Vacuum Torr to PSI

[crodriguez717]

I am working on an appoximation of pressures, but as extrapolate, I get a negative pressure (negative PSI).

For example: -100 PSI. I know this is wrong, but I can not figure out how to get around this.

If I am not mistaken this should be a logrithmic relationship, but I can not get the relationship. Any one have advise?

 

[hydtools]

1 torr = 1 mmHg = 0.01934 psi

Ted

 

[katmar]

The relationship between Torr and psi is linear and not logarithmic. The linear relationship given by hydtools is correct.

Where things can get a bit messy with vacuum is when people confuse absolute pressure, gauge pressure and vacuum. A standard atmosphere is almost exactly 760 Torr (= 760 mmHg = 14.696 psi) of absolute pressure. A pressure in Torr is almost always an absolute pressure, but it is best to specify exactly what you mean. A pressure that is 60 Torr less than one atmosphere could be specified as 700 Torr absolute, or it could be specified as a vacuum of 60 mmHg, or as a gauge pressure of -60 mmHg.

Similarly, this pressure of 700 Torr absolute could be converted to psi as 13.536 psi absolute, or a vacuum of 1.160 psi (= 14.696 - 13.536), or a gauge pressure of -1.160 psig.

So it is possible to get negative pressures in psi if they are gauge pressures (i.e. relative to atmospheric pressure) but you cannot get -100 psi under normal atmospheric conditions where an absolute vacuum would be -14.696 psig.

You can download a free units conversion program called Uconeer from the address in my signature below. Uconeer will do the straight pressure conversions for you, but you must keep track of the absolute or gauge pressures yourself.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Eltron]

You must also realize that your standard atmospheric pressure will vary. Here (in CO)we hover around 12.6psi.

Dan

http://www.eltronresearch.com

Dan's Blog

 

[JLSeagull]

Also, different standards reflect different pressures and temperatures. Also, occasionally standards change.

http://en.wikipedia.org/wiki/Atmospheric_pressure

 

[zdas04]

Katmar,

but you cannot get -100 psi under normal atmospheric conditions

Exactly when would someone be able to reach -100 psi? At sea level, -14.696 psi would be the total absence of anything. I have a hard time with the math to get less than nothing.

Eltron,
Where are you in Colorado? Atmospheric pressure on Black Ridge outside of Durango is 11.27 psia. At the highest producing well in North America (outside of Rifle, CO at 11,500 ft ASL) it is 9.588 psia. Your 12.6 psia corresponds to 4270 ft ASL and I wasn't aware that anywhere west of Yuma (Elevation 4,144 ft) was that low an elevation, even Greeley is 4,600 ft ASL (12.449 psia)

David

 

[katmar]

David, I gauge you to be absolutely right. I left the door slightly open on the possibility of having -100 psi because in jacketed vessels or pipes the differential pressure is very similar to having an extreme vacuum. Or if this post sits here for a few centuries and we have engineers visiting planets much larger than earth???

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[zdas04]

MUCH larger. Don't think I would want to get on a scale someplace with 7 times the gravity of earth.

David

 

[Eltron]

David, I'm in Boulder, and I was shooting a bit from the hip. I usually just check NCAR for the pressure.

Dan

http://www.eltronresearch.com

Dan's Blog

 

[rmw]

Well.... Katmar did say "normal atmospheric conditions". He just didn't define what abnormal would be to get a -100 psi.

rmw

 

[quark]

David touched it already. I just stop short of mentioning about black holes.

 

[25362]

Sometimes (not in thermodynamics) materials under mechanical tension are said to be under negative pressures. I remember the discussion on the upward sap flow in tall trees.