Please Explain The Physics

[shadders]

I was just playing around with pressure conversions and I noticed something I never noticed before. Atmospheric pressure is higher than typical serving pressure.

1 atm = 101Kpa according to beersmith. So what I don't get is why when you have a serving pressure of 50kpa does the beer come out at all? You got 50kpa pressure pushing it out and 100kpa pushing it back up the tap. Net pressure = -50kpa so the beer should be pushed backwards into the keg.

There's something blindingly obvious I'm missing here.

 

[Supra-Jim]

Your 50kPa serving pressure is 50kPa above atmospheric pressure. In absolute terms it is 150kPa.

Cheers SJ

 

[Hashie]

Your keg was already at atmospheric pressure before you sealed it. You then upped that pressure by 50 Kpa.

 

[shadders]

Your 50kPa serving pressure is 50kPa above atmospheric pressure. In absolute terms it is 150kPa.

Cheers SJ

so does that mean that 0kpa on my reg gauge is actually 101kpa?

 

[.DJ.]

pretty much, thats howI understand it..

0 atm would be a vacuum I believe...

 

[shadders]

pretty much, thats howI understand it..

0 atm would be a vacuum I believe...

That's a relief. For a minute there I thought I'd broken the laws of physics and was about to cause the total annihilation of the universe. Worth it for a decent beer though.

 

[jonocarroll]

There's something blindingly obvious I'm missing here.

There's a reason it's called 'gauge pressure' - http://en.wikipedia.org/wiki/Gauge_pressur..._zero_reference

You are always measuring relative to atmospheric, which is arbitrary for your needs. Good on ya for having a think about it though. :beer:

 

[Bribie G]

Yes I just checked it out myself to make sure the other comments are correct. Spot on lads.