Help with thermocouple

After having switched back to the Duet 0.6 I thought I may as well install a thermocouple whilst I have the printer apart, but I'm having problems. The temperature reads correctly but randomly jumps up to 2000c every few seconds. If I try to heat up the hotend I get a 'slower to heat up than expected' fault.
I've tried different thermocouples and different MAX31855 boards, which I soldered the needed 10nf capacitor to.
Just to be sure it wasn't a problem with my soldering or the Duet 0.6 I hooked a thermocouple up to my Duet Wifi and the daughterboard I got with it. Same result.

The Duet 0.6 is running RepRapFirmware-1.15c
The Duet Wifi is running RepRapFirmware-1.15-rc5

Attached a screenshot of the temperature jumps.

Is the thermocouple junction properly insulated from the heater block?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Well it's in a 3mm diameter cylindrical housing so I'm assuming it is, how would I check? Either way, I didn't mount the other thermocouples I tried into the heater block so it couldn't have affected them.
I've switched back to the thermistor, for the time being, on the 0.6, but I'd like to get the thermocouples working on the Duet Wifi.

Ah, so it's an E3D cartridge thermocouple is it? That should be OK. Can you confirm that the wire from the thermocouple to the interface board is twisted pair all the way, and kept well away from stepper motor cables? Preferably also well away from heater cables, unless they are twisted pair too. The thermocouple voltage is very low, so the wires can easily pick up interference.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

No they're actually from aliexpress, for just over a pound each I thought they were worth a try. I cut the two terminals off one to see if that made any difference and it didn't. The wire underneath is single core, I assume it's going to be the same all the way to the thermocouple? Stripping off that metal braid stuff looks tricky.

My guess is that the metal braid, and perhaps the casing too, is connected to one of the terminals and is picking up interference. But that's just a guess. The thermistors I test with are generic type K thermocouple from RS with unshielded twisted pair wires. I'll get an E3D thermocouple cartridge to test with too, and perhaps one of those AliExpress ones.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I'm getting 13ohms between the cartridge and the positive wire, and 9ohms between the cartridge and the negative wire. If I measure between the braid and the wire at the terminal end I get a couple of ohms increase in resistance. I think that means the connection between them must be at the cartridge end? I'm going to disassemble it and see what's what.

I yoinked the cartridge tube bit off (the ones I got don't have any heat shrink like they do in the listing photo) and the braid completely covers the junction. So I trimmed the braid, wrapped it and the junction in kapton tape and pushed it back into the cartridge tube. That seems to have solved the problem I've not tried heating it up with anything but body heat so far but it's reading the correct temperatures and not jumping at all.
Worth the fuss for £1.10 I reckon

I'm glad you solved it! That's one less thing for me to investigate.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
dc42
I'm glad you solved it! That's one less thing for me to investigate.

I really appreciate all your work on the Duet and whatnot, don't burn yourself out though!

I'll try and remember to update this when I get it hooked up to the printer. I might use some thermally conductive electrically insulated thermal adhesive stuff to keep the junction in its cartridge if I think it needs it.