2 zone cooking variation

[Basher]

I have been reading many articles around 2 zone kamado cooking from here and other sources and note the 23" can be set up for 2 zones, however, it has been suggested the 32" set up for 2 zones may be better as the larger zone areas can create a greater variation in temperatures within the zones(and more cooking area). This may have had me leaning towards a 32". I don't really need this much cooking space.

However, The 23", having a round fire basket, has the advantage of setting the zones front to back, or left to right. I have not been able to find any discussion about the variation of front to back zones may have on temperature variation.

Is this significant?

Surely this set up above will create a greater temperature variation to the set up below? through radiated heat from the dome alone?

In the same way reverse flow off set smokers distribute the heat more evenly. 

I am theorising, but wouldn't this make a significant difference?

Was the flute set towards the back for thermo design reasons or to distribute the weight over the hinge?

Sorry about the amatuer sketch and vandalising your artwork, I have fat fingers!

Edited May 24, 2019 by Basher

[Pequod]

The 23’s round basket has the advantage of left/right or back/front configurations. The latter is particularly convenient for rotisserie, and I also like it in conjunction with the new warming grate to provide easy vertical mobility as well.  

Note that two-zone is mostly produced by the difference in direct infrared heating between the zones, which has more to do with distance (squared) to the hot coals. So, the greater the distance vertically or horizontally, the more of an indirect zone you have.

The biggest benefit of the 23’s warming grate or the 32’s half main in 2-zone configuration is the vertical separation on the indirect side while providing a right over the coals direct side.

Edited May 24, 2019 by Pequod

[Basher]

I read your comments Pequod about the distance squared and 10 times heat in another article.

As with the reverse flow smoker, there seems to be a massive amount of heat carried through with the smoke- although the smoke just allows us to see the heat travel. Would you refer to the heat in an off set smoker as radiated heat? There is a big difference with the heat distribution in the reverse flow design.

They are trying to slow the heat movement so it disperses evenly.

[Pequod]

Offset is carrying its heat primarily through convection. Especially in a reverse flow offset, the firebox isn’t directly exposed to the cooking chamber, although it will be hotter and, therefore radiate.

Kamados, especially KKs due to the superior insulation over glazed pots, are characterized by being low flow. Not NO flow, but low flow. Convection certainly plays a role, but not the same role it plays in an offset.

Also, there is no such thing as two-zone in an offset...at least not on purpose. Goal there is even heat throughout. Two-zone is associated more with grilling, not smoking. The hot side of a two-zone grill is most definitely doing its thing via Infrared...and maybe a bit of conduction from the cooking surface which was heated via infrared.

Edited May 24, 2019 by Pequod

[Pequod]

Have posted this article before, but worth reposting here: https://amazingribs.com/more-technique-and-science/grill-and-smoker-setup-and-firing/how-control-temperature-indirect

Says it better than I have: purpose of two-zone is to have one “hot” zone dominated by infrared, with the other, cooler side dominated by convection. For convection to dominate, first must have low infrared due to distance squared or IR deflector. The 23 can do the job, but in the 32 the zones are more distinct and with larger area in each zone. 

Edited May 25, 2019 by Pequod

[Basher]

Thanks for further explaining Pequod. Based on this, the 2nd set up in the photos above will create more convection heat given the circuitous flow required for the heat to exit the KK. Meathead's description also seems to be focused on temperature control within the indirect area with minimal regard to temps over the direct area.

Am I right in suggesting you want the convection air flow around the food on the indirect area? Therefore you wouldn't set up with the fire directly underneath the top dampener where the heat has a more direct exit flow?

Do you set up the roti with the fire at the back or front of the KK?

Edited May 25, 2019 by Basher

[Aussie Ora]

I set the coals at the back for rotisserie cooking . I don't know why you would put them at the front

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[Pequod]

5 hours ago, Basher said:

Thanks for further explaining Pequod. Based on this, the 2nd set up in the photos above will create more convection heat given the circuitous flow required for the heat to exit the KK. Meathead's description also seems to be focused on temperature control within the indirect area with minimal regard to temps over the direct area.

Am I right in suggesting you want the convection air flow around the food on the indirect area? Therefore you wouldn't set up with the fire directly underneath the top dampener where the heat has a more direct exit flow?

Do you set up the roti with the fire at the back or front of the KK?

The key difference between the two sides -- direct and indirect -- is the heat transfer rate, not the temperature. I want a much lower heat transfer rate on the indirect side than on the direct side. Food cooks by energy being transferred to it, not by the surrounding temperature. That seems a bit counter-intuitive because we set our ovens and grills, etc. to a temperature, not a heat transfer rate. But bear with me.

This next bit is going to get a bit technical. Can't help myself -- being an engineer who once upon a time did fluid dynamics and heat transfer for a living (mostly over that now, having moved on to other, geekier things). Feel free to skip it. 

Ahem. There are three fundamental mechanisms of heat transfer: Conduction, convection, and radiation. All are always present. The overall heat transfer is the sum of the three. None is superior to the other, although one may clearly dominate in certain cases. Examples:

Conduction: food on a griddle or in a hot pan is cooked by conduction. We use the knob on the stove -- low, medium, high to tell us how hot it is (given time to reach equilibrium). When our grill grates get hot and we get grill marks on our food, this was achieved by conduction -- direct contact of the food with the grate. Note that the grate itself may be considerably hotter than the temperature reading on the grill. This is because the grate was heated, in part, by radiation...which doesn't care so much about your grill thermometer.

Convection: food in an oven or on the indirect side of a grill is cooked by convection where heat is transferred by the surrounding air to the food. Sous vide cooking is similar, but the heat is transferred by water. In both cases, the temperature of the fluid medium matters because the temperature determines the heat transfer rate. Note that temperature does NOT cook food. Temperature -- specifically the difference in temp between the fluid and the food -- determines the heat transfer rate which DOES cook the food. The temperature in your oven and thermometer on your grill are indicators of *convective* heat transfer, but are not the mechanism by which food itself is cooked.

Radiation: Searing directly over a heat source like a bed of hot coals is mostly done by radiation. The temperature of the heat source matters, but the distance from that source determines the heat transfer rate. The thermometer on your grill doesn't measure ANYTHING related to IR heat transfer -- only the temperature of the air which contributes to convection.

4 hours ago, Aussie Ora said:

I set the coals at the back for rotisserie cooking . I don't know why you would put them at the front

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This is what I do too.

[El Pescador]

8 hours ago, Pequod said:

Conduction: food on a griddle or in a hot pan is cooked by conduction. We use the knob on the stove -- low, medium, high to tell us how hot it is (given time to reach equilibrium). When our grill grates get hot and we get grill marks on our food, this was achieved by conduction -- direct contact of the food with the grate. Note that the grate itself may be considerably hotter than the temperature reading on the grill. This is because the grate was heated, in part, by radiation...which doesn't care so much about your grill thermometer.

Thanks, @Pequod - this is awesome.  Your point about conduction leads me to ask a question that's been rattling around in the back of my head - what's the best place to clip the pit probe for your controller?  Does clipping it to the grate give a misleading temp reading or is the device calibrated to account for the higher temp of the grate?  Maybe it doesn't matter because the grate and the metal probe itself are the same temp?

Geekin' out here...!

[tony b]

40 minutes ago, El Pescador said:

what's the best place to clip the pit probe for your controller?  Does clipping it to the grate give a misleading temp reading or is the device calibrated to account for the higher temp of the grate?  Maybe it doesn't matter because the grate and the metal probe itself are the same temp?

Very good question. It's important to NOT clip the temperature probe to the grate with anything metal or it will give you an erroneously high temperature to the controller. As far as I know, no one's software tries to account for this. BBQ Guru sells a "probe tower" to address this issue. Being a cheap-ass, I have fashioned myself a nice holder out of used wine corks (of which I have plenty!) Just trim a little off of each side on one end until it fits between the grate rods. I've then screwed a small screw in the other end to clip the alligator clip on my temperature probe to. The cork provide sufficient insulation that your probe is sensing actual air temperature and giving you a better idea of the cooking temperature for an indirect low & slow cook. Here's a picture of it. This is an older version where I was using a half of a toothpick, but they burned up after only one or two cooks, so I later switched to the small screw.

Found a second picture of one that was a new cork.

Edited May 25, 2019 by tony b

[tony b]

@Pequod Nice tutorial on basic heat transfer. Flashbacks to my 1st heat transfer course as an undergrad! 

Edited May 25, 2019 by tony b

[El Pescador]

Ahhh - Thanks, @tony b!  I like the cork idea - I'll have to give that a go!

[Pequod]

I actually splurged on the guru probe tree. Before that I would use a small potato with a skewer to clip the pit probe.

[tony b]

Many ways to skin that cat! I've heard of the spud trick before. Do you run into the problem of the potato cooking and getting soft enough in a long low & slow cook that the weight/tension from the probe pulls the wooden skewer out? 

[Pequod]

42 minutes ago, tony b said:

Many ways to skin that cat! I've heard of the spud trick before. Do you run into the problem of the potato cooking and getting soft enough in a long low & slow cook that the weight/tension from the probe pulls the wooden skewer out? 

Yes. That’s why I splurged on the Guru probe tree! 

[tony b]

That's why I came up with the wine cork alternative.

[El Pescador]

I suspect my pit probe placement may have been partially to blame for what ended up being a very long (too long, I think) cook.  I put two butts on - one maybe 5-6 lbs, the other a big butt - 8-9lbs - at 9:30 on Friday night, with my pit probe attached to the bottom grate.  I didn't hit target temp (203) on the small butt until 4:40 Saturday afternoon and by then, it had become a little dry (still good, just not as succulent as I would have liked).  I pulled the larger one off an hour and 10 minutes later at a temp of 191 (was time to head to the party I was taking them to!).  The large butt was perfect - in hindsight (no pun intended), I should have pulled the smaller one off earlier.  I suspect the slow cook time was a result of cooking at a temp that was lower than what I thought - the probe being too close to the fire and in contact with the lower grate.  I did pop the pit probe up to the top grate using the cork and pick method towards the end, after reading Tony's words of wisdom, but by then, it was a bit too late.  Lesson learned.

[Bruce Pearson]

Thermoworks makes a grate clip that works pretty good,if anybody is interested

[tony b]

Not a fan of metal clips like that, as they conduct heat from the grate and give you an elevated temperature reading. That's why I went with cork, as it is: 1) essentially free, 2) a way to recycle the used corks, and most importantly, 3) has a high insulating property so the probe is measuring true air temperature next to the meat. 

[Aussie Ora]

I rarely use my I grill probe anymore .my cooks have been consistent in time for ages being it collar butt ,pork ribs ,beef ribs .I just use my thermopen around the time I know they are almost ready

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