Sherman wrote:For those of us without access to a spot welder, would high-temperature silicone sealant provide sufficient adhesion?

I don't think adhesion would be the problem. The enemy here is the temperature gradient between the bottom of the roasting bowl and the sensor. I think the reason that spot welding the wires to the bowl was so successful is because the bowl itself becomes part of the thermocouple. So there is no gradient.

I have a few of these bowls made up and sitting on the shelf gathering dust. Send me a PM if you want one.

Jim

Jim -

I use a home-built capacitive discharge spot welder (several big electrolytic caps, a 24V PSU, and foot-operated SCR). It is a little bit like the one at this link: http://www.philpem.me.uk/elec/welder/

For welding the t/c wires to the bowls using my setup, I use a pair of needle nose pliers as the negative electrode. The positive electrode is a stylus-type, with a brass rod. Press the wire tip against the bowl using the brass stylus, squeeze the bowl somewhere with the pliers, and hit the foot switch to make the pfffft happen. I think I welded these at around 20V.

I don't think a butane flame gets hot enough. And I suspect an incredible amount of skill might be required to use an oxy-torch to weld the wire tips to the bowl without blasting through the metal.

I hesitate to offer any suggestions on building a spot welder. I think it's mostly blind luck that mine works.

Jim

I bet if you walked into a machine shop with thermocouple and dog bowl in hand, they'd spot weld it for less than $10. That is, after they stop laughing.

I get the idea of using a spot weld to make the bowl part of the junction, but that seems to boil down to ensuring simple physical contact, right? I can understand if using silicone may not provide sufficient contact, but as far as the temperature gradient issue, I'm failing to understand the functional difference between spot welding and soldering. Hmm... maybe a soldered junction with a blob of hi-temp silicone for insulation might do the trick.

JimG wrote:I hesitate to offer any suggestions on building a spot welder. I think it's mostly blind luck that mine works.

Thank [deity] for Instructables. In case nobody hears from me for a while, it'll most likely be due to the extended hospitalization resulting from me being incompetent with regard to electricity or because I've glued my fingers together with silicone sealant. Or because of the Libyan terrorists who want their plutonium back.

JimG, thanks for the quick lesson on thermocouples.

-s. "There are 0xFFFFFF kinds of people in the world. Those who understand lame math jokes, and those who don't care."

Sherman wrote:I'm failing to understand the functional difference between spot welding and soldering. Hmm... maybe a soldered junction with a blob of hi-temp silicone for insulation might do the trick.

If you can overcome two problems, solder would be perfect:

• Common solders melt at 400F or below. The bowl will exceed this temp.
• Getting solder to stick to stainless steel, as well as the alumel/chromel alloys in K wire, is difficult. I have had very limited success.

Jim

Again, JimG, thanks for the lesson. You've given me pause, but also opened another door - I did some Googling and learned about the possibility of high-temperature solder which I found at McMaster-Carr but from what I read, this path starts to lead to issues with stainless steel, as you mentioned. Further reading gets us into the issues with stainless steel, such as the problem with using acid flux. So if I'm hellbent on soldering, I have to pour muriatic acid onto the surface to dissolve the oxides and present a more solder-able surface (and risk health issues with leaching cadmium into the beans), use a high-temperature solder that melts >500°F. Would an off-the-shelf RadioShack soldering iron be able to produce the necessary heat? I digress...

We're still left with the problem of attaching the wire to the surface sans spot-welding. The Kester article made me think "why not use the same technique that was used for my Silvia?" (FWIW, I bought Auber's PID. No offense intended, I just couldn't afford your stuff.) I think that my next test (after sending you a PM about getting my hands on one of your spot-welded dogbowls) will be to drill a small hole, and mechanically attach the wire to a small screw & nut. Hopefully, my assortment of spare PC parts will produce something suitable, and the screw/nut will be conductive *rimshot* to this process.

-s.

another_jim wrote:Adding a complete TC via a screw has been tried (by Martin I think) and didn' work.

Actually, I tried a number of instruments and configurations but not attaching to them to the bowel. The best results were when I attached the TC or a particular probe-type "fold out) thermometer to the stirring wand where it would remain in the bean bed but not hit the bowl. Of course, I never tracked the temp other than keeping one eye on the ramp another on the timer, and a third on the beans. Working at my hardest and most precise, thermo-monitoring simply confirmed the other non-metered data I was processing.

I have to say that I got no new useful information about my roasts that I was prepared to act upon. So the question was not so much did it work (it did), but why bother? HG/DB is inherently a soft-focused, artisanal, feel-as-you go process. With a lot of experience you begin to triangulate the physics of the heat, stirring, batch size, and familiarity with the generic properties of roasting beans. A while longer, and blends and SOs start to make more sense.

There's lots more to be said about the good roasts and satisfaction of HG/DB roasting, but I don't hold much promise for its being modded into an econo Hottop.

Martin, thanks for the info. One thing stands out to me...

Martin wrote:Actually, I tried a number of instruments and configurations but not attaching to them to the bowel.

I'm not quite sure of what kind of instrument OR configuration you might need for this, let alone the possible requirement for sterilization

Martin wrote:With a lot of experience you begin to triangulate the physics of the heat, stirring, batch size, and familiarity with the generic properties of roasting beans. A while longer, and blends and SOs start to make more sense.

While I agree with this statement, my reasoning for pursuing this line is to provide a path to reducing the amount of time/experience needed to arrive at a point where consistent & acceptable results can be achieved; to make the learning curve more shallow, as it were. Don't get me wrong, I'm all for the artisanal method, but in my current state of mind I'd trade the warm fuzzy for some solid data and results that can be strongly correlated, neither of which I seem to be getting so far.

Martin wrote:There's lots more to be said about the good roasts and satisfaction of HG/DB roasting, but I don't hold much promise for its being modded into an econo Hottop.

Acknowledged. That said, I'm still searching for something that gets me Hottop (or better) quality at DB prices with HG-level control. I'm getting closer, but still need to do a few more roasts to confirm my preliminary findings.

Now if you'll excuse me , I'm going to go push this here boulder up that mountain...

Regards,
-s.