[wildknight]

I have set up a small chest freezer with a dual temp controller, heat wrap and thermowell for controlling fermentation.  After fermentation is complete, I crash the freezer down to drop all of the suspended yeast before I transfer to a corny.  Currently, I have the fermentor sitting on the bottom of the freezer.  I have noticed that if I drop too large of a temperature in a short time, then it gets some frost/crystals on the inside edges of the carboy.  I am almost convinced, too, that repitches from these beers contain a lot more dead yeast that past batches.  I believe that the direct contact of the carboy with the freezer bottom is cooling the glass colder than the set point.  Since the yeast cake is in contact with the glass on the bottom, it may be causing yeast to die and occasionally frost to form up the sides of the carboy.  By the time the temperature of the probe reaches the set point, the glass is effectively acting like the metal lining of the chest freezer and is well below the set point temperature. 

Has anyone else observed this with their system?  I figure putting a piece of foam on the bottom of the chest freezer will resolve the problem.  Is it common to do this?

[tygo]

I've had airlocks freeze before when dropping the temp down quickly.  So I usually step it down gradually so the freezer isn't running for a long period of time.  I like your foam insulation idea to solve your problem.

[Gribble]

I have in the past run into the same problem, and I lined the bottom of my freezer with reflectix.

[macbrews]

I have some wood slats across the bottom of mine to keep the carboys off the the bottom of the freezer.

[metron-brewer]

The pink foam works great for this, just watch out for moisture collecting/sitting under the foam.

[jaybeerman]

The pink foam works great for this, just watch out for moisture collecting/sitting under the foam.

+1  even just a few strips of foam help with carboys