Taking a chance on this one, but maybe it has to do with the way that lager yeast ferments from bottom to top, with the weight of a liquid on top of it?
To have something to go from, did the same thing occur with a sample of ale fermented in the same vessel?
Bouncing, like rising and falling in a rhythm like waves on a beach or like the rising and falling that happens when a hydrometer is plunked into a sample?
The vessel looks pretty nice and very functional the way it is, but maybe the correction is to add a side racking port, unless it places in jeopardy the integrity of the vessel. I have a Blichmann and it has a side racking arm that rotates, probably has something to do with the goop that falls into the bottom of the cone. Previous to that I used a racking cane. If the hydrometer was bouncy after using a racking cane and drawing from the bottom of a carboy, I never paid too much attention to it.
Try this the next time you brew the same beer, it has to do with lager, and "18 days it's not quite finished yet."
Don't lock into numbers on a recipe, malt is very inconsistent and you'd need to use the same malt that was used when the data was compiled. A spec sheet comes with each bag of malt. The data listed on a spec sheet comes from the various tests that are performed on malt and malt is tested because it is very inconsistent. The sheet is used for determining the quality of malt because to make ale and lager, depending on the level of modification, high modified malt may require the addition of enzymes to produce ale and lager. Skip a diacetyl rest, beer is krausened to make up for the beating lager yeast takes during a high temperature diacetyl rest. The rest is only a temporary patch, anyway. The precursor is still there and diacetyl reforms.
Let's assume that since OG is 1057 there is a lot of sugar in the wort, and that you are absolutely, positively, certain, yeast are still active. With yeast still active at 18 days, let's assume there's complex sugar in the mixture, because yeast would have ripped through simple sugar in 10 to 14 days, cranking out alcohol during the time.
Next time, after 10 to 14 days rack the beer off the goop into a fermenter, regardless of gravity, and transfer the beer into another fermenter. We want to remove the beer off the trub because yeast learns to love the crap and at 18 days it can happen. If CO2 is available purge the air out of the receiving fermenter. When the airlock is removed from the primary fermenter add CO2 through the opening as the beer drains out.
This is what happens during fermentation, yeast rips through glucose during primary fermentation because it really loves simple types of sugar and glucose is the biggy because it is one of the three building blocks of life. Yeast doesn't do anything with complex sugar, until all of the glucose is wiped out. The types of complex sugar that we are interested in are maltose, a disaccharide and maltotriose which is a tri-saccharide. The sugars form during mash conversion when Beta converts glucose into maltose and maltotriose.
Now, what takes place during secondary fermentation is that another type of conversion happens. During conversion, yeast absorbs maltose through the cell wall and an enzyme within yeast converts maltose back into glucose. The glucose is expelled back through the cell wall and it is used for yeast fuel, gravity reduces closer to expected final gravity.
So, now, were going to wait at least 10 days, no longer than 14 days, for the yeast to wipe out maltose, keeping fermentation temperature at 50 to 55F throughout secondary fermentation. After 10 or so days, transfer the beer into a keg, but do not add priming sugar, we're banking on maltotriose and yeast to naturally carbonate the beer which is much finer carbonation than sugar and CO2 injecting creates. Yeast does the same thing with maltotriose and during the carbonation/lagering cycle gravity reduces to expected FG.
Before filling the keg, pressurize the keg and soap the lid gasket, relief port and poppet valves and test for leaks. Purge the CO2 out the relief valve and keep the relief valve open while filling the keg through the liquid "out" poppet, bottom to top. Depending on the volume of CO2 in the beer it may foam in the keg and the foam comes out of the relief valve to let you know. Store the keg at 50F, and at two months lift the relief and see if gas comes out, if it does all of the ducks lined up. Blow off some beer and goop if there's pressure. With gravity at 1057 it may take a little longer than two months. A month later lower the temperature to 45F, a month later 40F for a month, then, to 35F for the duration of the clearing, carbonation, lagering phase. It's best to keep a pound or two of CO2 pressure on the keg with a CO2 tank and regulator because the beer will absorb CO2 when it cools down and pressure reduces. So, to make taking samples easier keep a little pressure on the keg when it's cold. A 1057 beer, around eight, nine months it rounds out.
I brew lager with Weyermann floor malt and use the decoction method. I haven't brewed ale for a bunch of years. When I made ale I used under modified malt similar to Weyermann floor malt with the decoction method and lager yeast. The decoction method makes all that stuff that I mentioned work out.
When you have a moment check out the recipes on Weyermann Malt website.
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