What's Actually Happening When you Pull a PRV? Like Sciencewise, Bro.

[doug293cz]

I wonder how much light enters the keg

“When you Pull The PRV? ...”

If you're worried about that, paint all of the "dry side" interior components of the PRV flat black. Any light that gets in has to be reflected off of something, since there is no direct path for light to enter.

Brew on

 

[Bobby_M]

This level of detail is unable to reach the types of folks that my explanationation was attempting to connect with.

While conceptually your example is correct, it may lead readers to believe that air molecules can travel significant distances before they collide with other molecules. This is not the case. The average distance air molecules travel before colliding with other air molecules is on the order of 30 nano meters (or 30 billionths of a meter) at room temp and atmospheric pressure. Each molecule collides many times per second.

"Having this approximately correct expression, what use is it? Consider the mean free path of an air particle. Atmospheric pressure (sea level) is about 760 Torr. Plugging this into the final expression gives a mean free path of λmfp = 3.4 × 10-6 cm. This is 34 nanometers, which is roughly half of the commonly reported value of 65 nm. Particles in air do not travel very far before they collide with other particles."
​

Brew on

 

[Moose_MI]

It took us until post #32 to be concerned about the level of detail here?

I love this place

 

[Qhrumphf]

Is a nanometer more expensive than a refractometer? Will it tell me my ABV?

 

[grampamark]

Is a nanometer more expensive than a refractometer? Will it tell me my ABV?

Why not? In another thread today a guy keeps referring to using his “hygrometer” to measure SG.

 

[Qhrumphf]

I thought a high grow meter was something your parents drew on the wall.

Unless, of course, he's truly looking for a specific gravity:

 

[IslandLizard]

O2 and N2 will try to move into the keg by diffusion (because their partial pressures outside the keg are higher than inside the keg)

I've always understood it's entropy (disorder) that's root to equalization/homogenization of different gasses (or mixtures), not selective motion due to "partial pressure" differences. IOW, molecules are not driven by partial pressure differences in bordering systems, randomness of motion is the big equalizer. Therefore equalization of (gas) mixtures requires time, it's a random process, and far from instantaneous.

 

[doug293cz]

I've always understood it's entropy (disorder) that's root to equalization/homogenization of different gasses (or mixtures), not selective motion due to "partial pressure" differences. IOW, molecules are not driven by partial pressure differences in bordering systems, randomness of motion is the big equalizer. Therefore equalization of (gas) mixtures requires time, it's a random process, and far from instantaneous.

Partial pressures are a stand-in for concentration (based on ideal gas law: P = nRT/V. Concentration is n/V.) Unless the concentration is zero on one "side", diffusion goes in both directions with the rate determined by the concentration on the side "behind" the direction of diffusion. If the concentration is equal on both "sides" then the diffusion flow is equal in both directions, so there is zero net flow. If the concentrations (partial pressures) are not equal then the diffusion flows are unequal, and the net flow is the difference of the two flows. The closer the two concentrations, the lower the net flow. When the concentrations are close to equal, the diffusion flow in each direction is very much larger than the net flow.

The gas mixing by diffusion does increase the entropy of the system, thus lowering the Gibbs Free Energy, and you can think of this lowering of the system energy as the driving force for diffusion. But, just the statistics of the random motions/collisions of the gas molecules will explain the mixing of the gases.

I'm sure this is more than the vast majority of participants in this thread want to know.

Brew on

 

[Moose_MI]

Partial pressures are a stand-in for concentration

Isn’t that the science behind a shart?
I can’t believe I learned something from this thread...Thanks guys!

 

[Jayjay1976]

Where is OP with all this good scientific discussion going on?

 

[Vale71]

I've always understood it's entropy (disorder) that's root to equalization/homogenization of different gasses (or mixtures), not selective motion due to "partial pressure" differences. IOW, molecules are not driven by partial pressure differences in bordering systems, randomness of motion is the big equalizer. Therefore equalization of (gas) mixtures requires time, it's a random process, and far from instantaneous.

It's random in the sense that each particle's motion is random, but the overall effect is not random and always goes from areas of higher concentration (or partial pressure, if you will) to areas of lower concentration at a rate that can be modelled very accurately. It's actually a very nice example of emergent behaviour.

 

[bracconiere]

LOL

 

[Vale71]

Nice. It would be more pertinent if they had done a time-lapse showing the balloon slowly sinking as CO2 mixes with air through diffusion until its density in the aquarium equals its density in the atmosphere.

 

[william_shakes_beer]

I'm not worried about diffusion, because it involves molecules. There at no molecules. I know because my high school physics teacher told me there's only 4 elements: Earth, fire, water and air

 

[seilenos]

I know because my high school physics teacher told me there's only 4 elements: Earth, fire, water and air

What an idiot!

The 4 elements are obviously water, grain, hops, and yeast!

 

[beersk]

Space is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.

I just finished this book series for about the 6th or 7th time. It's so good and entertaining!