Gauge Pressure usage in Gas Mixtures

[mechCB]

I guess I need a refresher. I have a 2 part gas mixture (40% Pa and 60% Pb).

I know that the total fill pressure is 10 bar absolute.

I need to fill with Pa first using gauge pressure. I was told that I have to subtract atmospheric pressure from the partial pressures of Pa and Pb to determine the patial filling pressures using gauge pressure. I do not think this is correct since I would be subtracting atmospheric pressure twice (3 bar rel. of Pa and 5 bar rel. of Pb) which cannot be summed to get the total filling pressure in gauge (9 bar rel.).

[Ptotal = 10 bar abs = 4 bar abs (Pa) + 6 bar abs (Pb)
my point: Ptotal = 9 bar rel = 3.6 bar rel (Pa) + 5.4 bar rel (Pb)
the other person's point: Ptotal = 9 bar rel but the partial pressure uf Pa is 3 bar rel and the partial pressure of Pb is 5 bar]

Who is right?

 

[quark]

You are correct.

 

[zdas04]

Are you starting with an evacuated container? If not, then you have three components not 2.

If so, then going to 3 bar(gauge) with A and to 9 barg with B gets you to the 60 40 mix.

David

 

[mechCB]

I am starting from a vacuum.

 

[zdas04]

Then it sounds like you've gotten the right mixture, but I'm still a bit confused about your example. Partial pressure is never relative to gauge pressure. It only has meaning in absolute pressure so your assertion that you have 3.6 barg of Pa and 5.4 barg of Pb doesn't mean anything (even if "true"). You have 4 bar of Pa and 6 bar of Pb.

When I was in school (think slide rules), we used the term "atmosphere" which was never defined as precisely as "bar" is defined in common usage today--but there was never any confusion about "gauge" and "absolute" because "atmosphere gauge" had no meaning. We've had this discussion many times in eng-tips.com about "what does a pressure gauge read at zero barg?". Some say zero. Some say 14.5 (psia) minus local atmospheric pressure. I don't think that there is an official answer to that question, but if it not the later then you get some squirrely results in vacuum situations at high elevations (where I live).

The fact that you are using a yardstick with the first foot cut off is not germane to how high the wall is.

David

 

[mechCB]

I am creating the mixture by first adding 1 gas to a tank. Then I add the second gas to complete the mixture. My pressure gauge reads in psig so my thought is that I need to fill to 52 psig with Pa (50% of 10 bar absolute converted to psi gauge) and then top off with Pb to 130 psig (equivalent to 10 bar absolute at STP). This is where the other guy is saying that I need to first fill to 43 psig (10 bar*14.5*40% - 14.7 psi) and then top off to 130 psig but then my ratio would be off. Am I correct or am I insane and the other guy is right?
Thanks.

 

[sailoday28]

I believe your calculation or method is based upon isothermal conditions. When finished adding gas "a" to the tank and tank is now at temperature a, how will you maintain constant temperature while filling with gas "b"?
Regards

 

[unclesyd]

When we used a lot of mixed gases that we mixed on site we took care of the temperature by filling the tanks in water bath. We mixed the gases by using a heat lamp directed at one spot on the bottom 1/4 of the cylinder.
If I recall correctly we filled the majority of the cylinders using pressure gauges in absolute pressure. The higher pressure gauges were Heiss gauges which I don't think were absolute.