Thermowell temps lower than "side probe" temps? huh?

[Ike]

Riddle me this, Batman:

I'm starting my second batch in my fridge-based ferm chamber. Like most "average Joes" on this site, I usually monitor fermentation temperature with a probe pressed against the side of the fermenter and held in place with some insulation and tape and/or bungee cords. I have the probe for the Johnson Controller submerged in a bottle full of water inside the fridge.

Just for grins, and because I'm a former engineer and the only thing better than data is MORE DATA, I spent a few bucks on a thermowell. My thought was to see how much warmer the wort was at the center of the vessel compared to the outside. The thermowell is a SS tube passed through the cap on the glass carboy, filled with water to give the best heat transfer between the probe and the TW.

Well, it is a bit early since we're only about 12 hours into fermentation, but I have steady blowoff tube activity (it's bopping away like an 80's rock band drummer) but my thermowell temps are LOWER than my side temps. WTH? This is the opposite of what I was expecting.

I swapped out the probes, thinking it was a hardware problem, but the situation still holds: the side of the vessel is reading 67 degrees, the thermowell is reading 64.

My only thought: the thermowell is well into the wort, but the shorter length of my probe (story of my life, dude) means it probably isn't below the level of the wort. I'm counting on the water in the thermowell to communicate the heat to the probe. It's possible I'm also sampling the temperature of the gas in the carboy. BUT, given that it is in closest contact with the wort and insulated by the glass carboy, I would guess the temperature of the gas would be similar (although admittedly not exactly the same) as the wort.

Any input? Is this normal? Something like the yeast migrate to the outside of the vessel, making it warmer than the middle? Have I managed to build a ferm chamber that invalidates the laws of thermodynamics? Notify Dr. Who!

 

[thekraken]

My first inclination is it's a calibration issue. You said you swapped the probes, are they two probes on one or two controllers? Did you try every combination of switching probes, switching probe ports, or switching controllers and probes (if applicable), etc. Did you try taking just one probe and seeing what it read at both locations?

Sorry, I'm not a Johnson Control user but a BrewPi user.

BTW as an engineer in search for MORE DATA you might be interested in BrewPi.

 

[hezagenius]

If your thermowell doesn't go into the wort, I'd be suspect of the readings you are getting.

 

[thekraken]

If your thermowell doesn't go into the wort, I'd be suspect of the readings you are getting.

Ah yes I guess I missed that part. I'd suggest getting that probe well below the level of the wort.

 

[bembel]

Riddle me this, Batman:

I'm starting my second batch in my fridge-based ferm chamber. Like most "average Joes" on this site, I usually monitor fermentation temperature with a probe pressed against the side of the fermenter and held in place with some insulation and tape and/or bungee cords. I have the probe for the Johnson Controller submerged in a bottle full of water inside the fridge.

Just for grins, and because I'm a former engineer and the only thing better than data is MORE DATA, I spent a few bucks on a thermowell. My thought was to see how much warmer the wort was at the center of the vessel compared to the outside. The thermowell is a SS tube passed through the cap on the glass carboy, filled with water to give the best heat transfer between the probe and the TW.

Well, it is a bit early since we're only about 12 hours into fermentation, but I have steady blowoff tube activity (it's bopping away like an 80's rock band drummer) but my thermowell temps are LOWER than my side temps. WTH? This is the opposite of what I was expecting.

I swapped out the probes, thinking it was a hardware problem, but the situation still holds: the side of the vessel is reading 67 degrees, the thermowell is reading 64.

My only thought: the thermowell is well into the wort, but the shorter length of my probe (story of my life, dude) means it probably isn't below the level of the wort. I'm counting on the water in the thermowell to communicate the heat to the probe. It's possible I'm also sampling the temperature of the gas in the carboy. BUT, given that it is in closest contact with the wort and insulated by the glass carboy, I would guess the temperature of the gas would be similar (although admittedly not exactly the same) as the wort.

Any input? Is this normal? Something like the yeast migrate to the outside of the vessel, making it warmer than the middle? Have I managed to build a ferm chamber that invalidates the laws of thermodynamics? Notify Dr. Who!

Even inside the carboy the air is going to change temperature MUCH faster than the wort, and if your temp probe was in a thermowell tube, that tiny amount of water around it is also going to change temp MUCH faster than the wort in the carboy. Personally, I think the underpants fairy was messing with your calibrations, or you might have taken your measurements at the peak of the swing in temp difference (the mass of the wort would be 2-3 degrees cooler on one side and 2-3 degrees warmer on the other side of each cycle, )

In either case, Kudos on the avatar, is that Milo drinking like Steve Dallas?

 

[Ike]

In either case, Kudos on the avatar, is that Milo drinking like Steve Dallas?

Milo! ...you saw B.B. is drawing strips again, didn't you?! The pic, as well as the name for my hard root beer, is from:

http://www.gocomics.com/bloomcounty/2013/07/10


WRT the probes: I've pretty much decided that when I thought the carboy would create a reasonably closed system that would keep the temperature of the wort and gas in the carboy at some degree of equivalence... well... I was wrong. SO, I guess I'm back to using a probe taped to the side of the carboy for temperature control, at least until I get a temperature probe that's skinny/long enough to reach all the way below the level of the wort.

 

[unionrdr]

That's my opinion as well with the thermowells. You need a probe skinny enough to get all the way to the bottom. I also read somewhere that silicon gel inside the thermowell helps it read the temp more accurately due to better temp conductance.

 

[ncbrewer]

My understanding is that typically, the probe is spring loaded so that the end of the probe contacts the inside end of the thermowell.

 

[thekraken]

That's my opinion as well with the thermowells. You need a probe skinny enough to get all the way to the bottom. I also read somewhere that silicon gel inside the thermowell helps it read the temp more accurately due to better temp conductance.

Silicon gel? I would have thought silicon would be more of an insulator?

I have tooth paste in one of my thermowells

 

[MindenMan]

I believe the reference was silicone like one would use on a heat sink, not the kind to seal things up with.

 

[drgonzo2k2]

Actually, diaper rash cream works really well as a thermal paste for home brewing. It's basically zinc oxide in a petroleum gel which works fine for the temperatures we're working with. The expensive thermal paste you see is generally zinc oxide in a silicone gel, which is rated for much higher temps.

 

[unionrdr]

Yeah, the comp repair near here fixed my heat sinks in the comp with it. Cheaper stuff might do just as well.

 

[kombat]

Am I the only one who doesn't put anything inside his thermowell other than the probe? I'd honestly never heard of that. I would think that the tip of the probe, if all the way down at the tip of the thermowell, is going to measure the temperature of the surrounding wort pretty accurately, no? What's adding a surrounding liquid going to do?

I'm also confused why the Johnson controller is in a glass of water. What's the probe on the outside of the carboy connected to? How about the probe in the thermowell? Who's actually controlling the fermentation temperature?

 

[doug293cz]

Am I the only one who doesn't put anything inside his thermowell other than the probe? I'd honestly never heard of that. I would think that the tip of the probe, if all the way down at the tip of the thermowell, is going to measure the temperature of the surrounding wort pretty accurately, no? What's adding a surrounding liquid going to do?

I'm also confused why the Johnson controller is in a glass of water. What's the probe on the outside of the carboy connected to? How about the probe in the thermowell? Who's actually controlling the fermentation temperature?

Thermal paste isn't a particularly good conductor of heat, but it's a lot better than air, which is why it's used. Adding thermal paste to fill the gap between the probe and thermowell wall will reduce the thermal resistance between the wort and the probe and improve the dynamic response of the probe (probe will respond faster to temp changes in the beer.) Since the beer has a large thermal mass, it only changes temperature slowly, and the improved dynamic response of the probe probably doesn't provide a measurable benefit.

Brew on

 

[GreyRaven]

I'm using the waterproof version of the DS18B20 sensors with my DIY BrewPi setup, and do not use anything around them in the thermowell. These sensors seem to fit fairly well without anything around them inside the tube, and as near as I can tell are reading accurately for me. They also have the advantage of having rather long leads on them so I can make sure that the carboy sensor is all the way at the bottom of the thermowell (12-inch) and at least 6 or 7 inches below the surface.

 

[kombat]

Thermal paste isn't a particularly good conductor of heat, but it's a lot better than air, which is why it's used. Adding thermal paste to fill the gap between the probe and thermowell wall will reduce the thermal resistance between the wort and the probe and improve the dynamic response of the probe (probe will respond faster to temp changes in the beer.) Since the beer has a large thermal mass, it only changes temperature slowly, and the improved dynamic response of the probe probably doesn't provide a measurable benefit.

We're talking about the rubber-coated probes that come with an STC-1000, right? In my experience, they react pretty darn quickly to changes in temperature. For example, if I have one dangling in my keezer keeping things at 2° C, and I lift the lid and remove the probe for whatever reason, the probe immediately reflects the rise in temperature, and within seconds, it's reading the ambient room temperature. I think this is much ado about nothing.

 

[doug293cz]

We're talking about the rubber-coated probes that come with an STC-1000, right? In my experience, they react pretty darn quickly to changes in temperature. For example, if I have one dangling in my keezer keeping things at 2° C, and I lift the lid and remove the probe for whatever reason, the probe immediately reflects the rise in temperature, and within seconds, it's reading the ambient room temperature. I think this is much ado about nothing.

I was talking about pretty much any type of probe in a thermowell. I have observed the same behavior as you for the common STC-1000 probe hanging in the air.

Brew on

 

[thekraken]

Am I the only one who doesn't put anything inside his thermowell other than the probe? I'd honestly never heard of that. I would think that the tip of the probe, if all the way down at the tip of the thermowell, is going to measure the temperature of the surrounding wort pretty accurately, no? What's adding a surrounding liquid going to do?

I'm also confused why the Johnson controller is in a glass of water. What's the probe on the outside of the carboy connected to? How about the probe in the thermowell? Who's actually controlling the fermentation temperature?

I don't think thermal paste is necessary in such a slow temp changing mass like fermenting wort. I use some paste in my thermowells for my RIMS system where the temps change very rapidly, even there it may not be necessary.