Measuring T diff above and below cooking grate

[DaveD]

Howdy folks, when I did some beef chuck ribs recently, they cooked super fast and one of the potential explanations was that because I had my cooking grate T probe clipped on to the underside of the grate, I was measuring a lower temp and the meat was seeing something hotter. Many posters recounted their view that one should expect a higher temp above the grate than below.

So to test this and measure the difference, I set up three pairs of probes, clipped at the same spot with one above and one below, and dumped in a lit chimney of Royal Oak Chef Select charcoal, what I happen to have on hand. Set the vents to what I have been using to achieve temps in the 250-275F/120-135C range, and let the Fireboard 2 log the data. After about 90 minutes the results are clear.



Here's where everything was set up. After about ten minutes, I closed the bottom vent fully.










Two things are clear. First, the temps are very similar to each other above & below the grate, very consistent. So the horizontal temperature gradient is very small. Second, sure enough, there is a difference ranging from 30 to 40 deg F between upper and lower probes at a given spot. Those deltas reached as high as 50F earlier in the test before things fully settled down.

Now, it's still a question whether temps 40-50F hotter (i.e. almost 300 instead of the 250 I was after) would be enough to make a rack of beef ribs that ought to take 8 hours to cook to get there in 2; I have a hard time believing that. But regardless, this is great info to understand and I won't be clipping my probes to the underside of the grate anymore, that's for sure.

It would have been ideal to have a piece of meat on the grate when I did this, but there really wasn't room with all those probes there. But I now have data... for SCIENCE!

[Huskee]

I know from experience on the actual grate it's 50-75 lower than a couple inches up, using the SnS. Much more if your probe is under the grate. Heat rises exponentially with the kettle/SnS combo.

[rlozinak]

I am surprised at the difference between the above and below grate temps. I would not think there would be that much of a difference. Good to know tho

[rickgregory]

NERRRRD Alert. Nice experiment. Of course, now you need to do it with meat there to see if cold meat affects things much. For science. And eat the evidence.

I suspect the difference was larger when you cooked the meat otherwise the 4:1 difference is a bit much to be explained by even a 50F difference, I agree.

[DaveD]

NERRRRD Alert. Nice experiment. Of course, now you need to do it with meat there to see if cold meat affects things much. For science. And eat the evidence.

I suspect the difference was larger when you cooked the meat otherwise the 4:1 difference is a bit much to be explained by even a 50F difference, I agree.

Yep, more data were needed! Nerds for the win. Repeating it with some meat in there will be the next step, now that I have the baseline established with an empty grate. I have a 4-ish pound pork butt that I can probably fit in there with all those probes in place (I won't bother measuring below the grate on that run though, that decision is made).

I updated the temp profile in the OP as well. This Royal Oak charcoal does seem to burn quicker than the B&B I'd used in my first cooks. But to be fair, I didn't attempt the low & slow configuration of lighting one end of the insert and letting it creep across, I just dumped a lit chimney in there. Would be interesting to see if the delta-T's would be any different doing it that way...

[rickgregory]

"(I won't bother measuring below the grate on that run though, that decision is made)."

No no, we need complete data!

[Red Man]

I would still measure below the grate when you do your test with meat in there. That was the whole point of this, right? The horizontal temp is very even but the vertical temp difference is pretty substantial, I bet it will be more if it’s loaded up with meat like when you had a rack of short ribs in there.

[Jerod Broussard]

That is why I used a raised grate with the SNS.

[Huskee]

Yep, you're in effect, "paying for" that heat up top, might as well use it. Then you can lower your actual grate-level temps when cooking at 'higher elevations', thereby conserving fuel and costs. I've done briskets on the elevated grate and it's fun seeing my grate-level probe read 170 or whatever it reads.

[bbqLuv]

Too complicated for me. I try to keep it simple.
But we do need to know our grills. And that will do it.

[RlsRls]

With my PK 360, Ive noticed a huge difdference of temps between the factory installed thermometer LOW on the hood, and the temp I would measure, with a probe dropped an inch or two from the top left vent, right where I would place some meats etc.. on my "little more" grate I bought separately! I use the "little more" grate a lot too speed up a cook. Just FYI!

[DaveD]

I no longer see the point of measuring below the grate with meat present. I think the view is unanimous that the presence of meat will make the gradient only more pronounced, not less. It's already pronounced enough with no meat present for me to rule out ever clipping below the grate again. All that will do is reinforce that conclusion. Plus, it took all six Fireboard ports to do this test, if I have meat in there I'll need at least one port for the meat temp (maybe more than one depending on what's in there).

What would be even more interesting is if I could rig something up to measure the temp another couple inches/cm higher, and see if it keeps going up. That would be useful info for those taller cuts, like pork butts or briskets...

[Red Man]

Ball up some foil to get the probe higher

[WayneT]

I’ve always heard/read you should put your probes on as close to the same vertical plane as the food but far enough away so that the food itself doesn’t affect the readings. Out of curiosity, where did you read or hear about putting them beneath the grate and was it the same application, i. e. A charcoal grill with an SNS insert?

[Sid P]

Great info, Dave, I didn’t expect that much of a difference.