Geeky post II: So THERE’S your problem!

I couldn’t wait. The weather was gorgeous today and I was just itching to light the Egg back up again.  Spring is upon us and I’m really looking forward to doing some long cooks. The trouble with testing stuff like this is you wind up cooking a lot of barbecue. That’s not a bad thing in theory, but it can get expensive. I’m also trying (at least a bit) to watch what I’m eating. Ribs or whatever for a second day just wasn’t a good idea. I adapted an idea from Dr. Greg Blonder, the guy who wrote about the stall that I obsess about so often. In his experiments he used sponges soaked in water and tied with twine to simulate meat cooking.  I rolled up a couple of towels, tied them tightly, then soaked them through. You can see in the picture below how the temperature probes are arranged.  The thermistor-type food probes are in either end of the “meat” while you can see the thermocouple grill temperature probe clipped to the grate at about 12 o’clock.

IMG_1153
Click to enlarge.

It looks like my hypothesis about the effect of the open charcoal bowl grate was right. Apparently I was letting in more air than SmoBot could account for. To recap, yesterday I had a bad problem with overshooting the target temperature. Once the temperature was exceeded, it was really difficult to bring the temperature down.  Difficult?  Impossible most of the time. The only way I could get the temperature back down was by shutting the lower damper. As I said yesterday, the country ribs I was cooking weren’t hurt a bit by the temperature variations, but this sure didn’t inspire confidence for letting a fire go all night.

This is the graph of the cook using the open grate.
This is the graph of the cook yesterday using the open grate. Click to enlarge.

Below is the graph of my experiment today.  I took out the open grate from the bottom of the charcoal bowl and put the stock grate back in. I didn’t add any new charcoal because I knew I wasn’t going to be leaving the fire going that long. This is what a cook graph is supposed to look like.

This is more like it!
This is more like it!  Click to enlarge.

You can see where in the initial heating there came a point where SmoBot was able to gradually bring the temperature right to the target. I made it a point to leave the lid open for several minutes, long enough to let the grill temperature drop below 200. The overshoot that followed was just about the same amount as yesterday, but it was brought under control in just a few minutes. Then it stayed planted at the set temperature of 250. I opened the lid again to simulate … well … opening the lid because that’s what I did yesterday to sauce the ribs. I got some overshoot that was followed by a little undershoot. I haven’t looked at how much fuel remains in the bowl, but I suspect it’s getting down a bit. I started it with only about a quarter of what I’d normally use to cook on. The thing to remember is that there are going to be variations in temperature all the time. Fuel burns out and other lights off. This isn’t an electric element in your oven. The fire is going to change. As long as SmoBot can control the airflow completely, the algorithm is solid.

This graph is also a very good illustration of the stall. Probe 1 was inserted in the towels a little closer to the outside than Probe 2. If you enlarge the image you can see that the temperatures diverge from around 2:15 PM until just before 3 PM. Probe 1 is heating faster than Probe 2 until that point. Then Probe 1’s reading start flattening out because the effects of evaporative cooling have started affecting the probe closer to the surface. A few minutes later Probe 2 starts experiencing the effect and the readings converge again. Both temperatures continue to rise, but slowly. They only would have begun rising again when most of the water had evaporated out. You can see the steam coming off the towel right along the top edge in this picture I took as I was shutting things down. That’s not smoke.  Nothing was burning, though the string was starting to get brown. 

One last note on the open grate. It let a lot of crap fall down into the bottom. Lots of stuff that wasn’t completely burned. I’d cleaned the ash out prior to cooking yesterday, but when I cleaned it out today you’d never have known it. Usually I pull out only fine-to-medium grain ash.  This was great big chunks of stuff that didn’t burn.

I think the only time I’ll use the open grate is when I really want to get the Egg hot, like for pizzas or steak. Otherwise I’m sticking to the stock grate.

Live and learn.

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