Thermometry for Heat Gun / Dog Bowl Roasting

[JimG]

To my utter amazement, a little holiday "I wonder if this will work" project did.

I spot welded two K-wires to the bottom outside of my dog bowl (DB). The wires are welded directly to the stainless, around 1/8" apart, so the bowl is part of the junction. The setup included a mixing bowl "enclosure" for the DB, both to protect the t/c and reduce heat loss.

Hooked the t/c up to a Fluke 54 and roasted a cup of beans.

What I observed on the thermometer really surprised me. Simply moving the heat gun (HG) a little closer or further away, while manually stirring the bean mass, allows temperature control easily within +/-10F. With a little bit of concentration, I think it could be controlled within +/-5F.

The captured roast profile is not quite as smooth as what I see from my PID'd Poppery I, but it is a helluva lot easier to set up the HG/DB operation. And making game-time adjustments as the roast progresses is much easier than modifying a ramp-soak profile in a PID.

FWIW, I observed that first crack took place when the Fluke indicated approx. 415F. This is around 15F higher than I usually see in in the chimney of my P-I. Second crack was at around 455F, again around 15F higher.

Even though a quick Google and HB search did not turn up reports of similar adventures, I'm confident somebody has measured some roast temps using a HG / DB roaster.

I'd be interested to hear about those results, in particular any lessons learned regarding differences in roast profiles measured this way compared to hot air roasts where the sensor is located in the air discharge stream.

Jim


Thermocouple placement on bottom of DB



Close-up of thermocouple welds



Captured roast profile

[JimG (original poster)]

Armed this time with a planned roast profile, and an improved (insulated double-wall) bowl, I roasted a small SO batch with my heat gun. I had had pretty good success with this profile in my popper and wanted to give it a try HGDB-style.

As I suspected, with just a little bit of care, it is possible to achieve a fairly precise roast profile with a heat gun and this simple "thermometric roasting bowl."

Planned roast profile:

200F    1:00
300F    4:00
400F    8:00
410F    9:00
410F   11:00
440F   12:30

The graph below shows how closely the actual measured temperature profile followed the planned profile. By watching the temperature, and adjusting the distance from the heat gun to the coffee accordingly, it was fairly simple to keep the temperature on track. I have never been able to achieve this level of temperature control using my PID'd popper.



I believe that measuring the temperature of the bottom of the improved roasting bowl is giving a good estimate of the bean mass temperature. I base that on the fact that first and second crack occur at the same indicated temperatures using either this instrumented roasting bowl or a thermocouple in the discharge chimney of an air popper.

I plan to continue testing and (hopefully) improving this system as time allows, and welcome comments or suggestions.

Jim

[jesawdy]

Jim- I love it. Did you purchase some sort of double insulated dog bowl or rig something up for your "improved version"?

[gscace]

To my utter amazement, a little holiday "I wonder if this will work" project did.

I spot welded two K-wires to the bottom outside of my dog bowl (DB). The wires are welded directly to the stainless, around 1/8" apart, so the bowl is part of the junction. The setup included a mixing bowl "enclosure" for the DB, both to protect the t/c and reduce heat loss.

Hi Jim:

Very clever. In case anyone questions the makeup of your junction, I'm here to say that while unorthodox, it'll work just fine and it'll give you the right answer as close as it needs to be for this application. Nice job.

-Greg

[JimG (original poster)]

Jim- I love it. Did you purchase some sort of double insulated dog bowl or rig something up for your "improved version"?

Strictly low tech. I nested a pair of Walmart dog bowls, with ceramic felt insulation in between. Then high temp silicone sealant around the rim(s) to hold things together.

In case anyone questions the makeup of your junction, I'm here to say that while unorthodox, it'll work just fine and it'll give you the right answer as close as it needs to be for this application.

Thanks, Greg. Before I got my thermofilter, I had used this same trick with a SS washer that I had mounted under the screen retainer screw on my Silvia. So long as the junctions with the 3rd metal are nearly the same temp, there shouldn't be much error.

K-wire seems better for this application. The copper lead in T-wire conducts electricity and heat too well for good resistance welding.

Jim

[mgh]

Would someone be kind enough to explain what this equipment is, how this is done?

I am a complete newbie trying the HG/DB with very inconsistent results. Very much need to figure how to measure my roasting temps. Trouble is I don't know what a thermocouple is. I guess the Fluke is a thermometer?

Any help is appreciated.

[JimG (original poster)]

Hi, Mike -

This thread had been inactive, and I did not notice your post for several weeks. Sorry.

A thermocouple is a pair of wires of specific metal alloys. The wires have known characteristics of generating small voltages when their ends are at different temperatures. By making use of this, thermometers (like the Fluke's) are able to remotely sense the temperature at the far end of a pair of thermocouple wires.

Measuring bean temps with HG/DB roasting is not straightforward, especially if you continuously agitate the beans with a dowel like I do.

So I welded a thermocouple to the bottom, outside of the bowl where it would be out of harm's way. But since the bowl is fairly thin, and because steel conducts heat reasonably well, the thermocouple is able to sense the temperature closely enough to be useful.

I am enjoying very good success with this setup. After a lot of trial and error, I have developed a roast profile that gives consistently good results with the Yellow Bourbon I am roasting these days.

I have a little chart at my "roasting station" that gives me a target temperature at 30 second intervals. It is not hard to be within 5F of target at almost all times.

This method is particularly adaptable to different batch sizes, weather conditions, and even bean characteristics. You just move the heat gun a little closer if you need to increase the temp, and vice-versa.

Hope this helps.

Jim

[another_jim]

I missed this thread.

I think Martin or one of the other early HGers tried a thermocouple sticking out slightly from the inside bottom of the bowl. It was postd on AC. This did NOT work. It seems making the bowl part of the junction, or at least using it as a heat capacitor, may be required to get this working right.

Would a surface sensing TC mounted on the bowl's underside, and insulated, be an easier solution for non welders?

[JimG (original poster)]

Would a surface sensing TC mounted on the bowl's underside, and insulated, be an easier solution for non welders?

My own success with stick-on sensors (in other applications, not this one) has been miserable. But if you can get the thing to stay put, and cover it with a pad of insulation (I used a ceramic felt pad that is 1" diameter, 1/4" thick, to cover the welded sensor), it should be a usable substitute for the welded junction.

Jim

[Sherman]

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

Curiously,
-s.