Temp sensor placement

I have been looking in this section every now and then to see if this comes up. I have been fermenting in a 22cf chest freezer now for 5-7 batches. After talking with the guys at my LHBS they told me to place the sensor on the outside of the carboy. It seems that with the sensor on the out side the temp swings quite a bit and the compressor cycles on and off a lot. I am wanting to know what every ones thoughts are on this versus putting the sensor in a thermowell in the center of the carboy.

I am using a ranco temp controller and a small personal heater in side that I picked up at walmart.


I don't currently have a thermowell.

for fermenting in a chest freezer, I've done tests where I put one down a thermowell and one taped to the side under a few layers of paper towel on the same carboy. there's no difference.

might be a different scenario if you're pushing hot air directly across the probe, that might be a different situation. but if you already have a thermowell why wouldn't you use it?

I personally just tape it to the side of the carboy and wrap it in bubble wrap for insulation. I have used a thermowell but couldn't tell a difference in the taste of the beer. Either way will work just fine!

Thermowell!!!

I cut the curved end off a SS racking cane and sealed the bottom with food grade sealant. I then run my thermistor down the tube as far as I can get, fill the top end around the wire with paper towels or foam to keep the outside air from getting inside the tube.

That set up is run through the center part of my carboy cap, the other outlet on the cap has a blow off tube on it. The end of my thermistor is usually dead center, vertically and horizontally, in the beer and from the edges of the carboy.

I've left a separate thermometer taped to the side of the carboy to measure the difference and it was dramatic. The sensor on the outside of the carboy seemed to cool too quickly and if running my fridge, would have cut the cooling before a noticeable drop in temperature inside the carboy. The glass conducts the cool, outside air too well no matter how well you insulate the probe from air. In a matter of minutes the thermometer taped on the carboy showed a drop in temp of 5 degrees that would have shut off the fridge were it in control, however the wort temp as registered by the thermowell had not dropped a single degree at that point.

It works better than nothing, but without a thermowell you can't measure the wort temp, you're still just measuring the carboy temp.

It's not much of a jump to create a little thermowell and I'm convinced it maintains a MUCH more accurate reading of the actively fermenting wort than the other methods.

Of course, this is applicable only to my experience with GLASS carboys. I would think buckets may not introduce as much of a variable in the readings but that's speculation on my part with no experience to back it up.

EDIT: It appears other people have had different results. Perhaps I'm not insulating the thermistor as well as you guys, but the above reflects my experience.

I just have a 2 liter bottle filled with water and the thermo probe is stuck down between the wrapper and the bottle wall. I really didn't feel like taping and untaping. Then worrying if the tape would fall off the fermentor. The 2 liter bottle doesn't take up that much room and will sit there forever. Works good too!

i strap the temp probe to the side of my bucket with a luggage strap and a gel freezer pack - i periodically check the temp of the a419 display and with a thermometer dipped into the wort and at most there is a 1° variance

Diver165 said:

I just have a 2 liter bottle filled with water and the thermo probe is stuck down between the wrapper and the bottle wall. I really didn't feel like taping and untaping. Then worrying if the tape would fall off the fermentor. The 2 liter bottle doesn't take up that much room and will sit there forever. Works good too!

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That's great for measuring the ambient temp. The point of taping/untaping or using a thermowell in the fermentor is to control the temperature of the actively fermenting beer, which we all know can swing upwards of 10 degrees above ambient temperature. Your solution is conveniently low maintenance, but your beer is swinging through a wide range of temperatures during primary fermentation.

I have mine duct taped to outside of the carboy insulated by a thin layer of packing foam. I set mine +/- 1 degree and it doesn't cycle on or off very often. If you find your cycling often, then it's being exposed to too much open air. It takes quite a bit to change the temp of 5 gallons, so you should be able to set the temp pretty tight.

I keep mine in the air, about a foot off the bottom of the chest freezer and and inch or so off the wall. I also have a separate thermometer taped and insulated to the side of the fermentor to measure the beer temps. I can easily keep the beer temp +-1F. When I taped and insulated the controller probe to the fermentor I found the temp of the beer would drop much lower then the set point. After the freezer would cut out at my set point, the freezer was still so cold that the beer temp would keep dropping.

This argument is always endless with no clear winner. I think several methods get you to the same point. I've noticed having used both the thermo well and the insulated bungee cord around the fermenter that once the wort gets to set-point both methods are the same. I attribute that to the temp change in the fermenter being a bit slower to equalize throughout the container, but once it gets to it things stabilize. My current set up is without thermo well and it works fine.

bigbeergeek said:

That's great for measuring the ambient temp. The point of taping/untaping or using a thermowell in the fermentor is to control the temperature of the actively fermenting beer, which we all know can swing upwards of 10 degrees above ambient temperature. Your solution is conveniently low maintenance, but your beer is swinging through a wide range of temperatures during primary fermentation.

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Hmmm, well I am new to the dedicated fermentation chamber. I'll move the probe to the outside of the carboy/buckets. I think I may look into a thermo well setup and monitor the wort temp inside the fermentor as well. I'm already tickled to death with the fact I can keep my ambient temps stable. Now I guess it's time to take it to the next level.

Jamil discussed this on a BrewStrong a while back. In short, if you insulate the probe on the outside it's just as good as a thermowell. He cited studies one guy did where a probe was directly in the wort, in a thermowell, and taped on the outside. All were essentially the same. JZ tapes down some bubble wrap on the outside of the probe. I just fold up a section of paper towel and do the same thing. Works great. If you're using a space heater in there, I'd keep the probe (insulated or not) away from it. I wonder how much that kicks out. Might want to check out the fermwrap which is pretty cheap and very low voltage. I can dial in to within a degree with my controller easily.

I have a freezer and have seen when using the thermowell that the freezer has enough cold mass to continue to remove heat from the fermentor after the low set point has been reached. The swing isn't too bad once the yeast gets cooking but is greater when there are critical times where the yeast can go dormant.

I moved the probe to the fermentors wall and slightly isolated to the ambient air. At times I add a heat source to the inside of the freezer.

I've used a thermowell for my last two batches and I haven't really noticed a difference between using a thermowell vs. taping the probe (with some insulation) against the fermentor.

I use part of a diptube from a bad corny keg for my thermowell. I have tried taping the probe to the side of the carboy/bucket/keg/ but there's always 3 or 4 degrees difference. If what I want to measure is the temperature of the wort the thermowell cannot be beat. leaving the probe on the outside will always give you a less accurate reading. Have you ever had a nurse press a thermometer against your arm and call that good?