Carboy in cooler

For my 4th batch I was going to try putting my BB carboy in a cooler as some have done here. Over the past few days I did some tests on the temperature using frozen water bottles. My plan was to put two water bottles in the cooler in the morning then switch them out again at night for the duration of the fermentation.

My tests show the water temperature goes to around 50F when the bottles are first added. But around 10 to 12 hours later (as the ice melts) the temp can warm up to 60F or so. I've read others who post that you can expect the brew to be warmer than the water temp but that isn't what worries me.

Is the swing in temperature every 10 hours or so going to stress the yeast? Would it be better to use no ice so the temp remains constant? I think my previous batches were too warm just sitting at room temp so I'm trying to do something to lower it but I'm not ready to try something extreme like a home built chiller just yet.

How much water is in the cooler and what size water bottles? I have one the large square coolers with extensible handle and wheels that a 6 gallon carboy fits down in exactly. The water comes up maybe halfway up the side of the carboy. Once I get it to a temp I want a one to two 20 oz water bottles a day allows me to maintain a steady temp on the carboys adhesive thermometer so I don't think you'll actually see temperature swings that drastic. It's just too much liquid to quickly heat/cool. Just pay attention to ambient temperature and make sure you get a ahead of it. If you want to keep it 70 degrees, and it currently says 70 degrees, but your house is 74 you know you need to keep ice in. I have found I rarely use even two bottles at a time, but it depends on your temps and cooler.

Good point, I forgot the details.

The water bottles are 20oz each. The cooler is probably close to this Igloo 60 quart but mine doesn't have wheels or a handle. It's almost the entire height of the carboy. Right now I only put maybe 5" of water to run some tests. I didn't want to waste water but now that I think about it, my test isn't too accurate without more water. After putting two bottles in this am, in only maybe 6 hours the ice appears melted already.

I need to get one of those sticky thermometer things for the carboy.

I don't think you'll see swings like that once you get everything in unless your place stays really warm (my place is between 72 and 76). The final result will be a cooler full of probably 7+ gallons of liquid (remember the brew itself is included). That doesn't change temp fast.

That being said if you find it does, your going to have problems with stalling fermentation and off flavors. Definitely drinkable and probably could be corrected by longer fermentation times or aging, but won't be the best you could do. Yeast don't really like temperature changes of even 5 degrees if it happens fast (<12 hrs).