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08 August 2014 @ 11:52 pm
Sparky -- Phase III  

Today was the day! I took a break from the office in order to fire up Sparky. I wanted to season Sparky's innards and verify whether adequate heat will flow through the structure for proper barbecue making. I needed a stretch of several hours with no rain and today was the day.

I set three temperature probes in the smoking chamber -- one positioned in the airstream entering the chimney, one in the middle of the chamber at grate level, and one opposite the chimney at grate level. I readied my trusty infrared gun, set the lid on the smoking chamber, and lit my first batch of charcoal briquettes in the lighting chimney.

I used a mixture of Kingsford briquettes and natural lump charcoal. The briquettes provide a longer, steadier burn; the lump charcoal burns hotter and faster.


At 11:50 a.m., the first chimney of briquettes was ready to dump. I poured them onto the grate in the firebox and covered them with about half again as much lump charcoal. The lid was lowered and the air intake vent fully opened; the chimney was also wide open and unbaffled. Ambient air temperature was 78 degrees.

The chimney drew beautifully; I could immediately see the light smoke from the igniting lump charcoal billowing from it and could feel the hot air jetting out. Within ten minutes each probe was registering 175 degrees.

By around 12:30 p.m., the smoking chamber had plateaued at 200 degrees. Plainly, my firebox was under-powered. I lifted the lid to add more unlit charcoal -- perhaps 30 briquettes and an equal volume of lump. I also decided to add a handful of hickory chips so that I could readily see where things were not smoketight.

The smoking chamber temperature dropped to about 170 degrees due to the length of time I had the firebox open. But a beautiful, thick hickory smoke quickly began to flow through and out the chimney. A fair amount escaped around the smoking chamber lid, but none was escaping through the block walls. That was a welcome discovery!

The leaking around the lid is due to the lid being pretty perfectly flat and my blocks, not so much. But it's nothing that a little fiberglass rope gasket won't fix. It is ordered and on the way; I expect to take delivery on Tuesday.

By 12:50 p.m. I topped out at 225 degrees. For proper barbecue, you need the cooker to sort of hum along steadily in the 225 - 275 degree range for many hours, never getting much hotter than that upper range or any cooler than the lower. Two hundred fifty degrees is a real sweet spot, but my target method is to get the cooker to hit a peak at 280 or so and then slowly coast down towards 225, tweaking the temperature up or down as needed by manipulating the fuel, chimney baffle, and intake vent throughout the cooking session.

Moving quickly, I lifted the firebox lid and added a fairly healthy dose of unlit briquettes and lump charcoal -- perhaps as much as originally went in. The temperature dropped only to 220 degrees this time. By 1:20 p.m., I hit my target temperature of 280 degrees on all three probes. I set the chimney baffle and closed the intake vent about half way.

At 1:40 p.m., the temperature had dropped to 260, which was a faster declining curve than I wanted, so I opened the intake vent fully.

At 1:50 p.m., I was back up to 278 degrees in the center of the chamber. The temperature stayed steady for about 15 minutes and then began a more gradual downward arc of about 1 or 2 degrees every five minutes or so.

When the temperature dropped to 269 degrees, I removed the chimney baffle to see how quickly it would recover. Within 5 minutes it was back up to 280 degrees. Beautiful!

I replaced the baffle and let it coast down for the next two hours. At one point I added a single chunk of hickory, about 1.5 cubic inches, just to see how it affected things. The temperature quickly rose 5 degrees.

As the thermal mass of the structure thoroughly heated, I could see that it would take much less added fuel to keep the temperature clicking along in the 225 - 275 degree zone. Thus, after about an hour, adding 5 unlit briquettes and an equal volume of unlit lump and resulted in a five degree temperature rise and a moderate slowing to the declining temperature curve.

At about 4:30 p.m., the smoking chamber was at 239 degrees. I baffled the chimney (I had removed the baffle when I added the last bit of fuel) and closed the firebox intake vent. I wanted to see how long it would take to drop the temperature under those conditions. Within twenty minutes the temperature had dropped 20 degrees.

A few random fun facts:

  • The temperature never deviated more than 10 degrees across all three probes.

  • The lid of the firebox did hit a scorching 650+ degrees. It actually bent upwards from the heat, but as it cooled it resumed its flat shape.

  • The lid of the smoking chamber never got above 200 degrees.

  • At about 9:00 p.m. the coals and ash inside the firebox were still maintaining a residual heat of about 400 degrees.

Bottom line: I can cook on this! It is responsive to the controls. It will not take an inordinate amount of time or fuel to heat up and keep the temperature going for 12 hours (at least, during warm weather). It looks like my design works as intended. It will be a few weeks before I have time to actually try it out with a couple of pork shoulders, but I'm looking forward to it!