I am adding digital monitoring to my manual coffee roaster. I am using an arduino uno to control (2) K type thermocoupler's with (2) max6675's. I have it working just fine with (2) 1 cm thermocoupler's. I bought a 20cm probe with 2 meters of wire and added it. All I get is 0 or random numbers. I thought I had a bad thermocoupler, because the readings are perfect with the 1 cm probes. So i bought (1) more, and same result. Then I bought another one, same result. The code is fine, wiring is fine. Any ideas?
After all the input, I believe i have been duped and sold (3) j type thermocoupler. I used a heat gun, don't know the exact temperature, I timed it at 30 sec, but the rise on the 20cm probes was much more rapid, and was 60mv. The 1cm probe only made it to 30mv, and was slow.
Update: The 20cm probes are grounded thermocouplers. Thanks to Jim-P, i figured this out. They are not supposed to work with the Maxx6675 module, but I discovered they will with an isolated power supply. The terminations were done very poorly, and caused erroneous readings in my meter, see images below. After I removed the spade connectors, it was very easy to troubleshoot. Thanks for all the input!
There shouldn't be any difference between the lengths of the probes, 1-cm or 20-cm in the way they operate. The longer probes just have longer outer sheaths and longer internal wires,
Have you tested all the thermocouples with a meter?
Connect the long probe directly to the MAX6675 where the short probe worked (same wiring) — does it still fail?
If it fails, replace the 2 m cable with a proper K-type extension or get a probe with built-in extension of correct thermocouple alloy.
Post an annotated schematic if you can along with your code using code tags. Following is a chart showing the output voltage of the different thermocouples. You might also look up cold junction, that is also important. From what I can see the Max interface boards have that built in.
My wording is not good in that sentence. I bought a compete thermocoupler assy that had a20 cm probe with factory 2m of wire. This is a complete factory assy.
my wording is bad in that statement. I added a complete factory assy to the project, i didn't add any seperate wires. I purchased a 20cm probe with 2m of wire attached. Sorry, my statement was a bit confusing.
According to the documentation, they are. But I'm believing that they may not be. I ordered (2) from the same company thinking the first one was bad. Then I ordered (1) from a different company.
There was someone else on the forum that had a similar problem. I think he bought something that was advertised as a meat thermometer, a long stainless steel probe on a long lead.
I saw that, but thanks for bringing it to my attention. I moved the ground to multiple places, power, arduino gnd. I even added a pull down resistor, thinking it was susceptible to noise with the longer leads. No change.
Thanks for the graph, it helped me significantly in troubleshooting. But I'm pretty sure that they are J type thermocoupler and not K as sold to me. I am retired and live in Vietnam, so ordering items can be a challenge, specially getting what you ordered.
Would not matter. Use an ohmmeter to see if you have a grounded thermocouple. Connect one lead to the metal case and one lead to either of the wires. If it's grounded you will get a low reading.
The only thermocouple fails I have seen are the welds coming apart. Welds are used for consistency in manufacturing. Test thermocouples can be made by just twisting the wires together.
