I just used a BrewJacket for the first time. Here are the device specification that might help you decide how you would want to use it. It’s rather slow at cooling, so best suited for maintaining temp.
From the spec sheet:
0-10 degrees F Above/Below Ambient: 0.5 to 1 degree/hour
10-20 degrees F Above/Below Ambient: 0.25 to 0.5 degree/hour
20-30 degrees F Above/Below Ambient: 0.1 to 0.25 degree/hour
“Maximum achievable temp from ambient +/- 25 to 35 degree F
Performance is dependent on the fermenter/ jacket combo. Carboys and buckets generally see better performance than conical style fermenters.”
My own experience:
During the height of fermentation we ended up throwing two cold packs down into the insulated jacket to help with the cooling, mostly because the unit was running a lot and the AC adapter was getting hot, which bothered my engineer husband. It does use 85-95 watts while cooling. We were able to maintain a steady temp of 68 F with the max ambient temperature of 78 F.
Now that primary fermentation has trailed off, the temp inside the vessel stays steady at 65 F and the unit rarely runs.
We have the temp probe taped to the outside of the bucket. What I observed is that if you do add a cold packs to the chamber, the air temp is of course going to drop quickly, which does not equate to the temperature of the liquid. So watch the immersion unit to make sure it doesn’t start to heat your beer! I noticed this right after my husband dropped in some cold packs, and the air coming out of the immersion unit was cold, not warm. If this happens, temporarily adjust the set point lower or unplug the unit while the temp equilibrates.
Another thing to be aware of is that the rod must not be directly exposed to pH levels below 4.0 or above 8.0 but you can use a protective sleeve (mine came with 2). Sanitizing agents like Star San and PBW could damage the metal rod. The instructions include alternate methods of sanitation. Iodophor or IO-Star are recommended. You could also sanitize a rod sleeve with Star-San, which is what we did.