a car tire, it will say to fill the tire to about 32 PSI (220 kPa). It is telling you to fill the tire to a pressure that is 32 PSI greater than the air outside the tire (so the instruction is given in gauge pressure). Most things in life are actually given in gauge, not absolute pressure. Does that clear it up? The 220 kPa is the tire's gauge pressure, and therefore the difference between its absolute pressure and Patm.

Glad it was helpful!

Glad to hear it!

+Daryl Daly Thanks for the feedback!

Thanks for the comment bro. You're welcome form Canada =)

Stop studying this vid immediately! Within first 2 min he states. Atmospheric pressure rises w altitude. Grain of salt at best! Please dont watch this and play w a pressure vessel u will win a darwin award

Awesome glad I could help 🙂

Lol 😅

Awesome glad to hear it Bishoy!!

@@Engineer4Free so much wrong in this vid from 1st 2 min "atm pressure rises w altitude" see digram bottom left. It doesn't! Its the exact opposite of that.

You're welcome! Thanks for the comment! =)

Awesome Hud, glad to hear it!! =)

Yes, in this case the air from the atmosphere would actually go into the tank if the valve was opened.

And 2 kids die instantly as a small town is reduced to rubble when they bled off 4200lbs of propane cuz they thought it would suck in air.

Oh I see your confusion. The little graph thing in the bottom left corner that you're looking at has nothing to do with hight. It's like a column graph, where the taller the column, the larger the value. A pressure with a greater value is further away from zero, that's all it's really representing. Does that clear it up?

The complete absence of air is a perfect vacuum. Space is a perfect vacuum for our purposes. A cylinder that has somehow had every bit of air sucked out of it is a perfect vacuum. A perfect vacuum has zero pressure absolute. Atmospheric pressure is the ambient air pressure at sea level on Earth. You can always assume it to be 101 kPa absolute for these problems. "Gauge" and "vacuum" are just used to indicate "above" and "below", respectively compared to atmospheric pressure. So 111 kPa absolute = 10 kPa guage and 91 kPa absolute = 10 kPa vacuum. Atmospheric pressure = 101 kPa absolute = 0 kPa gauge. There will never be a time when earth's atmospheric pressure = 0 kPa absolute. If that happens we'd all be dead yo.

@@Engineer4Free I see. Thank you.

You're welcome :)

Thanks!!

Thanks Merobot!! =)

The magic of RU-vid :)

You're welcome, thanks for the comment!

Glad you found it helpful!

Thanks!

Hey thanks for actually watching to the end and thanks for the correction. Yes I do say "more than absolute pressure" when I technically should have said "more than ambient air pressure" or " more than atmospheric pressure." I was pointing at 101 kPa while saying this, and in my head it was clear what I meant, but I should have been more explicit. Thanks for contributing to the discussion, hopefully if anyone is confused by my choice of wording your comment can clear it up for them =)

You're welcome

You're welcome =)

Thanks for the feedback!! :D

Atm pressure DOES NOT INCREASE WITH ELEVATION! Yet he says oposite and goes on to draw a broken diagram he uses for reference throughout vid.

:)

I do read all the comments, thanks for taking the time to post one! Glad you liked the lesson enough to say it!!! Casual learning ftw :)

Engineer4Free I'm Goin thru some series of yours right now. Learning a little bit of everything is something everyone should enjoy😊

Love the attitude! Enjoy and don't hesitate to ask any questions!

Engineer4Free awesome! Thanks again. Lookin forward to future stuff👌👌

What is vaccum pressure in last example is it inside the cylinder

Glad you liked it!

Atmospheric pressure can vary depending on where you are or the current weather conditions etc, but often in thermo/fluids problems we use 101 kPa (absolute) unless otherwise stated. If atmospheric pressure is being talked about as a gauge pressure, it would be 0 kPa (gauge), because gauge measures how far away we are from the reference, but if Patm IS the reference, then it is 0 kPa different than itself. Does that clear it up?

@@Engineer4Free Okay that 2nd statement went completely over my head. Can you please explain it again ?

@@dahaka_scares_me909 Put it this way, there's always going to be a background air pressure. The air pressure is not necessarily going to be 1 atm everywhere on this planet, because it will vary with altitude and numerous weather factors. However, in any academic problem, we assume background air pressure is 1 atm, or 101 kPa, unless there is other information given. This is the accepted value that we use for the global average of air pressure at sea level. The concept of gauge pressure is always based on what the background air pressure is, which is not necessarily 1 atm. If you are given that a problem takes place at high elevation, and the background pressure is 85 kPa, then 85 kPa absolute is the zero point for the concept of gauge pressure. Pump a tire to 250 kPa gauge at this high elevation location, and absolute pressure will be 335 kPa. Pump a tire to 250 kPa gauge at sea level, and the absolute pressure will be 351 kPa.

jacked

Thanks Abhay!

101 kPa absolute is often the absolute pressure found at sea level yes Patm is written as an absolute pressure that we then set as a reference point to take convenient gauge pressures from. Wings create a pressure difference, and that pressure difference results in a force called lift. The difference between the pressure at bottom of the wing and the top of the wing is essentially what is important.

Thanks for watching :)

+Hatim A Nobi Thanks for the feedback!

Yeah

This part confused me. Both are absolute.

@@marvindizon2912 101 KPa is the absolute Atmospheric pressure ie the exact pressure of the atmosphere. 321 KPa is absolute pressure meaning the sum of gauge pressure and atm pressure.

@@dahaka_scares_me909 hi can you help me please? I have this tank operates at atm pressure, 1.01 bara. And the design pressure is set at 10% additional from operating pressure. Therefore, I have design pressure around 1.1 bar. So my question is, what is the unit of 1.1, is it barg or bara?

@@fixjustin2699 Given 1 bar of background pressure, and 0.1 bar of pressure above the background pressure, this would make the absolute pressure be 1.1 bar. The corresponding gauge pressure would be 0.1 bar. I suppose it would be 1.1 bara and 0.1 barg, although I've never seen the units written that way.

Thanks!

+Dominic Illenberger haha thanks and thanks for watching :)

+kenny63477 Glad to help, thanks for watching!

Neither at all.

no ...it is gage in all text books

"Gage is the spelling of an obsolescent word meaning a pledge, a challenge, etc. Gauge is the spelling to use when you measure measurement, estimate, or standard." -1.08 sec google search....

and if you have studied engineering before then you will know that all "scientific text" books call it gage ..... whether it is wrong or right ... that is what they put in all books

In the engineering and scientific world, it is GAGE. If you're not used to it you're probably not in the group.

It's GAGE in Engineering world

Thank for commenting!