[Biran]

I have noticed some swings with my fermentation fridge that is plugged into a controller.  I brewed on Saturday and had the controller set for 66*f.  I monitor the temps every few hours and found that the wort temperature had reached 68*f at one point and the fridge was not turning on  Eventually it brought the temp down to 65 which activated my heater to bring it back up to 66.  Is the controller or the fridge to blame?  In case it matters I use a thermowell that is submerged in the fermenter.

[Stevie]

Having a probe in the middle is nice, but you have discovered its fatal flaw. It takes too long for the cool/heat to get to the middle. By the time it kicks off, the fridge (or heater) is still doing its thing. A smart controller (PID) would learn the overshoot and compensate.

Try attaching your probe to the side of your fermenter. I've been doing this for a while now and like it well enough.

[duboman]

Agreed, I use a block of pink foam insulation with a slot cut in for the probe. This gets bungeed around the fermenter and works really well.

I also question the need for both heat and cooling. If set up correctly there should be no need  to use the heat, the freezer should just kick off at set temp. You may be causing things to cycle too much in both raising and lowering temps repeatedly.

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[Hand of Dom]

I use plumbers clay to attach my probe to the side of FV.  It remains sticky across multiple uses, and helps insulate from the ambient temp in the fridge.

[narvin]

There is nothing wrong with using a thermowell.  The temperature of the wort is what matters, and you should be able to keep it within 1 degree of your target, since it has a lot of thermal mass.

It sounds like something may be wrong with your setup.  If the cooling delta on the temp controller is set higher than 1, it won't come on until temperature reaches your set point plus the delta.  As for the heating, you have it set too tight. Do you even need heating?

[kramerog]

Are you reporting temperatures as measured by the controller or something else?

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[Biran]

Do you even need heating?

I have experienced the temperature dropping below my target over night so I added a heater. 

[Werks21]

heating and cooling makes for a all season ferm fridge that can handle lagers and ales and D rest and cold crashing with ease, so yes you do need heating and cooling to have a fully functional ferm fridge in most parts of the country.

on the subject of set point vs beer point, this is an issue I struggled with after converting a fridge with an stc-1000 and a low watt heater. The problem is you set one temp and get another. Very frustrating. I had read about many probe placements in ferm fridges including themowell, air, taped to FV with insulation, Zip tied to a tall boy, and submerged in a cup of water. I went the air route because it was easy to implement and maintain however I was not able to adequately control my wort temp despite my best efforts. So I launched a study.

I used a bucket of water with a thermometer in it and recorded the following obsessively for quite a while.
STC set temp
CA value (temperature correction value setting on STC)
STC indicated temp
Air temp (inside ferm fridge)
Liquid temp

The study was conducted in cooler weather using the heater with a set point of 65 F
I collected data for several different CA values at the common set point of 65 F
I carefully poured over, processed, deconstructed, and evaluated the data for the 5 factors and each CA value was compared against each other. results are as follows.

For my particular fridge, probe placement and STC, CA- 4 turned out the best results by maintaining liquid temp the same as the set point within +/- .25 degree (hows that for a "dumb" controller!)
All CA values maintained decent control over liquid and air temps (however far off they were), but the CA values closest to 4 did the best in that regard as well.
The air temp was often way off from the set point and liquid temp which is confusing at first, but once dissecting the data I realized that this did not matter. only set point and wort temp need to match.
That is the relationship that matters most. make them match and you are done. I REPEAT, MAKE THEM MATCH AND YOU ARE DONE.  Air temp and indicated temp will bounce around will probably not match up or make sense at a glance (Temp wise). Just mind your set point and liquid temp at any given CA. its that simple.

The down side to using air for the probe is that the movement of wort temp is slow if the wort is not near set point.
That is to say that if you put your FV in the fridge with a temp 15 degrees warmer than set point, the fridge will hold your set point (since the probe is in air the set point is reached), no cooler.
 This will take significantly longer to cool the wort than a fridge running full tilt (lets say 33 degrees) because the probe is in the liquid which is well above the set point, which in turn drives the fridge to keep cooling.

however if you are aware of and make sure to account for this short coming it is not a big deal, and saves fussing with thermowells and tape and such. Not to mention extremely accurate wort temps....

This is some hard earned info. Hope it helps.