I have found the Omega XCIB style 3 K probes to be well worth the extra cost. They have a shoulder that fits nicely through the newly drilled holes. I routed the cables under the main board.

Very quick response to temperature variants.

Hi All.....

Thanks for the responses. I pulled the trigger on my probes today. I made a pour-over of some pretty damn good Yirgacheffe that I roasted 3 days ago and spent some time talking with the engineers at Omega. I ended up taking a slightly different approach than the other mods I have seen so far.

I landed on an ungrounded SS type K thermocouple, 1/16" in diameter, and 2.5 inches long, rated at 500f, with a simple epoxy bead transition. This is essentially the same probe I used to make my homemade SCACE device, just shorter. I will use a 1/16th NPT by 1/16th tubing compression fitting (with a back nut and lock washer) to stick it through the back wall. Just drill the hole, stick it in and screw the nut on, no JB Weld needed. The whole compression fitting is just under an inch long, roughly half of it will stick out on either side of the wall. I will allow the probe to stick out at least a half inch (8 diameters) through the compression fitting which should eliminate any danger of "stem effect". The ungrounded junction with teflon insulated wires will eliminate any danger of ground loops. This fitting/probe works out to roughly 1.25 inches inserted into the drum, and 1.25 inches sticking out behind the drum with plenty of play to slide the probe in or out as needed for fine adjustments. I did all the math and took a boatload of measurements and unless I screwed up it should fit just fine.

It turns out that an ungrounded 1/16th probe has a response time of just under three seconds, the XCIB style 3 (the best I solution I have seen to date) has a response time of 2 seconds, so essentially both probes are pretty fast. I traded the slightly slower response time for the durability of the sheath and ease of mounting. I figure that with the thermal lag of that heater being around a minute, a 1 second difference in response wont make that much difference in controlling the process. That one minute thermal lag means you still have to do "predictive" profiling no matter how fast your probe is, 1 second wont make or break me.

What I am shooting for is durability, ease of installation, and ease of maintenance. If the probe ever needs replacement, just loosen the compression fitting and insert a new one.

I will post pictures once I get it all done.
Now just to wait for all the parts to come in........

mikepetro wrote:I landed on an ungrounded SS type K thermocouple, 1/16" in diameter, and 2.5 inches long, rated at 500f, with a simple epoxy bead transition.

This sounds like a very good solution. I agree with all of your reasoning for the selection. I would have gone this way myself but I could not find a supplier of such thermocouples. Could you provide the model number for the thermocouples?

The XCIB style 3 are fairly robust as they use very heavy gauge wire but they are expensive and have the disadvantage of being inconel braided which then requires insulation to ensure it does not short against exposed terminals within the roaster.

The only concern I would have with your solution would be the protrusion into the roast chamber for the BT probe. I have found that it is necessary to keep at least one coffee bean width clearance between the drum vane nearest the rear wall and the thermocouple. Less than this and beans can get trapped between the rotating drum vane and the thermocouple. This will certainly cause alarming bumps and may result in a bent thermocouple or even damage to the drive motor. Hottop have changed the design of the drum a few times but on my roasters the maximum protrusion into the chamber is about 0.5 inch. The compression fitting approach would use up some of this. Your drum may allow more room. I have turned over the inner edge of the vane nearest the rear wall to provide more clearance. It may be possible to bend more of the vane or even cut it to provide more clearance but this may affect ejecting the beans.

I look forward to photos of your installation and would be particularly interested in the drum vane/thermocouple clearance and the clearance between the thermocouple and drive motor in the rear compartment.

Hi Greenbean,

Yes, I may have to get a little creative there, I wont know for sure until I get the compression fitting in my hot little hands. I do have a 1/8th" NPT by 1/16th tube fitting that I used on my SCACE to do a rough estimate. The one I have coming will be even smaller. The point where the fitting sits in the wall is not exactly the half point. I do know the new fitting is .97" in total length, what I dont know is exactly how much is on either side of the bulkhead. The diameter of the fitting is .313" so half of that is .157". I figure I can always use the compression nut on the motor side of the wall if need be, and put the NPT side inside the drum as it is the shortest side, I can even file the NPT side down a little bit leaving just enough thread for the nut to fit flush.

Anyway, I figure I need 5/8" clearance to accommodate the largest Supremo grade beans. I am estimating that the line clear of oils is the inside diameter of the drum. My proposed location is 7/8" in from that line. The vane appears to be about a 1/4" wide. That would give me 5/8" of clearance, and I can make a creative bend in the probe if that isnt enough.

The biggest obstacle I see is the bean eject chute on the backside, but I think I am good there.

Stay tuned....

Almost forgot, I think the part number for that probe is HTTC24-K-116U-2.5

I say "think" because I ordered it over the phone and dont have the invoice yet. Its not a stock part but Omega makes custom configurations if you ask them to.

BTW, the back wall of that drum has a high steel content. A magnet sticks to it quite well. I am drilling a hole just big enough to fit a toothpick in a small magnet. When I have all the parts in hand and know precise measurements I will cut a toothpick to simulate the actual length I intend to use and stick it in the magnet. I will then use the magnet to simulate placement and test clearances, with the drum and motor in place, BEFORE I do any drilling.

Swiped a photo from the Hottop site. This shows my target area on the back side of the bulkhead.

Lots of photos of where to go through the wall in this thread:
Installing ET and BT probes in a Hottop The photos at the bottom of pg 8 show some good locations. If you aren't using a flexible T/C you'll want a location that requires as little bending as possible. Installing an ET T/C also?

Why choose that location? The location shown by so many others moves the sensor further away from the heating element, places it deeper in the bean mass, and gives more room to maneuver the wiring without fighting with other components.

The main reason is the drum vane that Greenbean spoke of. My probe will stick out at least an inch, therefor I cannot go under the vane, so I chose to go "beyond" the vane. In order to leave enough clearance so that beans dont get caught between the probe and the vane I had to go 7/8" inward from the inner wall of the drum. Really, I am only about 1/4" more inward than most of the other installations I have seen and am still well within the bean mass. Yes, this puts me in a tight space on the backside, but I can deal with it.

Keep in mind that the placement is theoretical at this time while I await the actual dimensions of all my parts. It is sort of a worst case scenario based on known measurement and some assumptions. For one thing the inner wall of the drum calculation is based on the oil stains, which is not very precise. Once I get my parts in I will construct a rubber magnet that mimics the probes dimensions and can then play more with the placement and get an exact clearance. I am also trying to figure out how I can attach a Sharpy to the edge of the vane on the drum and then rotate it to make a "vane depth" mark on the backwall that I can measure from. That would allow me more precise measurement. Hopefully after trial and error with the magnet I can arrive at a more friendly location that satisfies all the clearances, I would love to get away from the corner of the eject chute on the back.

OK, I got the exact dimensions of the compression fitting finally so I made my magnetic "mock" probe. I took one of those self adhesive rubber magnet business card backings and using a 5/16 punch (the exact diameter of the compression fitting nut) I punched out enough dots to stack up to the thickness of the fitting. I then drilled a tiny hole through the middle of the stack and stuck a paperclip in it that was cut to match the length of the probe. I now have a magnetic replica of the dimensions of my probe that I can move around on the back wall of the roaster.


I want 5/8 clearance between the vane and the probe so that no beans get stuck, so I made line on a piece of tape to mark off 5/8 and stuck the tape to the bottom vane. This way when I move my magnetic mock probe around I can check to ensure I have adequate clearance.


So I put the drum and front bracket back on and moved the probe around looking for the right spot where the tape wouldnt hit it. Unfortunately my idiot-proof camera proved I was an idiot as I couldnt figure out how to shoot a photo that deep into the barrel, it kept trying to focus on the front bracket. Anyway, I ended up almost exactly where I projected I would need to be. I havent torn the machine down to look at the backside yet but I suspect I will be right up next to the corner of the cover of the eject chute. I will do the same magnet trick on the backside to make sure I clear everything, but I still believe I will be alright.