Temp controller accuracy

I would say you're meat thermometer is likely to be less accurate than the temp controller. Try putting the probes in a glass of water, this will help them give you a reading quicker

 

Stick them both in a glass of ice water and see if they read 32F

 

Test your meat thermometer in ice water. I don't think the Ranco probe is submersible, but the ice water should give you a sense for whether the meat thermometer is off from 32 F. You can also test it with boiling water to get a two point calibration curve which you can used to evaluate your meat thermometer across the range of temps most significant to homebrewer.

 

I tape my probe between my fermenter and a big hunk of foam insulation, with the hope that it equilibrates to and the controller responds to the fermenter temperature.

 

You can buy stopper thermowells from a couple of online stores but its pretty easy to make one. If you're using a 6.5 gallon arbor, grab a #7.5 rubber stopper and drill 2 1/4 inch holes in it. I picked up SS tubing from here http://www.brewershardware.com/16-Stainless-Steel-Thermowell.html . SS tubing goes in one hole of the stopper, airlock in the other. And the temp probe of the controller just goes down inside the tubing into the center of the wort, giving you a pretty good read on fermentation temp.

 

I have thermo wells that I sometimes use. I just took old dip tubes from Corny Kegs and had the one end crimped and welded. Most of the time though I just wad up a few sheets of paper towel and use duct tap to create a sort of "bandage" that encases the probe to the side of the fermenter....

 

I used a computer fan to circulate the air in my keezer. Its big enough that I was getting a few degrees difference from the bottom to the top without it.YMMV

 

Yep. A computer fan in my keezer works wonders in keeping the temps close throughout the keezer.

 

The freezing water and boiling water two-point temperature test for your meat thermometer will be accurate if you are living within 1,000 feet of sea level, since water freezes and boils at different temperatures at higher altitudes. For example, water boils in Denver, Colorado at a bit over 202 degrees Fahrenheit, a full 10 degrees lower than at sea level. The affect on the freezing point is much less dramatic with changes in altitude.

 

I use a dif of 1, with the probe attached to the fermenter's outside with insulation surrounding it. It takes a lot to make 5 gallons go up or down 1 degree so I figure this isn't taxing the compressor too badly.

 

I use one degree pretty much all the time.

 

Do this. I also typically have about a quart of Star San in the fridge when fermenting. I use a blow off tube and the tube runs into a 1/2 full growler. I take temps from the growler and assume it's pretty close.

Am I reading that right that you are taking temps from an empty fridge and you expect the ambient air temp to be accurate or consistent? It's not. I'd be testing with a few gallons of thermal mass to see if you can get that gap down to 1 or 2 F.

 

It really depends on the difference of your fermentation temp and the outside air temp. Where I live it's really HOT, so I use an 8 - 10 differential, with the probe laying exposed on the compressor hump.

Everyone thinks the wort will vary 10 degrees but my outside air temps are so HOT the compressor cycles regularly and the wort temp barely moves. The key is to have the proper compressor cycle . . . you don't want it to run for hours then be off for hours, that is what allows the liquid to vary and everyone knows what happens if the compressor short-cycles. If I want to hold at something like 65 with a 10 differential the air inside the fridge will vary from 60 - 70, but this range is only over a period of 60 - 90 minutes (average compressor cycle time). That means the maximum differential from desired temp is 5 degrees but only for 30-45 minutes which has little effect on liquid. This is exactly how your kitchen fridge works and you don't notice the beer or milk changing temperature.

I measure my wort temp with an electronic probe in a thermowell and never see a change in temp of more than 0.1 to 0.2, practically nothing. That's because the wort is only exposed to these minor +or-5° changes for a short period of time. If I were to set my differential at 1, my outside air temps are so HOT if would force the compressor to cycle several times an hour and most likely would die.

In your case with an air temp of 68 and a desired fridge temp of 62 you could probably get away with a 2-3° differential (not really sure, I rarely see temps in the 60s). Just make sure the compressor is cycling in the range of 1 - 2 hours, this is key.

As mentioned, the Ranco probe is not made to be submersed.

 

The top of the compressor hump seems to be a nice median for temp in my temp controlled chest freezer...which is where I keep my temp probe and thermometer

 

I can barely get a #7 stopper to fit in my 6.5 gallon carboys and certainly can't get a #7.5 in them. I actually use a #6.5 in them.....