[CounterPressure]

Anyone care to take a guess on this?  I see a quick google search turns up a result saying as much as 8 deg F, but I don't know if that's at large commercial scale or home brewer scale. If this was a 30bbl batch, I could see 8 F. Not so sure on home brew size.  My go-to fermenters are either 6.5 gal Big Mouth Bubblers or I have one 15 gal stainless conical.  When I ferment, these go in a 20 Cu Ft upright freezer (can get the Conical and 2 6.5Gal BMBs in at the same time) and I attach the probe to whichever is most active at the time.  I tape or strap it to the side with a piece of foam over top of it to 'insulate' it some from the ambient temps.  But, I don't know what the real temp is inside since I have no thermowell.


Anyone know how much it varies, and does it matter?

[denny]

I have measured internal temp vs. side of fermenter temp several time with both carboys and buckets. The greatest variation I found was 1 F. Most were the same.

[Bob357]

Just prior to active fermentation would likely be when the temperature differential is greatest. Once the beer is in motion, when   the most heat is generated, the beer is moving around enough to reduce the differential and maintain a fairly even temperature throughout. My guess would be within a degree or 2.

[Drewch]

I have measured internal temp vs. side of fermenter temp several time with both carboys and buckets. The greatest variation I found was 1 F. Most were the same.

That agrees with my experience comparing floating TILT temperature readings to outside sidewall temps.

[CounterPressure]

Interesting. Thanks guys.  So why do so many people make such a big deal out of this? Even the marketing for fermentation vessels will often cite the ability to add an internal probe, implying that is some great advantage... And like I mention, the google search said 8 degrees is possible, but never mentions the size of the fermenter. Obviously it must be a pretty big one.

[denny]

Interesting. Thanks guys.  So why do so many people make such a big deal out of this? Even the marketing for fermentation vessels will often cite the ability to add an internal probe, implying that is some great advantage... And like I mention, the google search said 8 degrees is possible, but never mentions the size of the fermenter. Obviously it must be a pretty big one.

Because although homebrewers are the cheapest people on earth, but they also can't resist needless stuff that looks cool.

[BrewnWKopperKat]

the google search said 8 degrees is possible, but never mentions [,,,]

Consider avoiding search engines that summarize search results.

In "new brewer" troubleshooting topics, new brewers would often ferment in warm (e.g. 70F with A/C running) apartments with no other temperature control.  Responses would claim the fermentation would be +5F, +10F, or more above room temperature.   This is likely the source for any LLM generated "search summary" response. 

CB&B web site had an article a couple of years ago (I couldn't find it today with a quick web search) that went into the idea in more detail.  eta: looks like the site has moved a bunch of content behind a 'wall' (so bots can not access the content?)

With a modified "swamp cooler" approach (and 2 or 3 gal fermenters), I find that water temperature in the cooler matches fermentation temperature.   When raising or lowering the temperature of the water, I find it takes an hour or two for the wort to reach that temperature. 

If you have specific links to human generated content related to +8F, there may be more to talk about.  Otherwise fermentation temperature control techniques as simple as a "swamp cooler" often result in +1F (maybe +2F) variation during fermenting.

edits: formatting, more failed CB&B site search attempts

[Richard]

There are two issues here: Temperature difference from edge to center of fermenter and temperature difference between air and fermenter. As Bob pointed out, the internal gradients will be smoothed by the liquid motion during active fermentation. However, the interface from air to fermenter wall is not very good thermally, and the temperature of the beer can be different than the temperature of the air by quite a bit. A swamp cooler, with the fermenter sitting in a water bath, has much better thermal contact from the coolant to the beer. You can improve the heat transfer from air to fermenter by using a fan blowing on the fermenter, but it will still not be nearly as good as a swamp cooler. If you have a refrigerator controlled by something like an Inkbird with the sensor on the fermenter wall or inside a thermowell you can get an idea of the temperature difference by measuring the air temperature in the refrigerator required to get the beer temperature you want. I use a glorified swamp cooler, so I can't do that experiment myself.

[CounterPressure]

There are two issues here: Temperature difference from edge to center of fermenter and temperature difference between air and fermenter. As Bob pointed out, the internal gradients will be smoothed by the liquid motion during active fermentation. However, the interface from air to fermenter wall is not very good thermally, and the temperature of the beer can be different than the temperature of the air by quite a bit. A swamp cooler, with the fermenter sitting in a water bath, has much better thermal contact from the coolant to the beer. You can improve the heat transfer from air to fermenter by using a fan blowing on the fermenter, but it will still not be nearly as good as a swamp cooler. If you have a refrigerator controlled by something like an Inkbird with the sensor on the fermenter wall or inside a thermowell you can get an idea of the temperature difference by measuring the air temperature in the refrigerator required to get the beer temperature you want. I use a glorified swamp cooler, so I can't do that experiment myself.

I have several freezers controlled by either the one ink bird that I have or the older style temp controllers they used to fleece us for. In either case, I tape the sensor to the side of the fermentation vessel as I mentioned in the posts above. So the outside of the fermenter is more or less what controls the temperature. I throw a piece of foam over it to try to insulate it from the ambient air but I don't go overboard about it. It's physically touching either the glass or stainless, so what it reads is what it reads. If the air temperature inside the freezer fools the sensor into thinking it's colder than it is, well I don't know what to say then. Typically when I shoot the fermenter with a IR thermometer it will read within one degree of what the temperature controller says the reading is on the thermistor. But what I did not know was what is the actual temperature inside, and there's a consensus that says it's well within any tolerance I care about.