Temperature Differentials


 
L

Lee Morris

TVWBB Fan
I did my first couple of cooks with my WSM with only one thermometer measuring my temps about 3 inches inside the dome. I was using the info I had gotten off this site stating that the top grate temp would probably be about 15 degrees below the dome temp. Everything worked out great with the cooks.

Well yesterday, I did my first cook with 2 thermometers. I installed one of the BBQ Guru Eyelets and was measuring temps about 6 inches inside the cooker about 1 inch below the top cooking grate. I was using a Taylor candy thermometer that I beilive is accurate. All day long it was consistently 52 degrees below the dome temps. I was cooking 8 racks of spare ribs (4 on each grate) with a full Brinkman water pan.

Why would my temps be that off?

Even though I don't have thermometer readings to prove it, I don't feel like the temp differentials could have been that severe on my first couple of cooks. I successfully smoked two 8 pound butts on the bottom grate in 14 hours with 3 racks of spare ribs added up top for the last 5 hours with dome temps of only 240-250 on my last cook before this weekend. I don't see how I could have done that if it were really only 180-190 degrees on the bottom rack. I cooked them to an internal temp of 195.

The more I use this thing, the more confused I get. At least the food keeps turning out good.

Any info is appreciated.
 
Two thermos are as helpful as two wives...or husbands as may be the case. There can be quite a variation in temps based on position. Just test your thermo you prefer to use-use it the same way...if you get good results dont't bother with success.

In fact there is a rather wide range of temps that work. I started using 220 then 250 now I do not mind 275. Just get used to what works for you.
 
Some therms do not measure air temp all that well, being better suited for moist material rather than air. That might be the issue here. I agree that it is not likely that you could have successfully cooked 2 8-lb butts in the time and at the temps you cooked were the 180-190 temps the case.

I also agree with Steve's point.
 
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Two thermos are as helpful as two wives... </div></BLOCKQUOTE>
Steve's reply reminds me of the old tale of the person that had two watches.... he never knew the correct time....
 
Ray-that may be a more politically correct way of putting it.

Kevin-I agree with you 99.9% of the time. I do not understand the science behind your statement regarding thermos better suited to moist v air. My limited experience would lead me to believe that a temp is a temp despite the medium. I must be missing something.
 
I should have said, "Some therms seem to not measure air temp all that well...". I agree with you intellectually--because most of the time that's my feeling/experience as well; I've no science to support my assertion. But adding in some anecdotal experiences of my own, and those of a few others on occasion... . It's been rare but I've not been able to explain it to myself another way.

I don't know what happened in Lee's case. I have had odd similar expereiences over the years--where I've gotten strange ambient temp readings from one therm or another (that differed from another therm put in the same place), but that seem fine, to work 'normally' when placed in food. This hasn't really been a repeatable phenomenon--so I'm not saying anything for sure at all. I do know that some manufacturers--other than making the end of air-temp probes blunt--change some qualities of the metal used for them. No idea if I'm even slightly correct or totally out to lunch.
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I certainly stand to be corrected.
 
For the record, temp is more complicated than Steve's statement. You can google "wet bulb", "dry bulb", and "black globe" for some more info on it.

But, that is really overcomplicating what is going on here.
 
Kevin,

A simple test might be to let a bowl of water sit at room temperature for hours, and then check the air temp and the water temp and see if there is any difference (with the same therm of course). Be sure to check air temp first. If you check water first and do not dry the probe, the evaporative effect might change the actual temp.

Ray
 
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">A simple test might be to let </div></BLOCKQUOTE>
Kevin,

My analytical left brain would not let me sleep on this one ---- and I realized that my bowl of drinking water for my cats has been sitting at room temperature since I refreshed it at 7 AM today--- so it is at room temperature. Got out my Thermopen and ambient room temp next to the bowl registers 78 degrees and the water temp reads 74 degrees. Damned if I can explain it. Looks like you are correct again!!

Anyone out there have any idea why?


Ray
 
Ray,
The water in the kitty dish is contantly evaporating. Changing water from a liquid state to a gaseous state requires energy in the form of heat, so the water is cooled because of the reaction taking place. At least, that's my theory.
 
Thanks Dave. I had considered that the water at the surface might be cooler, but was really surprised that the entire dish of water was cooler.

Ray
 
Once there is a temperature gradient in the water bowl, natural convection starts to occur. The water moves around in the bowl and mixes as it receives and rejects heat to and from the air and various surfaces, resulting in a fairly well-mixed sample.
 
It's 2 am and I'm awake so I googled 'dry bulb'. I know more than I can use now. Perhaps some of this confusion is related to 'shielded from radiation'.

Perhaps I wasn't shielded from radiation....

Only a fool tangles with Kevin. Kevin, you know more stuff about stuff I didn't know existed. Oh well I do learn in the process.
 
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