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Grate Temperature vs Dome Temperature


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I have always placed the probe on the grate about 2" from the meat following the instruction that came with my Flame Boss 300 without giving it too much thought.  Yesterday I cooked 2 racks of beef short ribs on my Classic 3 using the SloRoller and thought I would give John's approach a try.  I attached the pit probe to the dome thermometer's stem, one meat probe in each rack and instead of using a third meat probe I used another pit probe fixed to where I would usually clip it to the grate.  I'm not sure how much of a difference it made to the cook but it was interesting to see the variation, I have attached an image of the graph showing the grate and dome temps, the red line is the dome temp and the black line is the grate temp.  Not sure if this adds to the debate but thought others may find it interesting. 

image.thumb.png.5912f35428ea67b348fa7aac9cae09c8.png

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6 hours ago, chris.beard82 said:

... the red line is the dome temp and the black line is the grate temp ...

Which one went to the controller?

From the data, I'd bet it was the red line... the dome thermometer is not inherently more stable than a grate probe. I bet a water pan would have brought them closer, likely crossing. 

 

Frank

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On 8/26/2019 at 3:07 PM, ckreef said:

Like @keeperovdeflame   explained swivel the thermometer so target temp is straight up and down. That was my old method.

 

I now use my Thermoworks Smoke or my Thermoworks ThermaQ devices. But not at grate level. I removed my standard dome thermometer and put the digital probe in the dome thermometer hole. I use either a slice of a wine cork or some foil to keep it in place depending on the grill. Nice accurate digital reading in the dome that I can take in the house with me. I even have this setup for my Wood Fired Oven. Single best upgrade I did to my grills. 

 

 

 

I started dong the same thing with my Smoke. I use the short probe and a piece of silicon hot pad as a wedge to hold the probe in place. 

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I've never really noticed much difference between grate and dome. However, the small differences between the two will nearly match the longer everything stabilizes. If I do notice a difference I will check to make sure the pit controller probe is not too close to the meat. I think people get too excited about 30 degree temp differences. If a recipe calls for 325 and the kamado settles in at 355 then I'm cooking at 355 degrees. I've heard of people making vent adjustments when the temp is 15 degrees off from target temp. Silly in MHO. I will say that when I fired up the KJ for the first time I used the pit controller to verify that the dome thermo was on par. Much easier than using boiling water. I probably should of clipped the pit controller to the dome thermo, but I just clipped it to the grate.    

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It’s the hot air in my Kamado that cooks my food. So, in most circumstances, I want to know the temperature of the air that is doing the cooking—the air closest to the food.  I’m not interested in the air below the food, roiling off of the coals.  Nor am I interested in the temperature of the air that’s about to go out the chimney.  Consequently, I place my pit temperature probes as close to the food as possible.

 

Sure, that air is cooled somewhat by the cooler food, but it’s that air that’s doing the cooking—The air's cooled by the food because the heat is transferring from the air to the cooler food.  The two are moving toward the same temperature (thermal equilibrium).

 

But when John Setzler, in his video, tells me to cook a Boston Butt at 300°, I see that he’s using the Kamado Joe dome thermometer.  It makes sense, then, for me to measure the air temperature at the same location, in the dome.

 

I’m not very bright, so I’m probably overlooking something, but I don’t see why anyone would do anything differently.

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11 minutes ago, pmillen said:

 

I’m not very bright, so I’m probably overlooking something, but I don’t see why anyone would do anything differently.

 

It's one of these things that doesn't matter that much as long as you do whatever you do the same way every time.  MY way is not the right way.  It's right for me though.  I just like to share my way so others can consider it :)

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22 hours ago, pmillen said:

... Nor am I interested in the temperature of the air that’s about to go out the chimney.  Consequently, I place my pit temperature probes as close to the food as possible.

You should be. What do you think your controller does when the cold food drops the reading from the pit probe? 

 

This is why....

23 hours ago, John Setzler said:

It ... doesn't matter that much as long as you do whatever you do the same way every time.

It's an SOP world. 

Frank

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3 hours ago, fbov said:

What do you think your controller does when the cold food drops the reading from the pit probe?

 

The cold food drops the reading from the pit probe because the heat energy in the air next to the food has moved to the food.  It's that air, the air next to the food, that cooks the food.  So I think the controller kicks the heat up a bit, and rightfully so.

 

People have made strong arguments on other subjects that have caused me to change my mind.  In this instance, your argument on this subject, didn't.  But I've read plenty of your posts and have come to respect your opinions.  I'll listen politely and attentively if you offer additional thoughts.  You may, yet, convince me.

 

3 hours ago, fbov said:

It's an SOP world.

 

Huh?  Standard Operating Procedure comes to mind, but that doesn't seem to fit.

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On 8/28/2019 at 11:50 AM, fbov said:

I bet a water pan would have brought them closer, likely crossing. 

 

Please tell me why would he would want a water pan in his Kamado Joe.  Water never gets above 100°C so it seems to me that he'd have a cold spot that would be sucking up heat energy for no good reason.  It would only cause the controller to call for more heat.

 

What am I missing?

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Follow up from my problem. It was totally location of the probes on the grate. I’m finishing up a butt right now that I set overnight. Grill maintained constant 230-245 through the night. My probe for grate temp is positioned much further away from the side and it has been within a few degrees (10 at most) of the dome thermometer for this entire cook. 

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On 8/30/2019 at 10:06 AM, pmillen said:

 

Please tell me why would he would want a water pan in his Kamado Joe.  Water never gets above 100°C so it seems to me that he'd have a cold spot that would be sucking up heat energy for no good reason.  It would only cause the controller to call for more heat.

 

What am I missing?

Analog temperature control.

 

Yes, it's sucking up heat, but that's a good thing in a low-n-slow cook. Moisture helps smoke flavor transfer, it helps air transfer heat, and it keeps air temps down. I get a better smoke ring, and longer, slower cooks that result in excellent results. I've long found it easier to control the fire, and only just figured out why. 

 

Water has a bunch of unique properties. For these purposes, we're interested in these two. 

- latent heat of vaporization

- volume ratio, liquid to steam: 1700:1 at STP.

 

I did some quick calculations, and found that it takes about the same amount of energy to heat up a 100 kg Kamado by 100 C as it does to evaporate 2 kg of water. Using a water pan takes out a lot of heat, so the fire needs more air. It runs hotter, and in doing so, it creates it own back pressure. 

 

Temperature control is a result of positive feedback from the water pan. 

- fire gets hotter, pan evaporates more water, making 1.7L of steam per 1 mL of water. 

- steam adds to positive pressure under dome, reducing intake air flow

- fire gets cooler due to reduced air flow, so less water is evaporated

- reduced steam production allows greater airflow which lets the fire grow

- fire gets hotter, pan evaporates ...

 

And, yes, SOPs are standard operating procedures, a way to do things the same every time. Recipes are another way. After a while, SOPs evolve. That's where this idea came from. The Rule of Halves* only takes you so far. I could control temp with just the bottom vent, but I lost my smoke ring - too little air. Went back to using both vents, and noticed I'd lose temp control after several hours... when the water pan dried up. Put water in the pan and control came back. Now I need to do it a few times, to standardize the procedure and confirm the positive feedback mechanism. 

 

Have fun,

Frank

 

* RoH: Only change vents by 50% of the opening. Close vents half way, re-open vents by 50%

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