# Need help understanding negative pressure

1. Nov 7, 2009

### deathlucky

so i understand possitive pressure eg 14.7psi is about twice atmospheric pressure so if you were to fill a 1L bottle with air to 14.7psi it would have 2L of air in it.

question is if you were to have a 1L bottle and suck air out of it until it read -14.7 psi how much air would be in the bottle?

another question what is the vacuum of space

2. Nov 7, 2009

### rock.freak667

Well if you sort of suck out the air from the bottle, the bottle might become crushed.

3. Nov 7, 2009

### deathlucky

i was just using bottle as an example lets say the bottle can not be crushed

4. Nov 7, 2009

### Danger

How is your gauge calibrated? Zero psi would be a total vacuum unless you have your instrument zeroed to some other reference. Negative numbers would be impossible in that case.
I think that you mean internal vs. external pressure differential, but one has to be sure of such things.

Last edited: Nov 7, 2009
5. Nov 7, 2009

### deathlucky

gauge is 0psi at sea level

im trying to work out how much air is going in to my car when when at idle and coasting

6. Nov 7, 2009

### Pengwuino

A bottle at 14.7 PSI is at 1 atmospheric pressure. Now, if there's a pressure difference[i/]of 14.7PSI, then yes, the bottle would have a PSI of 29.4PSI and have 2L of air normally at 1atm in it.

7. Nov 7, 2009

### Danger

Perhaps I'm missing something, but why don't you just multiply your swept volume by your rpm's? That's how you work out the required cfm for a carb.

8. Nov 7, 2009

### ernestpworrel

You can't have negative air pressure. There's no way. Inflationary cosmology contains the concept of negative pressure being the cause of outward expansion.

9. Nov 8, 2009

Staff Emeritus
Then your gauge is calibrated to read 14.7 psi less than the true pressure. So if your gauge reads -5 psi, you know the absolute pressure is 9.7 psi.

Let's answer the OP's question and not go off into cosmology.

10. Nov 8, 2009

### ernestpworrel

So anyway, this question has no answer. You can't have negative air inside a container.

11. Nov 9, 2009

### D H

Staff Emeritus
That is wrong, Ernest. The original post obviously is talking about gauge pressure. You are misconstruing this to mean absolute pressure.

The answer to the original post is "very little", with very little being relative to the amount of air at ambient conditions. How much (or how little) depends on the quality of the vacuum system used to evacuate the container. Even the container itself needs special treatment to create an ultra-high vacuum.

Note well: Scientists use absolute pressure rather than gauge pressure to measure the quality of a vacuum.

12. Nov 9, 2009

### ernestpworrel

But that's not even air pressure though. I don't understand why you believe that I am saying that absolute pressure cannot be negative. But we both know that air pressure can never be negative. Wow, would I be in trouble in your physics class or what?

13. Nov 9, 2009

### James Leighe

Don't forget the cassimer effect, isn't that a kind of universal pressure?

14. Nov 9, 2009

### Staff: Mentor

This post makes very little sense. Do you understand and agree with the following two statements:

1. Gauge pressure and absolute pressure are two different ways of quantifying air pressure.
2. Gauge pressure can be negative, absolute pressure cannot.

15. Nov 9, 2009

### Staff: Mentor

Maybe, but it doesn't have anything at all to do with air pressure.

16. Nov 9, 2009

### ernestpworrel

I'm okay with the first statement, but I have a problem with the second. If you're getting a negative gauge pressure, shouldn't your next step be to recalibrate your gauge? And a vacuum has negative pressure so am I to understand that negative pressure is not absolute pressure?

17. Nov 9, 2009

### D H

Staff Emeritus

18. Nov 9, 2009

### ernestpworrel

Okay, now what? Is it absolute pressure or not?

Last edited: Nov 9, 2009
19. Nov 9, 2009

### Staff: Mentor

You said you agreed with my statement that said gauge pressure and absolute pressure are different! So you should already know the answer to that question is no.

Ok, I don't know what you found when you googled, but the first link is to the wiki on the subject. Please read the entire passage describing the difference between gauge and absolute pressure:
http://en.wikipedia.org/wiki/Pressu...e_and_differential_pressures_-_zero_reference

 Btw, somehow I forgot about the third type, differential pressure (Danger used the word "differential"). It actually is probably redundant and the other two are subsets, nevertheless the way wiki describes it, gauge pressure usually has the negative sign dropped, whereas differential pressure doesn't. Regardless, it depends on context.