WSM Dynamics of Temp Control


 
A

AlanM

New member
I'm not trying to poke a cobra with this subject but have made some observations with my 18.5 that I'll share.

I love my WSM and have incorporated an Auber control system which works well in that it will regulate the temp adjacent to the probe. That's about all one can expect from a control system. I've spent considerable time in identifying the best placement of the probe for managing the cook. Common knowledge states this to be in the center of the top grate, adjacent to but not touching the meat involved. I'm not sure how one does this with larger meats since the "middle" isn't available for probe placement.

Youtube views of some of Harry Soo's WSM cooking showed him placing the probe through a hole in the side via a grommet. Apparently he removed one of the grill support screws and leverage this hole, or possibly drilled one, I couldn't tell for certain. I modified one of these holes to permit insertion of a probe for monitoring. This probe protrudes about 1 1/2 " into the chamber setting just below the top grate. What I found was somewhat surprising.

During stable cooking, the temp monitored at top grate center was reasonably constant at 325*. The side placement probe varied significantly from the grate center when cooking at 325*F. Exploring other locations around the perimeter of the grate revealed significantly higher temps from the the center but in my case the side location across from the bottom forced air inlet used by the Auber was the hottest at about 75*F above the 325*. I then explored other locations with varying differences. Monitoring the initial side position also revealed this temp changed significantly with time, as did the other locations. The only location which was not changing and tracked with the center grate temp was the analog gauge in the top. It consistently reported a steady cook temp although is was about 12 degrees different.

I concluded with the following understanding.

1) Thermal characteristics of the WSM results in hotter temps at the outer edges of the top grate.
2) These outer edge temps can change significantly with normal operation, fuel consumption, time, etc
3) Forced air thermal control most likely influences this, or possibly causes the differences?
4) The analog gauge may not be the villain most believe it to be given the dynamic thermal characteristics of the WSM
5) Optimum control probe location should be someplace away from the outer edges of the grate. Possibly the top of the dome?

Comments anyone?
 
It's been known forever that the outside edges of both cooking grates runs hotter than the center of the grates, due to the heat being concentrated there as it rises up and around the water pan. One can see this when cooking slabs of ribs laid flat on the grates...meat extending into those edge areas gets overcooked as compared to meat in the middle of the grate.

I don't know the extent to which forced air temp systems affect this phenomenon.

It's my personal opinion that measuring temp at the lid gets a bad rap from people. Yes, the Weber therm is not of highest quality, so replace it with something from Tel-Tru and you'll get more accurate readings. For some reason, we bake just fine in our kitchen ovens with a thermometer located somewhere in the top back, away from the food, but we MUST place our pit thermometer on the cooking grate right next to our meat.

In fact, in a few recent cooks I've found that my lid thermometer tracks nicely the grate temp registered on a ThermoWorks Smoke, to within just a couple of degrees. Many times I'm happy to just monitor the lid temp as I cook, and when I know I'm close to the end I start poking with a Thermapen to check the final internal temp.
 
Above a big lump of 40 degree meat, dome temp can read 25F lower, easy.

Also big variations in temp vertically and horizontally in smoker. The big cold hunk o meat affects everything around it

The good thing is, it dont matter. Within +/- 25F, only thing that changes is time
 
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Chris Allingham said:
In fact, in a few recent cooks I've found that my lid thermometer tracks nicely the grate temp registered on a ThermoWorks Smoke, to within just a couple of degrees. Many times I'm happy to just monitor the lid temp as I cook, and when I know I'm close to the end I start poking with a Thermapen to check the final internal temp.
Click to expand...

Maybe I'm not crazy after all, :). I've concluded that using the analog for gauging pit temp is probably as good as anything "once the cook is underway". I'm looking at mounting a probe in the center top for PID control for the same reason. Cold meat will have some effect at the start but I can adjust this out with the set point on my Auber.

Thanks for the feedback!
 
Here's an example from a whole brisket I cooked a while back on the top grate. Lid temp recorded by hand at time indicated from my 20 year old Trend/Wika industrial thermometer. Top grate temp logged using ThermoWorks Smoke air probe a few inches from side of brisket.

Observations:

1) Notice how little difference there is between measurement locations at many points during the cook.
2) Notice how at a certain point the difference flips, with top grate measuring hotter than lid temp.

TimeLid TempGrate TempMeat Temp
12:00 am--46
12:30 am22822553
1:00 am26025574
1:15 am27526783
2:00 am28027592
2:30 am288286122
3:00 am297297132
3:15 am300310-
3:30 am270292139
4:00 am244268146
4:30 am244257152
5:00 am300301156
5:30 am262282159
6:00 am271258162
6:30 am274273165
7:18 am260268170
 
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Chris Allingham said:
For some reason, we bake just fine in our kitchen ovens with a thermometer located somewhere in the top back, away from the food, but we MUST place our pit thermometer on the cooking grate right next to our meat.
Click to expand...

I've sometimes wondered if those folks also create 12 imaginary zones in their kitchen ovens and track variations, lol. I'm 50+ and never checked an oven's temp.
 
LOL, I checked ours for amusement one time.

To say the setting temp was an approximation was misleading at best. I don't remember the double digit number but I gave up trying to convince my wife to just add the offset to the oven setting. However, I haven't missed any meals just because the temp is incorrect.... :)
 
All of this makes me wonder how much the Hunsaker vortex plate evens out the temperature gradient at the grate level. Really, the way the temp distribution on the WSM is (in stock form) with the outside hotter is probably the opposite of ideal. The outer thinner parts of the meat, which are already prone to overcooking, are getting the hottest air.

Obviously this is really splitting hairs though because there has still been a ton of great barbecue made on WSM despite this sort of inherent design flaw. I’m sure some engineers at Weber have discussed it though, lol.
 
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Put pie pan on bottom grate with hole in center to force hot gasses up thru there., Blocking near wall

Actually has been on my list of things to do.. basically a ring to block the current near the wall and help them diffuse. Tinfoil could even be used for starters
 
When I first got my WSM I logged grate temp and lid temp. I had results similar to Chris. I just accept the fact that the outer edge is hotter and cook accordingly. That just seems to work and I only log one temp now..
 
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