I think purple is the right answer for lots of things!:cool2:
how much air in the tank?
- Thread starter Crush
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I think purple is the right answer for lots of things!:cool2:
OP
Crush
Contributor
Besides dialects, I thought you folks up there had only two languages?Very good.
But to go sideways whilst going forwards and to put spg use into basic perspective
You learn the idiosyncratic behaviour of the gas gauge in your car so it goes you could do likewise with a spg.
The main one being when attaching your spg to a different sized tank, the gauge may read the same but the quantity will not
How long does a tank last?
Purple
jaybombs25
Contributor
You are right with the Van der Waals equation and your calculations. However what does this really accomplish?? You could use the redlich-kwong EOS, Clausius EOS, Beattie-Bridgeman EOS or any other equation of state that sticks in coefficients to to account for a gases deviation from ideal behaviour. When it comes to the bottom line though your SPG does not rely on theoretical values proposed by various equations, it works based on mechanical parts and therefore reflects the TRUE pressure of the gas with varying degrees of accuracy of course. Though you can use many different equations of state to get to closer and closer values to true value, only via measurement i.e your SPG can you truly say what the pressure of the gas in the cylinder is and thereby how much gas in truly in the cylinder.
OP
Crush
Contributor
Your SPG can tell you the pressure of the gas in your tank, but it can't tell you how much gas is in the tank.
We assume that pressure is directly proportional to the amount of gas in the tank, i.e., the relationship is linear. I knew that there were subtle non-linearities, but I didn't know how significant they were. As it happens, they are comparable to the inherent uncertainties in most diving SPGs.
It satisfied my curiosity and the calculations are easier to do than the actual experiment. The results are meaningful if you keep in mind the limits of the theory and the range of pressures, etc., across which the parameters were obtained (empirically).
OP
Crush
Contributor
Did that yesterday and the hose exploded albeit very slowly right in the middle somewhere.
No obvious damage, just pin prick. Must investigate further, probably starts at first end. No hose protectors?
Small price to pay for cheap SH set.
Yep. Permeation of the rubber under the swage and up the outer casing, missed by that much, worn out hose syndrome..
No obvious damage, just pin prick. Must investigate further, probably starts at first end. No hose protectors?
Small price to pay for cheap SH set.
Yep. Permeation of the rubber under the swage and up the outer casing, missed by that much, worn out hose syndrome..
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Its been over a decade since my thermo class, but I think what you have shown is merely the difference between two gas models. I don't think its correct to say that the van der waals equation is correct and thus the difference is the error in the ideal gas law. At 3000psi we are far away from the triple point, the gases are not close to phase change, the ideal gas law is very accurate. Van der waals (I had to wiki this to remind me) is to improve accuracy at very low pressures, or when near phase change. From wiki, which we know is always right...
"Above the critical temperature the van der Waals equation is an improvement of the ideal gas law, and for lower temperatures the equation is also qualitatively reasonable for the liquid state and the low-pressure gaseous state. However, the van der Waals model is not appropriate for rigorous quantitative calculations, remaining useful only for teaching and qualitative purposes."
How are you getting standard errors from calculations?
"Above the critical temperature the van der Waals equation is an improvement of the ideal gas law, and for lower temperatures the equation is also qualitatively reasonable for the liquid state and the low-pressure gaseous state. However, the van der Waals model is not appropriate for rigorous quantitative calculations, remaining useful only for teaching and qualitative purposes."
How are you getting standard errors from calculations?
OP
Crush
Contributor
1. My a and b parameters were parametrized within the pressure range that I was calculating. There was no extrapolation. Since standard errors were not given, I assumed the last digit to be +/- 1.
2. I did a propagation of errors analysis (Taylor series expansion). All the usual caveats apply: no cross-correlations, no higher order terms, etc..
3. Critics have thus far failed to seize upon a major issue - there are not attractive "a" terms in the vdW system for hetero-molecular attraction. I state this for the second time and offer my numbers as the best-possible right now. They are an approximation, but they are better than the ideal gas approximation.
4. The take-home message is that, even if my numbers have infinite accuracy, they indicate that the break-down in ideal-gas behaviour is commensurate with SPG accuracy.
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