Curious if anyone has successfully insulated a Hottop- that is with a measured increase in rate-of-rise for a given amount of power- and if so, what the installation process was.

There have been a couple references to a fiberglass paper (McMaster 9323K21) some have used for roaster insulation, so I picked up a roll, $30 shipped. The quickest way to install wasn't the most thorough but should have helped: cut 5"x6" pieces, removed the top filter, and slid them down on either side between the 2 sheet metal walls outside of the drum, leaving a 1/2" gap at the top to allow venting of steam/smoke.

This had a very noticeable effect in that the side walls were no long radiating heat, which I'd expect would result in an increased rate-of-rise when compared to pre-insulated roasts, but if there was a change, it was insignificant.

It seemed the majority of the lost heat was through the top vent. So out of curiosity, I added another layer of insulation over the top (which is permeable enough to steam/smoke it seemed but will need replacement eventually as it clogs up), and that didn't help much either, despite even less noticeable lost heat to radiation.

My power measures ~ 122V with no load, 117V under load, and I'm seeing ~ 15 F/min rate-of-rises during the middle of the roast with the power on full and fan off. This is for 7oz batches, in a drum with extra fins which aid in agitation/heat transfer.

Thoughts?

I've insulated a home built air roaster and the M3; the effect on each has been different. In the air roaster, you blow hot air into a chamber with the beans. By insulating that chamber, less heat is lost and more transfers. So you can roast faster or cooler, especially at the end of the roast when the air needs to be hottest. On the M3, the effect far less dramatic. I can roast reduce roast times by a few minutes, but mostly where you don't want to, after the first crack. However, it does let me finish the roast at very cool drum temperatures; and that is a benefit for a lot of the DPs.

I'm guessing the effect is more complicated for drum roasters in general, since the beans are inside the drum, not inside the actual case that is insulated, like they are in an air roaster

rama wrote:Thoughts?

Where is your BT probe? Mine is poked through the chute lid, a la Randy G.

Similar to John, I have no trouble achieving 20F per min rise with 225g batches.

Jim

The BT probe is through the back wall, in the lower right corner.

Hi, Rama -

I can think of a couple of reasons, unrelated to actual roaster performance, why my BT probe might show a higher rate of rise:

• Since only about 1 inch of the probe on my roaster is buried in the coffee, my readings are probably influenced by environmental temperatures which rise faster than bean mass temperatures.
• Or, maybe your probe is thermally lagged to the rear wall of the Hottop? Can't really make out the details in the photo.

My BT readings are almost always between 380F and 385F when FC starts. How does this compare to your readings?

Jim

Hi Jim,

• I've installed bare wire probes, with about 1/4" between the bead and the JB Weld holding it in the rear wall. I can't imagine there'd be thermal lag as a result? In retrospect I wish I had left more wire exposed, but feared they would bend and become displaced with time.
  
• The BT installation location was chosen by the very obvious "clean" spot the beans scrape away at the rear wall. IIRC, your installation was much higher- and possibly not sufficiently in the bean mass when the beans are still small in size?
  
• First crack typically is reported at 325F BT. Annoying low compared to most people's readings, but easy to compensate for when comparing numbers given its consistency. I estimated the end of the drying phase to be at 245F on my readings, for example.

This is a typical roast for me (the image should look familiar ), with first crack beginning close to 9min:

Hi Rama

The insulation helps most in cold ambient rooms. It will help you roast to the same level using colder drum temperatures. It will also give you bigger rates of change to shorten your final time in colder weather. Basically, it pays off in the winter.

Is the green color curve the rate of change? If so, you could make it much less jittery by sampling using longer times (instead of 1 second use for example 5 seconds).

Cheers

Arpi, yes the green plot is the rate-of-rise, and needs to be filtered a bit heavier.

Living here in central California, it sounds as if my money would have been better spent on a Variac than insulation.

Although your measured temperatures (both ET and BT) are around 50F lower than what I read on my HT, you are reaching FC in 9 minutes. My roasts rarely get to FC that fast.

This suggests to me that the heater output is similar to mine, but for some reason your temperature sensing system is reporting results that are a lot different. I know we are using identical logging hardware and software, so the difference must be the sensors, right?

With only 1/4" projection of the TC's into the roasting chamber, your readings might be influenced by the temperature of the rear wall of the roaster. I wouldn't have suspected that with bare bead TC's, but I'm having a hard time coming up with an alternate explanation.

Jim