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Old 06-23-2011, 03:49 AM   #1
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Default Johnson A419 controller - question about setting temp.

Just got a Johnson A419 controller that is being used on a 5 cubic feet chest freezer for a fermentation chamber, and I have a question about setting the temp for fermentation.

The controller is currently set at 63 degrees, but earlier today it was reading a temperature of 46 inside the freezer (which is empty, just testing it out). That seems almost unacceptably too different. At the moment it's reading 58 inside.

Can anyone explain the big difference? When it actually comes to fermentation I'm going to have the probe on the side of the carboy covered in bubble wrap for insulation from the air temp as much as I can. If I want to ferment at 65 degrees, should I set the controller to 65 exactly, or something different to compensate?

Going to be fermenting with it for the first time tomorrow, and just want to make sure I have my ducks in a row. Thanks!

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Old 06-23-2011, 04:16 AM   #2
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Was the reading of 46 taken shortly after you set the temp? Unless you opened the door to let warmer air in, then the fridge basically just sits until the cold seeps out. That can take awhile depending on how good the seals are. You have to remember, your fridge doesn't have a heating mechanism in it (unless you installed one). All the temp controller is doing is cycling on when the temp goes above your indicated temps. Once it hits the temp you need, it shuts off. If your temp outside is colder than the set temp, it'll be colder.

I use two of these, and they've been fairly accurate for me. I get temp readings within 1-2 degrees of my set temp, and that's perfectly fine for me.

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Old 06-23-2011, 04:28 AM   #3
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Originally Posted by Suthrncomfrt1884 View Post
Was the reading of 46 taken shortly after you set the temp? Unless you opened the door to let warmer air in, then the fridge basically just sits until the cold seeps out. That can take awhile depending on how good the seals are. You have to remember, your fridge doesn't have a heating mechanism in it (unless you installed one). All the temp controller is doing is cycling on when the temp goes above your indicated temps. Once it hits the temp you need, it shuts off. If your temp outside is colder than the set temp, it'll be colder.

I use two of these, and they've been fairly accurate for me. I get temp readings within 1-2 degrees of my set temp, and that's perfectly fine for me.
Thanks for the reply! Actually no, the 46 reading was kind of random. It held pretty steady at 60-61 reading for awhile, then just checked it at one point and it said 46. Not sure what the deal is.

Curious, but what's a good differential setting? It's just at factory setting at the moment (I think a 5 degree differential).
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Old 06-23-2011, 01:07 PM   #4
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I set mine at 2. I'd love to do 1, but I'm worried that cycling the compressor on and off that much might do damage to the fridge.

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Old 06-23-2011, 04:20 PM   #5
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That was my thought as well. I think I'll switch it to 2 or 3 degrees, but like you said I'd be a little hesitant to go with 1.

Hopefully the strange reading last night was just a weird error, since my wit is going into the chamber tonight!

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Old 06-23-2011, 05:02 PM   #6
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I have mine set at 59° w/ 3° diff. Once it hits 56° it shuts off, but afterwards, the temperature continues to fall all the way to 50° before it stops and starts creeping back up. Your thermometer may be measuring the metal wall temp which get pretty cold once the compressor's been on.

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Old 06-23-2011, 05:03 PM   #7
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Where was the temp probe? The controller is going to be forcing the compressor to run until the probe senses the cutout temp. Actually, that brings up another point. You can select whether it operates as a cut in or cut out temp which basically determines which side of the setpoint the offset sits at. Hmm, this is confusing to explain.

If you have:
Setpoint: 60F
Cut In/Out: Out
Offset/Dif: 2F

Once the temp rises to 62F it will close the relay and hold it until it senses 60F.

If you simply change the cut in/out to "IN" then it will close the relay until it senses 58F.

Also keep in mind that the ASD time will limit how soon the compressor can run again once it has shut off. This may cause some confusing operation if you're not aware of it.

If you set the ASD to 5 minutes, there's no reason you couldn't set the dif to 1F as the ASD will prevent short cycling as will keeping the probe attached to the vessel with all that thermal mass.

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Old 06-24-2011, 05:01 PM   #8
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Thanks for the reply, Bobby. I mentioned the settings to my brewing buddy (who has the freezer at his place) so he can take a look as make sure everything is in order.

I asked him what the temp was earlier today, and he said it held steady at 64 for a couple of hours (we have it set at 65), but then at one point he checked and it was reading 61. The beer is already in there, so I'm hoping to get this sorted out soon so the fermentation goes smoothly without too much temperature fluctuation. It's a wit, and that yeast is slow enough without sudden temp drops.

For reference, we have the probe on the side of the fermenter insulated from the freezer air temp as much as we can.

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Old 06-24-2011, 07:24 PM   #9
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So what I think you have going on is based on the offset/dif. You might have it set to 65 set point, cut in (turns on when it hits 65) and the dif might be 4 or 5 degrees which means it will run to 60 or 61F before it shuts off.

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Old 07-09-2011, 03:32 PM   #10
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Bobby - do you prefer setting the A419 to cut-in or cut-out mode? Is one method preferred over the other?

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