Thoughts & advice....


 
J

JKalchik

TVWBB Emerald Member
I have an Oklahoma Joe style horizontal smoker. I assembled a 4.2.4 HeaterMeter a couple of months ago, with a Tom Kole barrel server V2 with the requisite 60mm 10CFM blower. I've ran a couple of smokes, and I'm a bit confused by some behavior. Last weekend, in particular, I smoke a whole packer cut brisket and a couple of pork butts (about 34 lbs. of meat in total.) I was more than a little unhappy with the brisket, the pork turned out fair. The pit temp last week by the thermocouple (Thermoworks) never read above 225 and the temp was significantly higher. I know I had a problem thermocouple and bought replacements for it.

So, today, I ran an open pit, no food. 1 load of charcoal in the basket (Minion method,) and aside verifying my original thermocouple was bad and opening the pit to swap it out (and shuffle the other probes,) here's today's graph.

Screenshot_20160910_194720.png


The pit thermocouple is close to the center, offset away from the firebox towards the stack.
The firebox probe is about 6" from the firebox end of the cooking chamber.
Firebox #2 was between the pit thermocouple and the first firebox probe.
The stack probe was about 5" from the stack elbow (draws from the cooking grate level, not the top of the chamber.)

I did get a fairly even 275 degrees from the pit thermocouple today, and the other probes seemed to have nearly identical curves. How on earth does the thermocouple probe maintain 275, when all of the other probes dropped well below? The charcoal was pretty well burned out by the time the pit thermocouple dropped below the setpoint. Is this an indication I've got other issues with my HM?

I probably should run a boiling water test with all of my probes, I suspect. I did that shortly after I assembled the HM, and the probes all showed the expected values.
 
Hard to say really, the TC line is so flat but the other two follow the same path, hard to tell which is accurate... or they could all be accurate and it could be due to air flow conditions in your pit.
If you are attempting to test the HM's accuracy and/or troubleshoot problems with probe accuracy I would suggest you take all of the probes and wrap them together in some foil and place them in the grill. Watch this graph, if the TC shows a flat line while the others go up and down I would suspect you have issues with your build, if they all track together than you have air flow issues in your pit....
 
After you do the boiling water test on all your probes and the thermocouple probe, adjust the offset value so they all give you the exact same temperature. Then, try changing the position of the thermocouple probe to the other probe positions and see how the new thermocouple position impacts the curves of the other probes.

After that, I would say that the temperature differences may be related to the airflow through the cook chamber. You might try experimenting with deflectors to alter the airflow through the cook chamber and see how the curves change.

Please post your results so we can all benefit from your experiments.

Mike

Edit - I take too long to type, but at least Ralph and I have similar thoughts.
 
Good points. I'll probably skip the boiling water test, and run the bundled in salt test later (doubt I'll have time for it today.)

I've been reading http://tvwbb.com/showthread.php?671...n-all-the-fuel&p=745567&viewfull=1#post745567. I've got a fair amount of space around my charcoal basket, and have already been wondering about airflow. The air inlet is already below the bottom of the grate the basket rests on, I think all I need are some flat plates (mild steel, not galvanized,) to keep the air directed up through the basket.

I do have a set of baffles in the bottom of the cooking chamber, to try & distribute the heat flow a little more evenly. I do see on the graph that the temp distribution from end to end does diminish as the cook goes on. I did seal up the chamber with high temp stove caulk when I assembled it.
 
I have done a bunch of experiments with probe placement and did a formal sort of writeup about it. That might add a little insight into temperatures in general you might be able to use in your own experimentation. Here's the quote:

I needed to do some other testing so I thought it would be fun to illustrate how probe placement is a little important, but dome vs grill level doesn't matter. Behold the test setup! 3 probes at grill level, run through wooden clothespins and held roughly an inch above the grill at different locations. 3lbs of lump charcoal were loaded into a freshly cleaned Large Big Green Egg and lit with a weber starter cube. All probes have undergone a 1000+ point calibration to be within 1F of each other.


  • Center - A probe located centrally over the big green egg ceramic disk (or is it a disc?). This probably would be the A#1 duke of positions to place your control thermometer it weren't taken up by all the inconsiderate meat we're trying to cook. We'll use this probe as our reference we're trying to track.
  • Far - This probe is located off to the side, but with the last 3" of probe shielded by the ceramic. This is a typical location a probe would be placed if the grill had food on it.
  • Close - This probe is also off to the side but you dumbass, the tip is in the draft of the fire. This probe experiences most of the unshielded heat of the fire and is where the "Far" probe might end up if things get knocked around
  • Dome - The orange line is a bare thermocouple in the dome, which I am claiming to be "just as good as at grill level".


There's a lot of churn here over 6 hours. Clearly the gray probe ("Close") is never in a good position to asses the temperature compared to the light green ("Center" ) probe. The dark green ("Far") probe is closer, sometimes following the Center probe, but sometimes moving in the opposite direction with temperature differences exceeding to 20F. The big dip down in the middle is where I opened the lid to take a picture of the probes.

Let's compare average temperatures starting at the first time the orange probe passes the red setpoint line.
Code: 
Avg      Error from Center
Dome -   249.2    -7
Center - 256.2    0
Far -    262.0    5.8
Close -  280.4    24.2

Given that we can't place our thermometer right in the middle of the grill, the dome temperature is 1.2F difference from being at grill level. 1.2F over 6 hours is virtually the same, especially considering the probes are only calibrated within 1F of each other. The dome temperature has less maximum deviation from the center location too. As a caveat, "Dome" is not the dome thermometer. That thing is a piece of s--- and read anywhere from 240F to 290F through the course of testing despite being literally attached to the thermocouple.

In conclusion, I assert that you do not need to measure your BBQ temperature at grill level. If you can get it situated centrally, that's probably the best. However, a shielded position at grill level or a thermometer located in the dome, with only a 6 degree difference in either place, roughly equivalent. A probe tip which is exposed in heat currents is definitely a bad idea. Unless you've arranged the majority of the meat to be exposed to those heat currents in which case this would be the ideal place to put your thermometer, which you couldn't because the meat is there and you'll have to read it from somewhere else which won't be the same temperature so you're f'ed again.
 
<scratches head> There's an INTERESTING thought..... there's a [worthless] pit thermometer in the top of the chamber. That might be an interesting spot to clip a thermocouple. Guess I'm going to run at least 2 tests this week, one with all of the probes bundled together, then one with a thermocouple on that thermometer. G/f will be out of town this coming weekend, I want to run another brisket (and hopefully NOT ruin this one.)

BTW, Bryan, I told Garrett H. last weekend when he was here that I didn't know whether I should shake your hand or kick you in the shins for this project, it's gonna eat up a LOT of my time. I've known him for 35 years.
 
JKalchik said:
<scratches head> There's an INTERESTING thought..... there's a [worthless] pit thermometer in the top of the chamber. That might be an interesting spot to clip a thermocouple. Guess I'm going to run at least 2 tests this week, one with all of the probes bundled together, then one with a thermocouple on that thermometer.

BTW, Bryan, I told Garrett H. last weekend when he was here that I didn't know whether I should shake your hand or kick you in the shins for this project, it's gonna eat up a LOT of my time. I've known him for 35 years.
Click to expand...
I'd love to hear more of your data as you mess around with the probe placement. I'll tell you, I used to just put a bunch of charcoal in the Egg, light it up, and check on it every hour or two or three but now that I have built HeaterMeter I have definitely spent 100x more time watching graphs, running experiments, and trying to correlate data to control models than I ever spend actually eating. This whole "where is the RIGHT temperature" thing was the biggest revelation I had, I thought the temperatures would be much more consistent overall. Something like, Dome is always 10 degrees cooler than center. Center is always 5 degrees cooler than 3 inches to the right of it. The fact that even given the massive ceramic mass of the Egg, temperatures can flip flop was a revelation and I realized that I should really just relax and stop trying to get perfect uniformity because it just wasn't going to happen.

So Garrett wasn't kidding when he said he was going to help a friend put together a HeaterMeter! I was texing back and forth with him a few weeks ago, just seeing what was up with him since I never see him since he moved away, and he offhand mentioned it but had scurried away before I could ask about it. He could have gotten you a friend discount at the HeaterMeter store... or sent him some referral commission. He has the right spirit for this project, he's always messing with some piece of hardware or IoT or home automation bit.
 
I've kinda seen this go both ways... for the first year I spent a lot of time figuring out the HM and my grill rig, studying air flow and temps, watching cook graphs, working on dampers etc... Eventually things became routine and I could predict behavior enough that I can light up the pit, let it heat up, then throw on some food and take off and tend to another project or leisure activity. Sometimes I'm gone for hours on end, never worrying about what's happening with my food, I just know it's gonna be great.
One thing that came clear into view in my experimentation is forcing air into your pit has good and bad points. It's a great way to control temps overall, but can cause more air currents in the pit, cooler air currents of ambient air that you wouldn't normally have with a passive convection burn. The negatives of extra air are increased variance in temperature in the grill, more drying out of food, burning more charcoal and the tight top vent holds in smoke longer than I like it to hang around my food.
As a result of this realization a lot of the experimentation I have done has been aimed at limiting the forced air flow and/or making the most efficient use of the air that is forced into the pit. The air burner uses forced air, but the small air holes limit the flow and direct the air right at the coals where it can actually stoke the fire like a fireplace bellows, making more efficient use of the air that is forced into the pit. The roto damper takes another approach, thanks to changes in the HM config settings I am able to turn off the blower completely after the pit has been stoked to temp and go back to convection flow stoking the fire. This reduces the cool air currents in the grill and allows me to open the top vent wider to allow the smoke to flow out more freely, as a bonus I burn less coals.
At any rate, I have thoroughly enjoyed the experience (for the most part) and consider it time well spent...
 
Garrett & I have known each other for 35 years (split an apartment for 6.) We've been around the block a time or two..... I'm not worried about discounts. You've got way more time sunk into this that I would have, I don't have any problem paying the freight.

Back to the business at hand. I've got 1 thermocouple and 3 thermistor probes (all Thermoworks) in a salt bundle right in the middle of the pit. Interestingly enough, the thermistors are running 4-5 degrees below the thermocouple, otherwise are following on just about exactly the same graph. I think I need to spend some time with a known temperature source and each probe. Actually, 2 of the thermistors are pit temperature probes, one is a food probe. I think I also need to go dig out some wooden clothes clips (have a few stashed in the garage) as the alligator clip thermocouple doesn't really want to clip on the grates (won't open far enough.)

Ralph, you're probably already where I am probably going to end up: learning the vagaries of this pit. Based on this, I think that I'll give the top part of the chamber a try for pit control. And probably some goofy air currents inside the chamber. I'm also seriously wondering if I just overloaded the pit (very little extra space on the cooking grates.)

I took a good look at the air vent into the firebox when I fired this test run. It's bisected by the charcoal grate, so making a baffle will have to cover half of that opening. I may still entertain that, but down the road. The blower over the last couple of test runs has been running at 25% or less. And I'd also have to spend some time figuring out how to seal up the ash drawer or just fabricate a new one. Might be easier to build an entire new firebox.
 
The mighty Casey steps up to the plate.....

Time to swing for the fences. Picked up a 12 lb. packer brisket ($2.89/lb, prime from Costco.) I've got the pit probe clipped to the top of the chamber (thanks for the tip, Bryan,) probes on the grate at the stack and firebox end, and 1 in the brisket. Aside from one odd blip, the stack and firebox probe graphs have nearly identical shapes, albeit lower than the pit probe.
 
The brisket turned out pretty good, albeit a bit crusty (Aaron Franklin's method.) That might be due to how long I ended up holding it warm after pulling it from the smoker.

I can pretty consistently get 3-3.5 hours out of a basket of charcoal, almost set an alarm by it. I don't think I'm going to make any changes to the firebox airflow.
 
I smoked another brisket (15 lb. packer cut) this week (office potluck.) I turned down the pit temp to 250, and much to my amazement, it ran from just above fridge temp to 165 from 11:30 AM to around 5:00 PM when I wrapped it in foil. It started to rain around 5:30 PM, so I just put it in a 250 degree oven (and took the HM inside to keep it all dry.) Brisket finished around 7:30 PM. For a 15 lb. brisket, that sounds awfully fast to me, but it did turn out pretty well (a few slices were just a bit chewy.)

Might smoke something this weekend. More data to follow.
 
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