Johnson Analog Controller Accuracy Question

I recently bought a Johnson Analog Temperature Controller to control the spare fridge in my garage. The goal is to main fermentation temps in summer... But for now my target temp for this fridge is 40-42f for cold conditioning some ales I brewed and also cold crashing a 6 gal batch.

I have the probe in my spare fridge (which is set to it's coldest setting), buffeted by the cardboard box and bubble wrap it came in.

However, in order to get that fridge to stay on and reach that 40f temp (as verified by 2 stem thermometers inside the fridge) I am having to set the Johnson Controller to 30f, otherwise, it is switching off and rising above 40f.

When I had set the fridge to 40f on a fairly warm (70f) day, it crept as high as 60f!

Does that seem correct? Should I need to crank the thermostat that far below my target temp? I would presume that if I wanted my fridge to kick on at 40f....I would set the thermostat at 40f.

Do you think I have an inaccurate probe?

 

Try it without insulating the probe. Tape the probe to the side of fridge about 1/3-1/2 up from the bottom. Allow time for temperatures to stabilize and try to keep the door closed as much as possible for the first day or so then check it. I think that the insulation over the probe is causing the problem.

 

I'll give that a shot. What kinda tape are we talking about here?

 

I had the same problem with mine and it was just down in the spare fridge in my basement. Had to keep it somewhere between 50f-55 just to keep my fermentation temps in the low 60's. I'm a little disappointed with it.

 

The dial on mine isn't terribly accurate but the 4 deg differential range is. I just go by the thermometer inside the fridge and don't pay much attention to the dial.

 

Just use some packing tape and tape it to the side of the carboy/keg or whatever you need to keep cool. I have been using mine for 3 years and my fermentation temps stay within 1-2 degrees of what it is set at. I have checked it against my digital thermo and fermometer stuck on the carboy. You need to focus on the temperature of the liquid. Just slap it on the side with some tape and you'll be fine.

 

Any kind. It doesn't even need to be taped, just something to keep it in position and out of the way. You could also attach it to the fermenter or keg as others have suggested. I make adjustments based on the thermometer readings instead of relying on the controller dial alone.

Be sure to allow plenty of time for the system to stabilize when evaluating any changes you make, including and especially when adding/removing fermenters or kegs from the fridge or freezer.

 

I pulled it out of the box & bubble wrap and that almost immediately fixed the issue. I outsmarted myself with that one.

Thanks All.

 

I had the same problem with one out of my two. The one that was accurate had the whole capillary inside the fridge. The first one I built I just had the probe in the fridge and the rest of the capillary was rolled up and taped to the outside. I don't know if this was why it was off but I plan on fixing it soon to find out.

 

I have three of the analog Johnsons and the excess capillary tube is coiled up and outside on all of them and they have been working just fine that way. Only the bulb part needs to be inside.

 

Bummer. Well I guess it's no big deal to just compensate for the in accuracy.

 

Let it settle out through out the day and it is back to its old tricks.
It thinks:
30 = 40f
40 = 50f
I guess I'll just need to invest in some tiny numbers for the thermostat controller wheel.

 

Morebeer.com stepped right up and shipped a replacement controller to me today (with a return tag). Hoooray for Morebeer.