I’m wondering if I’ve out-thought myself. It wouldn’t be the first time. I’ve certainly done it before. You might even call it my specialty.
You know I’ve been beta testing the SmoBot robotic damper controller. I’ve written more about it here and here. For all of you TL;DR folks, it attaches to the air outlet of most popular kamado-style smokers (like the Big Green Egg). A temperature probe monitors the internal temperature of the cooker and opens and closes the top damper to maintain a consistent internal temperature. I’ve used a couple of different iterations of the hardware and software since November and I can say with absolute confidence that the thing works.
One key difference between this controller and others is that there’s no blower or anything else that forces airflow. It’s a strictly passive system. The basic idea is that you leave the bottom damper in a set position and then don’t move it. The top damper does all the work. That leads immediately to a question: how much should the bottom be opened? It’s not a trivial question. I’d argue that the fundamental skill you have to be able to master on an Egg (or any other good-quality cooker) is airflow control. A self-sustaining fire requires air, fuel and heat. More air supports more fuel combustion, less air supports less. Combustion is where the heat comes from Basic stuff. Go on any of the Egg online forums and you’ll see thread after thread discussing the finer points of damper control. The fact people have such trouble with it is a reason a device like the SmoBoT is such a great idea.
When Eric, the guy who developed the device, first demoed an early version he ran his bottom damper wide open. As the device went through several iterations in hardware and software he changed his practice to running the bottom damper half-open. The idea was that there was still plenty of air available when needed, but wind would be much less likely to affect airflow. That’s what I did when I first got a unit and it worked fine Then I made a change to the Egg. It’s been a bit of a struggle since.
In a kamado-style smoker the lump charcoal is placed in a bowl that sits at the bottom of the unit. The bottom of the bowl isn’t closed. In the case of the Big Green Egg, there’s a grate that looks like this:
You can see the holes aren’t very big. The disc is 9-inches across and the holes are maybe 3/4 of an inch across each. When you let a fire go for a long time and ash drops to the bottom of the bowl, those holes can get plugged up. Not good for airflow. I replaced my stock grate with this:
No way that’s ever going to get plugged up. But I think it may be too efficient. Here’s a graph of tonight’s cook:
I’m cooking country ribs here. They don’t take long and they’re incredibly forgiving. Carla had a late night at work and I wanted to have them waiting for her when she got home. I’d normally do these at 225F, but I got a little bit of a late start so I went ahead and set my temp to 250F. That’s where the forgiving part comes in. Anyway, I let the Egg come up to temp to make sure the smoke was clean. I had the bottom damper open about 1/2, a little under if anything. I get 5F of overshoot on the initial climb which is within spec. Anything +/-5F is assumed to be on target. We’re talking about charcoal fires here. 5F is close enough. If you squint you can see that just before I open the lid to put on the meat (the first dip) the temperature had actually settled back to 250 on the nose. Then it got crazy.
When I opened the lid to put on the meat, the cooking chamber got flooded with air and more fuel lit off. That’s to be expected. What should have happened, though, is that once the lid was closed SmoBoT could do its thing and reduce the airflow enough to starve the fire back down. But that didn’t happen. The temperature overshot and stayed overshot regardless of what SmoBoT did with the top damper. It ran a good 10F hotter than it should and showed no trend downward.
I tried reducing the set point to 240F and opened the lid briefly to drop the temperature. I also reduced the bottom damper to about 1/8 open, though the top damper was still partly open. That brought the temperature down. I sauced the ribs (3rd dip) and felt guilty about shutting the bottom damper so much. I returned it to it’s original position. And, as you can see, the temperature spiked like crazy. I quit feeling guilty and shut the lower damper down to this:
That’s the same 1/8 it was open before, but this time the top damper was completely closed when I shut it down. It undershot, but was starting to settle in when it was time to take off the meat so we could, you know, eat it. (Delicious, by the way). No country ribs were harmed in the making of this blog post.
My hypothesis is that the new grate is a mistake for low and slow. Yes it can get plugged with ash, but it takes a long time to happen. I’ ve never managed to do it yet. All that’s needed to clear it is a wire coat hanger. Since Joan Crawford isn’t using my Egg I have no problems doing that. People criticize the stock grate, but I’m beginning to think more thought was put into the design than I initially thought. Something like SmoBoT can only work if closing the top vent will kill the fire. With enough repetitions I’m sure I could come up with a sweet spot for the lower damper with the new grate (and I might have already). But why? The stock grate acts as kind of a restrictor plate. Open the damper full, open it it half. It’s all the same. The bottom plate is only going to let so much air through. (It occurs to me that the width I had the bottom damper open at the end isn’t that much wider than a single hole in the stock plate. I wonder if there’s some kind of inverse relationship going on here? )
I’m just going to have to smoke something again this week, but this time with the stock plate in the charcoal bowl and the bottom damper half-open. Oh darn. But it’s for science!