My thought when I cobbled that all together was that a lower mass probe, located away from the rear wall might give better readings. Part of the problem is that with a 250 g load, in the early portions of the roast the bean mass is not very deep. The way I inserted the probe was trying to get it into the beans mass, away from the rear wall, and at the same time to clear the drum.

My motivation at the time was to be able to easily import data into my computer with my datalogger, but as it turns out, the Serial interface on it no longer works, so I have to manually transfer all the data from the meter's display into the spreadsheet..

In reality, I do not think that what probe used or how it is mounted matters as much as using one with as little mass as possible, and one with a mount that offers consistent readings from batch to batch.

I came through the wall lower to keep the bt t/c in the bean mass at all times. With the 227g loads I roast it works well.


It's hard to see in this photo but the t/c extends into the drum about 1".

Dieter01 wrote:I've been searching this jungle trying to find a thermocouple that looks like the one in the picture but I can't find anything that matches... Suggestions?

Just use one of the thermocouples that come with the meter.

NOTE : The thermocouples may absorb coffee down the wire. It does not effect the reading until if fills the meter with coffee. A word to the wise : Use a short extension so any coffee does not leak into the meter.

CafeIKE wrote:NOTE : The thermocouples may absorb coffee down the wire.

Yeah, I read the Scace device thread one more time over last night. My first experiment will just be to calibrate the water temperature of the PID on my Duetto. After that... Who knows Thanks for the tip about the datalogger though, I didn't know it also came with a set of thermocouples.

Randy G wrote:In reality, I do not think that what probe used or how it is mounted matters as much as using one with as little mass as possible

Randy, do you think the probes suggested in my shopping list are the best option or do you have an alternative suggestion?

johnB wrote:I came through the wall lower to keep the bt t/c in the bean mass at all times. With the 227g loads I roast it works well.

John, perhaps you could help me with the shopping list... What fittings am I looking for?

Dieter01 wrote:John, perhaps you could help me with the shopping list... What fittings am I looking for?

I used 1/8" brass pipe nipples to mount my two t/cs in the wall. Most hardware or plumbing stores will have them & they are easy to cut to size. The nipples are held in place with JB Weld which is also what I used to pot the t/cs in the nipples.

I would recommend a thermocouple which was:
- stiff so that it stays in place.
- closed so as not to admit any coffee particles or oils.
- as low mass as reasonable for faster response
I had previously looked at the ones you linked from Omega for my project and besides the cost, I didn't see any I liked. here's what I did. I took some thermocouple wire and tightly twisted the end together for about 1/8" and crimped them. I used a stick thermometer's stainless tube probe and bent it to about the shape I needed. I squeezed just a little thermal heatsink grease into it and slid the thermocouple wire pair into it until it hit bottom. The upper thermocouple was from eBay with a supplier that makes them in the far east. I found the smallest one that fit into a threaded opening and use that for the ET. Elegant? No. But they work well enough for what I needed.

cafeIKE wrote: Thermocouple response times in air @ Omega

I also came across this which looks like a slightly more informative version of the same thing. The "Time Constant" is defined as the time required to reach 63.2% of an instantaneous temperature change. Multiply by 5 to get 100%. For beaded-type and ungrounded junctions multiply time constants by 1.5.

OK, so trying to relate that to roasting. A 1/4" grounded probe has a time constant of ~40. Does that mean that if the temperature change is less than 40 seconds / degree the probe will keep up and measure within the 63.2% acceptance criteria? In order to measure 100% correctly the change needs to be less than 5x40=200 seconds / degree? For a 1/8" probe the numbers would be ~13 seonds / degree (63.2%) and 65 seconds / degree for 100%.

If I got that right I assume a 1/4" probe would not be accurate enough for roasting purposes but a 1/8" probe should do fine.

Randy G wrote:I had previously looked at the ones you linked from Omega for my project and besides the cost, I didn't see any I liked. here's what I did

Randy, have you tried doing a roast comparing this latest BT installment with your chute approach? Would be interesting to see how much they differ.

Randy G. wrote:I took some thermocouple wire and tightly twisted the end together for about 1/8" and crimped them. I used a stick thermometer's stainless tube probe and bent it to about the shape I needed. I squeezed just a little thermal heatsink grease into it and slid the thermocouple wire pair into it until it hit bottom. ...Elegant? No.

And possibly not very accurate, depending on the composition of the heat sink compound.

For DIY probes, twist the wires together at the bead end. Place BOTH bared [ or connector pins ] on the NEG terminal of a 12v battery. Slowly bring the bead end close to the POS. When the spark jumps , jerk the probe away. Voila ! Fused connection. Every bit as accurate as store bought
Also see DIY thermocouple

cafeIKE wrote: ... of a 12v battery.

Don't do this using an automotive lead-acid battery (at least not the part where you strike an arc directly on the terminal). There's probably only a small chance of igniting the H2 around a modern sealed battery, but it is not worth the risk.

Jim

I've done it many times on automotive batteries in the shop, not in the car.

In a well ventilated area, when the battery is not on charge, the risk is next to nothing.

Consider that on a modern automobile, with all it's electronic gadgets, a small spark is drawn when the battery is dis/reconnected for/after service.

Probably 1000's of times per day cars are jump started from a fully charged to a completely discharged battery and there are no explosions when the spark jumps.

You can use a 30v 10A desktop supply if you have one. Take the center electrode out of a AAA battery and stick in the POS bindng post to prevent damage to the post if making a lot of probes.