Surprising result of first experiment with fermination chamber

[sensibull]

I'm in the process of converting an upright freezer to a fermentation chamber. A Love controller is on its way, but in the meantime I decided to do a little experiment last night.

I filled an ale pale with a couple gallons of water and put the probe of a digital outdoor/indoor thermometer into the water. Then I plugged in the freezer. When the temperature registered 65 I pulled the plug to see how long it would hold that temp (I was curious about compressor cycling times). I figured the temp would continue to drop slightly before leveling off, but was shocked when nearly a hour later the temperature was still dropping. It went nearly all the way to 50 before it started climbing back up.

Can I chalk that result up to the energy required to go from ambient (77) to 65, and assume I won't have such a wild swing when the temps have steadied more? I mean, I use the same method for my kegerator and haven't gotten any frozen beer.

Just trying to fine tune the process before I actually begin to ferment...

 

[motobrewer]

well, was the freezer in steady-state when you pulled the plug?

if you have 77F water in a 77F freezer and plugged in it, the freezer starts taking the freezer air temp down. the water temp will start to drop as air temp drops obviously, but it will lag a bit.

so, your are measuring water temp. by the time the water gets down (lot of mass, relatively inefficient cooling compared to freezer to air, etc), the air inside your keezer is way lower than 65, probably below 0. so that's why you saw the big dip below.

take a look at this pic:
http://en.wikipedia.org/wiki/File:Change_with_Kp.png

follow the red line from zero, you basically cut the power at 4. if you let it settle itself out, it probably won't fluctuate much around 65F

 

[Airborneguy]

This is just an assumption, but I think that what you probably experienced was the water being slower to cool down to the temperature of the freezer. So when you shut it off, the freezer compartment was already near 50, but the water hadn't gotten that cold yet. As time went on, the water chilled to the temp of the freezer, even though the compressor was off.

 

[sensibull]

well, was the freezer in steady-state when you pulled the plug?\

Hmmm... I don't think so, because it was operating with its normal "freezer" thermostat, i.e. trying to get to 0.

In any case, I think you've answered the question, which means I should have the freezer running and in a steady state (i.e. liquid measured at 65 and holding steady) before I put the brew in to ferment.

I've read of some using freezer pack gel to mount the temp probe in. Would a jug of water essentially do the same thing?

 

[motobrewer]

well, what do you do with your kegerator?

the issue is that the beer is going from 77F to 55F or whatever. so, if you set the air temp to be 55F, there will be no undershoot but it'll take longer. if you put the probe in the beer there will be undershoot but it'll be quicker. if you set the air temp to 0 and constantly monitored the beer temp, switched it over to monitoring beer temp right at 55, then you'd be perfect. but you'd have to stare at your kegerator for awhile.

 

[sensibull]

well, what do you do with your kegerator?

Good question. I've just been using air temp in the kegerator, but I keep wondering whether placing the probe in water would reduce cycling.

 

[passedpawn]

Good question. I've just been using air temp in the kegerator, but I keep wondering whether placing the probe in water would reduce cycling.

Uh, yes. Best thing to do is tape it to the side of a femermenter (with a piece of insulation, cloth, etc over it). But putting it in a jar of water works well.

 

[jason.mundy]

Measure the fermenter, not the air.

You said this happened with "a couple of gallons". I am hoping the results will be less dramatic with five gallons.

Yeast produce heat. This should also lessen the effect.

 

[Spintab]

Measuring the fermenter is the reason he is getting this result. If the probe is in the water, the freezer will cool to the lowest temp it can until the probe senses the shutoff temp. The water will cool much slower than the air so like others have mentioned the freezer will be at a temp much lower than the water by the time the compressor shuts off. If all you are doing is turning the freezer on and off(normal behavior) the best thing to do is stick a probe on the fermenter or even better in the fermenter to MONITOR temps. Then use the thermostat to regulate to whatever AIR temp keeps your wort closest to your target temp. This is such a huge misnomer I see here all the time and I don't get it. If you stick your thermostat probe on your fermenter, the freezer is going to drop way down every time it wants to cool the temp around the probe, taking your wort temp down with it.

If you want your temp to stay dead nuts on your target, you need 2 probes and a circuit or software to dynamically regulate your temps. It would have to know how much to compensate your target air temp when your yeast are really cookin and how far to drop the air temp in response to wort temp rise.

Of course this all depends on how big and how well insulated your freezer is. It just may so happen that I sound like a jerk because just enough heat gets back in once your wort reaches it's temp to regulate perfectly.

 

[motobrewer]

yeah, exactly.

the main issue is that a freezer isn't something that can be proportionally controlled like a propane burner. you can't run a freezer at 50%, it has to be full on or full off.

also, when you cut power to a freezer, it will still continue to cool because there is still coolant in lines.

the best way i think is to monitor beer temp and not pitch until the system is at steady state. you'll still get variation, but it's minimal.

 

[jason.mundy]

I guess I am not having the problem with the over shoot since I am using a fridge. It is not trying to go down as low as his freezer.

So that could be one reason why I am not seeing the overshoot.

I am usually within 5F of my target temp when I put it in the Fermentation Chamber as well.

Jason

 

[Spintab]

Exactly why I put in my caveat at the end. Under the right circumstances it works fine.

I personally use a son-of-fermentation sort of thing but wrapped in ply with hinged lids. I think I prefer it to a fridge or freezer because it can cycle more often. An Arduino, thermistor, and relay turn the fan on and off a minimum of every 30 seconds. Between cycles, the air in the chamber may change as much as 4-5 degrees but the wort stays within a degree once the bulk of the fermentation settles. While things are really cooking I do have to babysit it a little but I usually drop the target temp a few degrees and it's fine. And of course I have to change out the jugs of ice every couple days but coming from changing ice packs in a swamp bath every few hours, that's fine with me. Who doesn't peak at their baby at least once a day anyway?