Here are the maths

You can see it's much more efficient to use sanitizer and push it out (10-12psi). Also trimming your gas diptube flush with the top wall of the keg and overfilling the keg in a closed transfer will be your best bet.

I'd be curious to see what 5, 10, and 15 seconds of 15psi on the out post would be.

I've been looking over some past threads on purging. It's been mentioned several times that there may be info on purging with a flow from the bottom, but nobody's posted a link. Anybody please?

(Liquid still seems to me the surest thing. Dalton' s Law and all.)

You mean like this?

“So, what happens if instead of doing pressurize/vent cycles, we flow CO2 into a vessel that originally contains air? Does the flow improve the dilution and removal efficiency of O2 compared to the cyclic process? We can argue that if the CO2 inflow is fast enough that CO2 comes in faster than it can mix with the air, then it could form a sort of gas piston that would push air ahead of it towards the vent, and that this would push out more O2 per volume of CO2 than if complete mixing of incoming CO2 and existing gas occurred (as it does in the pressurize/vent case.)

The best case for non-mixing of CO2 and headspace would be if there were absolutely no internal "air" currents, such that the only mixing of CO2 with headspace gas would be via diffusion. So the question comes down to: Is the linear CO2 flow rate faster than the diffusion velocity of CO2 in air? If the CO2 flow rate were much faster than diffusion, then mixing would be limited, and continuous flow would be more efficient than purge/vent. If CO2 flow rate were much slower than diffusion, then gases would be mostly mixed, and continuous flow would not be any more efficient than pressurize/vent. If the flow rate and diffusion rates were of the same order of magnitude, then there would be significant, but not complete, mixing, making this the most complex scenario to analyze.

To start we need to get an estimate of the diffusion velocity of CO2 in air. If we limit our analysis to one dimensional flow (say from bottom to top of a keg, uniform velocity across the width), things will be much simpler, but still valid. Fick's first law of diffusion is (ref:

https://en.wikipedia.org/wiki/Diffusion):[indent]Flux = -D * (ΔConc / ΔDist)

Where Flux is in mass/area-time,

D is the diffusion coefficient, and

ΔConc / ΔDist is the concentration gradient[/indent]If we divide Flux [mass/area-time] by density [mass/volume] we get linear velocity [dist/time] which is what we are looking for.

The diffusion coefficient for CO2 in air is about 0.15 cm^2/sec (ref:

http://compost.css.cornell.edu/oxygen/oxygen.diff.air.html) Now if we make some assumptions about gradients we might encounter, we can estimate a linear CO2 flow rate due to diffusion. We will use approximate numbers for simplicity, since we are only looking for order of magnitude estimates of velocity.

A corny keg has a volume of about 20 L or 20,000 cm^3, and a height of about 55 cm, leaving a cross sectional area of about 20,000 cm^3 / 55 cm = 364 cm^2. The density of CO2 at STP is about 2 g/L or 0.002 g/cm^3 (ref:

http://www.engineeringtoolbox.com/gas-density-d_158.html.) If we assume 2.5 cm of pure CO2 at the bottom of the keg, and 2.5 cm of air at the top of the keg, and a uniform concentration gradient from the bottom to the top, the CO2 gradient becomes:[indent]ΔConc / ΔDist = (0 - 0.002 g/cm^3) / 50 cm = -4.0e-5 g/cm^4[/indent]The CO2 flux becomes:[indent]Flux = -D * (ΔConc / ΔDist) = -0.15 cm^2/sec * (-4.0e-5 g/cm^4) = 6.0e-6 g/cm^2-sec[/indent]And finally the linear velocity of CO2 due to diffusion is:[indent]CO2_Diffusion_Velosity = CO2_Flux / CO2_Density = 6.0e-6 g/cm^2-sec / 0.002 g/cm^3 = 0.003 cm/sec[/indent]

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