MaKoMo wrote:See WikiPedia on Thermocouple and especially the chart showing the characteristic functions of the different TC types


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i found the chart you were talking about. is there some information in here i should use to configure my phidget or my PID?

CarefreeBuzzBuzz wrote:2mm is often subject to wide swings. The PID has what probe. Now they are different and you can expect them to read differently.

the PID has 2mmx60mm type k thermocouple. same probe is daisy chained to my phidget (as BT in the profile above) with type k thermocouple extension wire

MaKoMo wrote:If you swap a k-type TC for the j-type TC connected to your PID you need to tell your PID that the signal is now coming from a k-type and not a j-type TC such that it converts the detected voltage correctly to temperature values.

yes i did that. i told the pid that it was a type k.

i stuck the 2mm directly into the phidget and i seem to be getting somewhat accurate data at first, but the longer that the probe is hot, the lower the temperature reading. the 3mm will be giving me 350 and so will the analog, and the 2mm, but after 10 min of being hot, the 2mm will be saying 280 or less while the other 2 are reading 350.

is there any reason that a probe would continue to decline the longer it was hot?

debadano wrote: i stuck the 2mm directly into the phidget and i seem to be getting somewhat accurate data at first, but the longer that the probe is hot, the lower the temperature reading. the 3mm will be giving me 350 and so will the analog, and the 2mm, but after 10 min of being hot, the 2mm will be saying 280 or less while the other 2 are reading 350.

is there any reason that a probe would continue to decline the longer it was hot?

The thinner the probe, the quicker it will respond to changing temperature. It might be that the 2mm probe is responding faster to temp changes than the 3mm and analog probes, which have more mass and thus are slower to respond to change. One way to test this would be to warm up the roaster with no beans in it at a low temp/fan where you try to maintain a steady temperature state. Change Artisan (if needed) to graph each probe and see if the different probe temps converge/diverge or stay a steady distance from each other.

Also, have you tested your new probes in both boiling and ice water? This is a great way to know the probe itself is not inherently off calibration (which you can apply a correction factor to in Artisan by the way)

Also, I would expect the BT probe (2mm right?) to be cooler than the ET probe (3mm right?) regardless of probe diameter. With my drum roaster, as you move higher into the drum, and closer toward the exhaust port, the temps go up due to hot air rising. This can also be affected by where incoming air comes into the drum, which in mine is from the back/bottom of the drum. FYI; Here's an example profile from my roaster where the dark blue line is the BT probe (in lower inside of drum within bean mass), the red line is the ET probe (in upper inside of drum) and the thin black line is the MET probe (outside of drum close to exhaust port).