There will always be a temperature gradient in the freezer from top to bottom unless you install a fan to circulate the air. Putting the probe in a container of water causes the controller to lag considerably. What happens is that the compressor will turn off when the temp drops to the set point minus the differential. The way you have it set up, the compressor will turn on at 40 and shut off at 39.9 with the 1 degree differential. The problem is that the freezer evaporator (chilling ) coils are in the walls of the freezer. The walls get very cold and stay that way for some time after the compressor turns off. The temp will continue to drop for awhile and undershoot your target temp. The reverse happens as the freezer warms. The water bottle with the probe will not warm up very quickly and as a result you will overshoot the target temp before the compressor turns on. Warm air rises and the temp near the top of the freezer where the beer lines and taps are located will likely be warmer than you want. This can cause foaming problems and the beer won't be very cold initially out of the tap. So, you might think that turning down the controller temp might help, but that can sometimes result in the bottom of the kegs freezing. The bottom is where the beer out dip tube is located. If it gets blocked by even a small amount of ice, you won't get any beer out of the tap at all. I've been through all this, so I'm speaking from experience. My solution was to install a muffin fan inside the freezer to circulate the air and I have the temp probe near the top of the kegs. I usually have the controller set to 40 F with a 5 degree differential, so the air temp drops to 34.9 very briefly and then slowly rises to 40. I've arrived at these settings through trial and error. You may prefer different settings depending on how cold you want to serve your beer. You can get a good idea of how well everything is working by measuring the actual temperature of a poured beer. It's nice to have the first beer pour at the same temp as the subsequent ones.