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Old 04-12-2011, 03:49 PM   #1
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Default Johnson Digital Controller Problem - Not holding temp

So I keep setting my Johnson digital temp controller (attached to a new GE 7.2 cu ft chest freezer) to a specific temperature but it is continually stays 2-5 degrees (f) cooler than what I've set the controller to. For example, right now I have it set at 42 f (I'm not fermenting right now, only serving) but last night the controller read 38. This morning it says 40. For serving, this isn't a big problem but I'm about to brew another batch and I can't have a 5 degree swing in my fermentation temp! That's why I got the freezer and controller in the first place!

FYI, the controller and freezer are both less than four months old.

Am I missing a setting or something on my controller? It seems like it's fairly straight forward.

Thanks in advance for your help guys!

-Jason

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Old 04-12-2011, 04:08 PM   #2
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Assuming you are talking about the Johnson A419 controller, you probably want to adjust your Differential setting. I believe the factory default is something like 10 degrees F.

So, you can shrink that differential down, but what it going to happen is that it will likely cycle your fridge way more than is healthy for it. If you adjust the differential down, you ought to increase the anti-short cycle delay to make sure it's at least 10-15 minutes.

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Old 04-12-2011, 04:13 PM   #3
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Where is the sensor?? I have heard of different locations, but I think having it somewhat insulated, attached to the side of the fermenter, will be the best place.

The fermenting wort cannot change temps as fast as the air outside, due to it's thermal mass. So what you want to do is average your temps outside within a range that promotes the proper temp inside.

Also on my Johnson there were various settings beyond the setpoint. You can have it set to CUT IN or CUT OUT, set or Heating or Cooling and any combination of that. Then you can set the Differential and Offset. And there is an anti-short-circuit feature that tells the device to not cycle any more often than, say, 2 minutes, to help prevent your freezer from killing itself.


I'd have to read the manual again on the settings to remember exactly how it all goes together. I know that after I got mine set up, my heater inside my freezer, kept my temps withing a degree or so for the whole fermentation.

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Old 04-12-2011, 04:45 PM   #4
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Just tape the bulb to the side of the fullest keg halfway down. I set mine for a 5 degree diff, from 39f to 44f, giving me a nice 42f pour. Remember the ambient air will be cooler than the actual beer temp. After the freezer cycles it will drop a few degrees cooler than it's set at. The actual beer temp is the only important factor.

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Old 04-12-2011, 11:49 PM   #5
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Thanks guys! Looks like I can tweak the settings to dial it in a bit. Good call on the cycle times. I don't want to kill my freezer. This site and all if you on it are the best!

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Old 04-13-2011, 12:11 AM   #6
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If i remember correctly, you have to watch out for the cut in vs. cut out setting. In other words, if you set it to "cut out", it will stop running the compressor at your set point. It will start x number of degrees above the set point where x=offset. If you set it to "cut in", it will run the compressor at the set point and continue to run until x degrees below the set point where x=offset.

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Old 04-13-2011, 10:38 PM   #7
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I also discovered another factor you should try and consider: the thermostat setting on my fridge needs to be tweaked to work well with my controller setting as I found that if I had the fridge set too cold I would overshoot the temp setting on the controller and I would get the results you describe. My controller would cut out when it was supposed to but there was a settling out period afterward where the temperature would drop further. I fixed that by dialing back the fridge setting so it wouldn't run quite so cold.
Once you have looked over all the settings the guys above have mentioned you might want to watch the temperature as it rises back up to see when the controller kicks in. It might actually be cutting out at the temp you set but just overshooting afterward.

-Andy

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Old 04-14-2011, 01:35 PM   #8
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Quote:
Originally Posted by swfc68 View Post
I also discovered another factor you should try and consider: the thermostat setting on my fridge needs to be tweaked to work well with my controller setting as I found that if I had the fridge set too cold I would overshoot the temp setting on the controller and I would get the results you describe. My controller would cut out when it was supposed to but there was a settling out period afterward where the temperature would drop further. I fixed that by dialing back the fridge setting so it wouldn't run quite so cold.
Once you have looked over all the settings the guys above have mentioned you might want to watch the temperature as it rises back up to see when the controller kicks in. It might actually be cutting out at the temp you set but just overshooting afterward.

-Andy
You should be turning the fridge/freezer controller all the way down, so that it's always on. The temp controller will turn the fridge/freezer on. At that point the thermostat built into the fridge/freezer does nothing at all.
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Old 04-14-2011, 04:14 PM   #9
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Quote:
Originally Posted by Homercidal View Post
You should be turning the fridge/freezer controller all the way down, so that it's always on. The temp controller will turn the fridge/freezer on. At that point the thermostat built into the fridge/freezer does nothing at all.
+1 This is correct. The refrigeration compressors are either on or off. The OEM thermostat is just that; a thermostat, and basically so is the controller. There is nothing to be gained by wiring them in series.
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Old 04-14-2011, 04:29 PM   #10
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+2. Freezer/fridge @ max setting. The controller takes over on when to turn on/off.

Also, just b/c the johnson temp says 38 or 40+ doesn't mean the liquid inside the keg is fluctuating as wildly. 5 gallons is a decent amount of thermal lass to cool/heat so the liquid temp swings are usually only a few degrees.

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