Temp. Control - Can\'t Seem To Get The Temp Right

PortlyPorterGuy

Ok I recently picked up a fridge so that I can have temp. controlled fermentation. I bought the Johnson controller, set it to 50 deg and taped the temp. probe to the side of the plastic fermentation bucket and I taped some fiberglass insulation over the probe. Next time I'll put the probe in the beer but for this batch it'll do.
The problem is that the stick on thermometer on the bucket is reading 56 deg when the controller has the fridge turned off. The differential on the controller is supposed to take the temp down to 4 deg less than the setting so that isn't the problem. I'm thinking that the stick on thermometer is accurate, so why is there a 6 degree difference? If the stick on thermometer can see that it's 56, why can't the controller's temperature probe? Should I just trust the stick on thermometer and lower the setting on the controller?
-Portly

codewritinfool

I'd trust the probe more than the stick-on.  Do you have another measuring method?  Anything?
The controller will have hysteresis by design, knowing the size of the dead-band will help to explain the trip points.  The danger of lowering the controller's temp setting is that the lower limit will drop too.  Maybe someone that has this controller will know how accurate a temperature window you can keep with it.
My two cents, probably worth less than that.code

PortlyPorterGuy

I put the trusty Radio Shack thermometer in the fridge and it agrees with the controller. Guess I'll think twice about relying on the stick on. I'm not sure what made me think that the $1 stick on thermometer would be more accurate than the $60 controller.
thx

Speyedr

Are you using a "Fermometer" as your stick on?  Which color are you using as your reading?
From my aquarium days I recall that you use the brightest color for your reading, usually a green or blue.
Just a thought.
Rob

PortlyPorterGuy

Yup, it's a Fermometer. The Radio Shack thermometer in an 'indoor/outdoor' type w. a lead and probe for the outdoor reading which I tape to the bucket under some insulation. It tracks closer to the controller than the Fermometer.