Temperature controler

I've put together a temperature controller for my freezer using factory circuit boards (ebay) and a couple wallwort power supplies(goodwill).  It works wonderfully for remote temperature control (wifi html interface or ip) but I've run into a problem I'm  not sure how to solve.  My temperature probe, DS18B20 has a lag time of several seconds reporting temperatures.  I can target 60 degrees F and turn off at the exact temperature but the delay in reporting causes an over shoot of up to 10 degrees, usually 5.

Does anyone know the formula for anticipating over shoot and compensating?

BTW: all the parts come in under $100 but require assembly ( drill, cut, solder, glue ).  I wonder if enough brewers were involved if it would be viable to add a forum for just such a project?

Do you have anything in your freezer?  You need some form of mass, ie a carboy full of water.  Without that the air temperature will swing widely and rapidly.  This is experience talking

That, and taping and insulating the probe to the carboy, so it's reading that specific temp more than the general ambient temp of the air in the freezer may help.  Liquid changes temp much more slowly than air. 

My next step was to put some warm water into a bucket and put it inside my freezer.  I'll just have to do it before I get the temp swing under control.

I have three temp probes, one for ambient(garage), one for freezer and another for the wort.  If I use wort temp for my control my freezer will keep running on until the wort chills then shut off but the box will be frozen and the wort temp will continue to drop.  If I use the free air temp probe it will take lots of time to get the wort temp right.  Maybe the temp over shoot would be good in that case...

hmmm.  I do love a puzzle.

I ran a test with 5.5 gallons of water and here's what I got.  I realized that a fan inside the box would make the probe react to temperature changes more quickly and I had a little battery powered desk fan so I put it in there to.



INITIAL
TIME 12:00
AMBIENT 80.4
FREEZER 84.3
WORT 101.8

TIME 1:00
AMBIENT 81.5
FREEZER 80.0
WORT 97.3

TIME 2:00
AMBIENT 82.0
FREEZER 80.7
WORT 94.0


MAX OVERSHOOT 1.2 Deg F.

It's obvious that the closer the freezer temp is to the target temp the slower the change in the wort temperature.  more figuring...

Chasin'
You wouldn't normally use the freezer to actually chill the hot wort.  That specific heat of air is very low, in other words it is bad at transferring heat.  You should be place room temperature wort in the freezer and only moving the temperature a few degrees.  The freezer is mainly to maintain a temperature.  Plus or minus 1 to 2 degrees is pretty good.

For my test run I used hot tap water, 101Deg F.  In a real live temp control I would do as you say, a couple degrees. 
I did find a formula for heat transfer and the rate of transfer is directly proportional to the surface area exposed and the difference in temperature.  I'm mostly convinced that the temperature over shoot will be helpful rather than a liability.

My plan now is to use the wort temperature to control the freezer temperature but use the difference between the two to calculate a reasonable over shoot so the wort temperature will react more quickly. 
So I would turn on the freezer when the wort temp rises above my target temp then turn it off based on the freezer temperature.  The freezer turnoff temp would be calculated using the rate of heat transfer so the turnoff temp would gradually rise while the wort temp gradually decreases to target.  hmmmm.