Pressure gauge mounted in bottle cap - Page 7 - Home Brew Forums

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01-31-2013, 11:09 AM   #61
dbsmith
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Quote:
 Originally Posted by Bobby_M I don't know why you would suggest that shaking in a closed system disrupts equilibrium. Le Châtelier's Principle requires that you change something like temperature, pressure or concentration. Shaking does none of this.
Pressure is changing when the bottle is shaken. It is obvious because the walls of the container become harder to move, because there are more molecules of gas in the headspace colliding against the container's walls. I don't see what more evidence you need. Given time, equilibrium will become reestablished and the container will return to the original hardness.

I think that your assumption of a closed system is a false premise. Closing something off in a bottle doesn't shut the system off from the universe, as I can shake it, heat it, pass light through it, make gas come out of solution, etc. As CO2 leaves solution, the concentration has changed, and thus the equilibrium shifts.

Look at the graphs above. See the dip off in pressure? That indicates that at some point in time the pressure was higher than the pressure at equilibrium. That is because initially the yeast were producing CO2 in solution (reactants). This causes the reaction to shift to create more products. After the yeast have ceased making reactants, the reaction shifts back towards creation of the reactants (because of the excess products that were made earlier..indicated by the beginning of the 'dip'), and eventually equilibrium is reached (meaning that the creation of reactants and products is happening at the same rate).

01-31-2013, 11:34 AM   #62
Hex23
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Quote:
 Originally Posted by dbsmith Pressure is changing when the bottle is shaken. It is obvious because the walls of the container become harder to move, because there are more molecules of gas in the headspace colliding against the container's walls. I don't see what more evidence you need. Given time, equilibrium will become reestablished and the container will return to the original hardness. I think that your assumption of a closed system is a false premise. Closing something off in a bottle doesn't shut the system off from the universe, as I can shake it, heat it, pass light through it, make gas come out of solution, etc. As CO2 leaves solution, the concentration has changed, and thus the equilibrium shifts.
When I first saw this discussion, I tried this out, and I have to say that I did not notice any change in pressure by feel in a 16 oz pop bottle. It wouldn't be too hard to use measurement system like this to see if there is any pressure change from shaking.

01-31-2013, 06:09 PM   #63
Bobby_M
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Quote:
 Originally Posted by Hex23 OK, trial 3 is finally done. It turned out to be a pretty tasty beer too! (afterall, this really is still about making beer) It started as a Goose Island Mild Winter clone, but I decided to add a little more rye than normally recommended. I tastes nice, but I know I wouldn't add any more rye than I did. Anyway about the experiment. I built an entirely new cap/bulkhead to try to eliminate the leak issue: Attachment 97711 I pressure tested it to 49 PSI for about 2 days and it didn't appear to leak at all: Attachment 97707 The priming was carried out in my fermentation chamber at around 70 F for 3 weeks. It does look like the pressure built to a plateau of 39 PSI and then diminished to settle at 35 PSI. The priming sugar amount should have put this at 2 volumes. At 72 F (as measured) that should have make a pressure of about 23 PSI at equilibrium. This is obviously well above that. Attachment 97708 Attachment 97709
This is an excellent effort. Did you ever explain where the temp probe was? I still think that the spike may be influenced by temperature or some other anomaly. I was searching for the thread that showed anoldUR's similar experiment and he didn't get the spike. It would be nice to figure out they the results vary.
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01-31-2013, 06:12 PM   #64
Bobby_M
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Quote:
 Originally Posted by dbsmith Pressure is changing when the bottle is shaken. It is obvious because the walls of the container become harder to move, because there are more molecules of gas in the headspace colliding against the container's walls. I don't see what more evidence you need. Given time, equilibrium will become reestablished and the container will return to the original hardness.
It is not obvious to me because I do not feel the walls of a soda bottle get firmer when I shake. What makes you think the CO2 molecules are coming out of solution faster than they are reentering? What would convince me is to measure pressure at stasis, shake and measure again.
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01-31-2013, 07:50 PM   #65
Hex23
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Quote:
 Originally Posted by Bobby_M This is an excellent effort. Did you ever explain where the temp probe was? I still think that the spike may be influenced by temperature or some other anomaly. I was searching for the thread that showed anoldUR's similar experiment and he didn't get the spike. It would be nice to figure out they the results vary.
Thank you.

You've probably sensed that I'm not ready to draw any conclusions just yet.

For one, the shape of the decay doesn't look like what I might expect. I suppose to really rule out leaking, I'd have to run a big study to establish how likely it is to leak (gasket placement, screw torque, etc) or get a CO2 detector. Neither of those is going to happen
Based on the results of the leak test I'm fairly confident in the capability to avoid leaks.

The temperature measurements came from an RTD with very small thermal mass (Auber Instruments) which was tucked behind that spounge you see in the picture. I decided against a true thermowell because of how complicated (with my skills anyway) it would have been to construct.

While I was getting ready to run the experiment, I thought about the physics behind the problem and I was convinced that the "rise to a peak, then fall" phenomena could happen, given one assumption. If the yeast generated CO2 faster than the surrounding liquid could absorb it, given the current equilibrium conditions), then it would evolve (not bubble) off and pressurize the headspace, then go back to equilibrium. But honestly I didn't study the absorption/evolution rate vs yeast CO2 production rate rate enough to know whether this was possible.

Absorption is a pretty slow process (as seen by common kegging practice) and evolution is a relatively fast process (as seen by the small amount of time it takes a beer/pop to go flat).

Your point about the other graphs people have drawn is a good one, which I had considered before I ran the experiment the third time. Up until 1/26, I just figured that my experiment was saying the same thing as theirs. But once the pressure started going down the question opened back up.

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01-31-2013, 08:09 PM   #66
Bobby_M
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Well, I love a good mystery so thanks for that.
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02-05-2013, 12:46 AM   #67
dbsmith
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Quote:
 Originally Posted by Bobby_M It is not obvious to me because I do not feel the walls of a soda bottle get firmer when I shake. What makes you think the CO2 molecules are coming out of solution faster than they are reentering? What would convince me is to measure pressure at stasis, shake and measure again.
I went to the store today and bought a soda bottle, just to be sure about this. Cracked it open and closed it back up tightly. The bottle was very easy to squeeze. I shook it up and now it is incredibly firm. The only explanation is that shaking caused gas to come out of solution. I don't think that there is any other explanation as to why the container is firm.

02-05-2013, 12:54 AM   #68
passedpawn
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Quote:
 Originally Posted by dbsmith I went to the store today and bought a soda bottle, just to be sure about this. Cracked it open and closed it back up tightly. The bottle was very easy to squeeze. I shook it up and now it is incredibly firm. The only explanation is that shaking caused gas to come out of solution. I don't think that there is any other explanation as to why the container is firm.
Did it get warmer?
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02-05-2013, 12:55 AM   #69
passedpawn
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I could do this test really quickly buy just filling my bottle w/gauge (see post #1 ), capping, shaking, and watching the needle.
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02-05-2013, 01:01 AM   #70
dbsmith
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Quote:
 Originally Posted by passedpawn Did it get warmer?
On a molecular level, molecules are colliding with each other and transferring kinetic energy to each other. Some have higher amounts than others. The heat is proportional to the average of the kinetic energy of all of the molecules in the solution. So the shaking may have increased the overall kinetic energy, but I doubt that a typical thermometer would show any difference in temperature. But who knows? Maybe it would..sounds like another experiment