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Home Brew Forums > Home Brewing Beer > Equipment/Sanitation > Temp Controller internal resolution Johnson vs Control Products
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Old 01-06-2013, 04:24 PM   #1
pelipen
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Default Temp Controller internal resolution Johnson vs Control Products

Question about the internal resolution of Johnson specifically. I'm not sure. It's currently out of pocket.

Here is my issue:
I got the Control Products two stage controller, because i wanted a second, and the digital two stage is very price competitive with the Johnson single stage. However...

It seems to have an internal resolution of one full degree. If you have a setpoint of 60F with a 4 degree Hysteresis in cool mode, you would expect it to kick on at 60F, then kick off at 64F. What actually happens is it kicks on at 60.9x and off again at 64.x. It's like a floor function internally. 60.0, 60.1, 60.2 .... 60.9 are all represented as 60 for purposes of controlling the relay.

So if I want a setpoint of 60F with a 4 degree hyst., I actually have to set it for 59 with a 5 degree hyst. Doing that will kick on at 59.9x (effectively 60) and off at 64.

Does the Johnson A419 work the same way? I'm looking for another one, but at this point, I'm almost thinking to go the arduino route and build my own with 0.1 degree resolution. Then I could have better calibrations as well. I know this is off by 1.25 at the desired setpoint, but I can only set full degree offsets (A419 is the same i remember). I know these aren't commercial units which cost a fortune, but I would expect at least resolution equal to the accuracy of the probe.

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Old 01-06-2013, 08:24 PM   #2
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Sounds like questions for the engineers at those respective companies. The published accuracy of the Control Products unit is +/- 2 degrees F or C and the spec isn't listed in the Johnson Controls manual. I'm pretty sure the displays don't show tenths of a degree and I would think any rounding to a whole number for the display would also apply to the relay driver - but I don't know for sure. If you require high accuracy and resolution, I don't think you'll find it in the price range of these units.

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Old 01-06-2013, 08:45 PM   #3
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I think you're over thinking this. One degree isn't going to make or break your beer.

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Old 01-06-2013, 09:08 PM   #4
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Quote:
Originally Posted by ShinyBuddha View Post
I think you're over thinking this. One degree isn't going to make or break your beer.
hehe, yeah, no doubt. They easily keep my fermentation and keg temps where I want. I had another application come up where I wanted a bit more precision and just noticed this behavior playing around today. You can't set a 1 degree hysteresis with the control products unit or it just flutters like a butterfly. I need an excuse to buy an Arduino or Raspberry Pi anyway.
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