Temperature fluctuations in a conical

What is the differential variation on your controller set to?

 

I think this is what's happening...
When your controller responds to the (high) wort temp, it turns the fridge on until the wort temp cools down to the setpoint (or below, depending on your parameters). Because the fridge airspace cools down much faster than the wort, by the time your wort temp reaches the point where it will turn off the fridge, the fridge airspace has become way colder than the wort. Then the fridge airspace and the wort continue to move toward equilibrium, lowering the wort temp further than you want.

This is the reason I use a fridge, a heat wrap, and two controllers. I control the fridge airspace with one controller (with its probe in the airspace). I set this to maintain an average airspace temp about 5 degrees lower than the desired fermentation temp. Thus the fridge and its controller maintain the ambient temp. The second (heatwrap) controller's probe is in the fermenter thermowell and is set to maintain the wort at the desired fermentation temp.

 

Same question as tngolfer. Have you used the fridge for fermentation before? The fridge itself could just be going through its compressor cycle and have that amount of change.

 

The differential you have set on your controller is very key here. Not sure of the quantity you are talking about, but it takes a long time to lower the temp of 5 gallons of liquid a couple of degrees (think hours). Then it takes the same time for it to raise.

I echo Vikeman's ideas . . . even if your differential is only 1 degree, changing the temp of a large amount of liquid could cause larger swings than you expect.

An easy way to approximate the temp of your wort (before fermentation takes place) is to place a 750ml bottle of water with thermometer in your fridge. You will be surprised how stable it is, your wort will be even more stable because of a larger mass. I leave my controller's temp probe in the fridge's airspace with a diff of 8 and the 750 water temp is rock steady. Of course the water bottle doesn't account for the heat of fermentation.

You might consider using your temp probe in the airspace and put another thermometer in the thermowell. This defeats the automatic control of your temps and will require you to manually check the thermowell thermo and adjust the controller as fermentation decreases, but that is normally a once day deal only.

 

I wouldn't be too worried so long as you are getting consistent results. If your set up goes through the same process then that is just how your set up works. You should see the fridge kick on, when the temperature hits the right temperature it cuts out, and then there is a continued drop in temperature until the beer begins warming again. As long as the continued drop after the fridge cuts off and the warming afterwards is a consistent fluctuation then it's just the physics of your system. You can try to tweak but as long as you are getting beers you are happy with and consistent results there's no reason to be alarmed.

 

I think OP was saying that the wort temp is fluctuating by 4 degrees. If I would solve that if it were me.

 

Differential will take care of it.

 

You should be fine after changing the differential setting, since the wort temp swing you saw corresponds nicely with your setting. In my setup though, with a differential of one degree, I was getting a wort temp swing of a few degrees below the set point. The fridge (big chest freezer actually) would literally go below freezing by the time the wort temp shut the controller off, followed by a slow swing downward for the wort temp.

 

Even if you set your differential at 1 degree, that is too much of a swing for your wort temp. I suggest you remove your probe from the thermowell and replace it with a conventional thermometer to monitor wort temp. Then place your probe in the airspace of the fridge with a diff of around 8. You know fermentation raises the temp 4-6 degrees initially. Just use a set point to give you this average. I. E. if you want to ferment at 66 you want the wort to be about 61 (an average) to account for fermentation heat. So use a set point of 57 with a diff of 8. This should keep your wort at 61 plus the ~5 degrees fermentation will add. Use the thermowell thermo to cross check everything.

This method will have swings of less than 1 degree. As fermentation decreases you will slowly raise the set point, never more than once a day . . . using your thermowell thermo as a guide. It might take a session or two to find the sweet spot, but this method works well for me.

Keep your beer in the fridge while fermenting. A full fridge is actually more efficient than one that is near empty. Any liquid is perfect to help stabilize temps. Just insure there is enough room for air to circulate.

 

This is excellent advice. While probably not cold enough for serving (assuming you're fermenting an Ale), it's a good place to store the beer that might otherwise be sitting at room temp. Or simply fill the empty space with water filled milk bottles. Any thermal mass will do. This doesn't actually cause the compressor to run more efficiently, it simply reduces the amount of 'cold' that escapes when you open the door, keeping it inside where you want it, thus reducing the need for the compressor to turn on, which increases the overall efficiency of the system. (it also reduces the load on your home's A/C as it doesn't need to remove the extra heat generated by the compressor, but I'm only going to follow these dominoes so far )

 

But there's a butterfly in China waiting to hear what the imact on him will be.