Refrigeration - Air Temperature vs. Beer Temperature

So I bought a giant keg box with a Ranco controller. I am going to start kegging, but in the meantime (and afterward) I want to use this to cold condition ales, and ferment lagers and lager starters. So I figure if I can get everything I put inside this to 48°F, then I'm good. It's on the low end of what most lager yeasts can still ferment at, keep my on tap beers cold (but not too cold), and it should still help cold crash ales in secondary.

I have my ranco set at 48°, with a 5° differential (centered), so it will kick on the compressor until it reaches 43°, then shut off until it reaches 53°. In my mind it seems like the average temperature is 48°, so eventually anything I put in there will reach the same temp. I have calibrated this with several thermometers inside the unit, and the ranco probe is within a degree or two of agreeing with all of them.

But what I'm finding out is that it either takes a very long time for these masses of liquids to come down to temp, or the air temperature needs to be lower in order to get the beers down to the temp I want:

I put 7 gallons of doppelbock (in primary, but done fermenting) in, with a thermometer lead inside the liquid. It was 51° when I put it in, now it is at 52° after 24 hours.
I put 5 gallons of cream ale (in secondary, cold conditioning) in, again with a thermometer inside, and after 48 hours it has gone from 64° to 54°.

So I am not a physics major by any means, but are there any general rules for setting a thermostat on a fridge (when using the inside air temperature for reference) to control the temperature of large volumes of liquid? Should I set the temp 5° lower, just be more patient, or is it a trial and error thing?

I've found that measuring the air temp can be pretty inexact. I've had fermentations that are 5 to 7 degrees hotter in the fermenter than the surrounding air.

I prefer to tape the probe directly to the side of the fermenter, plastic buckets in my case. It may be a degree or 2 off from the liquid inside, but it's much more exact than measuring the air. When the liquid gets to the right temperature, the Ranco unit will turn off the compressor.

I dont know if this would work but a thought crossed through my mind, it happens now and then. What if you put a jug of water in the giant keg box and put your temp probe in the water. That should mimick the liquid in the fermenter.

@cdburg: Thanks - i understand that fermentation causes a heat increase, but in this case, there is nothing currently fermenting - I'm just trying to get my 2 secondaried beers to 48 degrees, and not understanding why they are not reaching equilibrium with the average air temp.

maltMonkey said:

@cdburg: Thanks - i understand that fermentation causes a heat increase, but in this case, there is nothing currently fermenting - I'm just trying to get my 2 secondaried beers to 48 degrees, and not understanding why they are not reaching equilibrium with the average air temp.

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Regardless of whether it's fermenting or not, if you can affix the probe to the side of the fermenter or keg, it should get the beer down to the temperature you want. You won't need to worry about the air and liquid reaching any type of equilibrium.

Both really good ideas guys - thanks! Cdburg - I see what you are saying now.

Just for my own curiosity, does anyone know the physics behind this?

I wouldn't tape it to the keg or fermenter, here's why:
liquid has a lot of thermal mass, and the freezer/fridge/whatever will bring the air to as cold as it can get until the temp probe reads your target temp. Meanwhile, when it cuts off, thermal inertial will draw the temp down further still.

At least that's my rationale. I let my temp probes free-hang and I usually set the temp to where it needs to be. For just kegs, eventually they get down to the required temp, and for fermenters I just set it to target temp, minus however much hotter the ferment temp will be.

It's what works for me, others have Great Success (tm) attaching the probe to their beer vessels.

I think you want Newton's law of cooling. I would say that you can ignore the fermentation vessel and assume the coefficient of heat transfer from beer to air is the same as from water to air (and I assume you can find that online).

However, if you are going to be opening the door of the fridge all the time to check temperatures you have really messed up the system because the ambient temperature will be fluctuating.