Casting my mind back to long ignored rules of physics:
Effect of Temperature:
The rate of gas diffusion is based on the kinetic energy of the molecules present in the mix. From memory, kinetic energy is proportional to absolute temperature. Therefore at absolute zero there would be no diffusion and no mixing at all.
At most average temperature ranges, the change in diffusion rates between highs and lows are not great. Temperature won't have a huge effect.
Incidentally, it is the kinetic energy of the molecules in a gas that cause it to have "pressure". Basically, the molecules have more energetic collisions at higher temperatures and so the force exerted on whatever they run into (tank walls, gauges, each other) is greater. Pressure = force/area.
Effect of Pressure:
Not really applicable, but for completeness...
If there was some difference in pressure between the two tanks when the isolator was opened there would be an equalisation of pressure. The rate of equalisation is proportional to the difference in the pressure. This flow of gas would also agitate the mix in the receiving tank and help combine the gases.
Actually, this might explain why the analyser went silly while testing with the isolator open. The flow from the other tank through the manifold bar would be mixing with the contents of the tank being tested - but not mixing thoroughly hence the fluctuations.
Size of Orifice:
Rapidly getting out of my depth here... not a fluid mechanics scholar...
The cross sectional area of the orifice through which the gas must diffuse has a big effect on the rate of diffusion. In this case the manifold bar.
From memory (and it's very foggy in here) changing the cross sectional area effects the rate of diffusion as the square of the change. ie: twice the area = 4 x diff rate. I might be very wrong here so please don't quote me. Are there any fluid engineers out there who can remember orifice theory vs gaseous phase diffusion???
Other Factors:
One probably has to consider differences in molecular weight as well but given the small difference between N2 and O2 this might not be a biggie.
Mechanical mixing will probably make a difference. Using the displacement method of inerting large (60000m3) LNG tanks on a ship works better in calm weather. Rough weather results in mixing of the gases and the resulting dilution method requires a whole heap (4x or more) extra inerts. Then again, the degree of mixing through the small orifice in the manifold may be negligble.
Conclusion:
I don't know what the standard protocol is for filling twin tanks with mix but looking at it from the outside I think this is what I would do:
1. Open the isolator before filling.
2. Close the isolator before analysis.
3. Analyse both tanks separately - be worried if there is a difference in the analyses.
Then again, I could be full of s#1t!
For the record - my money is on the bouncing round in the back of the truck and exposure to temperature fluctations making bugger all difference!
LOL
Great experiment Rick. I don't think the open isolator measurements will give any meaningful reading because you are getting random portions of gas out of each side. But I did always believe they would mix sooner than that.
