bellybuster wrote: A good way to look at it is if we put a 75 degree fermenter in a chamber and wish it to be 68 degrees and we control by ambient temp, the fridge will run until it hits set point of 68. Logical.
Once the chamber hits 68 and the fridge turns off the beer will start to release its heat energy to the cooler air. Being almost 1000 times greater thermal mass it will take a better of minutes for that chamber air to warm up and restart the fridge. The beer will have released a fraction of a degree. The cycle will continue over and over again until they reach equilibrium.
Now lets do the same with controlling by beer temp. Same 75 degree beer same 68 set point. The fridge will run continuously until the beer hits set point and shut down. The air in the chamber will be much colder than the beer so will in fact continue cooling it. Considering the difference in thermal mass, the beer will lose maybe a degree to the chamber. Fridge will not run again. At least not until the insulation qualities of the chamber show too much loss or the exothermic reaction of fermentation warms the beer above limit.
The big difference is by controlling the beer temp we are removing heat directly from the beer, not the air surrounding the beer. The temp of the air around the beer is now left to the mercy of the thermal mass of the beer which is much much greater.
This part I agree with (up to a point), especially if there is nothing else in your chamber. If you have 4 or 5 kegs in there already at fermentation or lagering temperature, it will not take long for the new keg to cool down because of the thermal mass of the other kegs. It might cycle for half a day or so, but then become stable. You mentioned that if you put a keg at say 68 degrees in the chamber, the freezer/fridge will basically stay on for days. That was one of the issues I had - the air temperature in the fridge/freezer will bascically get down to freezing temperature or below, making it useless for storing other beers, etc. in the meantime. Adjusting the ambient temperature keeps it within a small range, so it does not affect any other contents in the fridge/freezer. That was one of the key points I tried to make earlier.
The second point was the stability of the temperature in the keg/carboy. I totally agree that if you first put it in the freezer/fridge, AND THERE ARE NO OTHER KEGS ALREADY IN THERE - then yes, it will take a long time to cool it down. The freezer/fridge will indeed stay on constantly until the keg/carboy has reached an equilibrium. However, once it has,m then I would argue that controlling the ambient temperature is a far more efficient way of adjusting the temperature in the keg/carboy than measuring the temperature directly inside the keg/carboy. In BOTH WAYS, the freezer will come on in more or less the same way if the temperature of the keg/carboy gets too high. Either the ambient temperature of the beer temperature will not go above a certain set range i both ways. It is only when the beer starts to cool where the difference begins. When measuring the temperature of the beer directly, the fridge/freezer wil come on when too hot (no different than the other way), BUT the freezer will stay on constantly until the temperature of the beer has reached its set low point. Meanwhile, the freezer/fridge will reach close to freezing again, if not below, making it useless for storing other bottles of beer, wine, etc. By adjusting the ambient temperature within a narrow range, the temperature of the freezer/fridge will not drop down to freezing temperatures. It will stay within the narrow range, and the beer temperature adjusts to the mean of that range, keeping its temperature very stable. The ambient temperature will fluctuate a little, but because of the thermal mass of the beer, its temperature will fluctuate very little. When measuring the beer temperature directly, the freezer/fridge stays on constantly, drops to freezing temperature, and the beer temperature will continue to drop until the temperature control device says it has reached its lower limit and urns the fridge/freezer off. Since the freezer/fridge is near or below freezing temperature, it will continue to cool the beer BELOW its lower limit until equilibrium has been reached. That is purely logical.
So let's compare the two approaches from a logical point of view. In BOTH approaches, the beer will not go above a certain set limit. So there is no difference as to the upper range of the beer in both cases. Now let's look at the lower temperature. When measuring the ambient temperature, the fridge/freezer turns off when it reaches its lowest point. Thus, the beer cannot go below that point. Because of its thermal mass, it will always be somewhat ABOVE its lowest set point. Now, when measuring the temeperature directly, as I have already pointed out above, the beer does reach its lowest set point - AND THEN CONTINUES TO DROP A LITTLE BELOW THAT POINT. So - simply from a logical point of view, which of the two approaches results in significantly les fluctation of beer temperature. There can only be one conclusion.
So, let's look at the pros and cons of each case and see if we can't come up with a compromise solution that results in the best overall approach.
Measuring Ambient Temperature
- less fluctuation of beer temperature
- can use fridge/freezer for other storage (beer, wine, etc.)
- takes a long time initially for beer to reach its desired set range of temperature, especially when the fridge/freezer is mostly empty of other kegs/carboys. If there are other carboys, kegs present already at the desired temperature range, then this 'Con' becomes much less of an issue.
Measuring Beer Temperature Directly
- takes significantly less time to reach desired temperature range
- more fluctuation in beer temperature range
- cannot reliably store other items in fridge/freezer - especially other carboys - because of a much greater range in ambient air temperature, possibly going down to freezing range
- basically can only ferment one keg or carboy at a time since only one is directly controlled by temperature.
How can we make the best of both approaches? Simple.
1.Make sure there are no other carboys, kegs, beer or wine bottles etc. in the fridge/freezer when you put you caroby/keg in.
2. Attach the temperature probe directly to the outside or inside of the keg/carboy. Leave it in there until the proper temperature range has been established.
3. Put all the other items (beer, wine bottles, etc. back in the fridge/freezer.
4. Take the temperature probe off the side of the keg/carboy or from inside, and leave it in the air chamber, about half way up the height of the keg/carboy.
Now you can cool a beer down quickly, keep it at a more stable temperature, and enjoy the use of the fridge/freezer for other items that either should not be frozen or cannot be subjected to large temperature ranges. Also, in this way, you can control the fermentation or lagering temperatures of many kegs/carboys rather than just one. Each will stay at the same temeprature range. I have done that many times.