@dougal1957 You lovely lovely person.

Asking me to post it made me go back through it with the appropriate paperwork in hand to check everything over before sending it through. And I spotted a rather obvious error, essentially trying to define the thermocouple type using the wrong variable.

Thanks for all your help folks. So handy to be able to bounce things off others in the know!

@dc42 Hey thanks for getting back to me.

I have experimented with moving the thermocouple and also installing a few capacitors here and there to try and remove the noise but nothing was working. Sadly I can't change the heating elements as they are custom made for our project and rather expensive. You can probably guess from the temperatures I'm dabbling with that we aren't using a 40w heater cartridge!

Well I'm going to tentitively say that I may have solved the issue I was having. I had setup the heating elements with PID control etc. However removing that and just having it setup as bang bang control with no reduced duty cycle or PID has removed the interference I was seeing around 950C. I can only guess that this was causing some signal to be induced in the thermocouple.

So for anyone else experimenting with super high temperatures with elements run off AC power just setup your heater with bang bang control.

An update here,

So I hooked up the indepedent reader to the thermocouple and it was reading the same value as the duet board throughout. Think I can safely assume that the duet board and the associated wiring is not causing my issues.

I am now suspecting that the interference is coming from the electric heating elements inducing a current in the thermocouple. They are unfortunately rather close due to design constraints. A small capacitor across the terminals has helped but I think I'm going to have to look at moving it and adjusting the heating characteristics of the elements as the interference drops when I lower the duty cycle of the elements.

Quick update for anyone following the thread.

I have a thermocouple reader on the way. Decided to splash out a bit more and get one that would handle the S and R thermocouples directly. That along with twisted pair shielded wiring should hopefully solve the issues I've been having but time will tell.

I'll update here with my findings.

@dc42 Thanks for the response.

Ok so it should be possible.

Interference was my first thought. I will go over all the connections however I can say that the thermocouple is fully isolated from the metal work which is also grounded.

I get the feeling i'm going to be chasing a gremlin that will be something incredibly simple in the end.

Hey folks,

After I got the previous issue actually reading an accurate measurement from the R type thermocouple I was using, I have come across a new issue. This has occurred with both R and S thermocouples. Essentially when the temperature reaches 1050C, the temperature reading starts to fluctuate by a few degrees, then it gradually gets worse until the swings are plus and minus 100C before an error message is thrown.

I have spoken to the thermocouple suppliers and they have assured me that they should be able to comfortably go well above 1300C. I am using a length of compensated thermocouple extension wire between daughter board and the thermocouple which has a ferrite ring towards the daughter board side. I did try shielding this cable however this caused more issues with the signal to the board.

Does the duet have the ability to read temperatures as high as I'm looking to read? Or am I going to have to find another interface to allow me to read these high temperatures? Is there likely something else I'm missing?

All temperatures have been verified using a high temperature flir thermal imaging camera so I'm confident that the temperature readings are accurate until we hit 1050C. I have attached a couple of images showing the behaviour.

@dougal1957 You lovely lovely person.

Asking me to post it made me go back through it with the appropriate paperwork in hand to check everything over before sending it through. And I spotted a rather obvious error, essentially trying to define the thermocouple type using the wrong variable.

Thanks for all your help folks. So handy to be able to bounce things off others in the know!

@3dpmicro yes the polarity is correct

Hey guys,

Long time lurker, first time poster.

Wonder if you can help me out. I am using an R-type thermocouple on a project that I'm controlling with a duet 3 and associated thermocouple daughter board. And yes, I'm using temperatures that need an R type thermocouple.

Anyway, the issue I am having is that i'm not getting a right reading from the thermocouple. I have a flir high temperature camera to take direct readings. The thermocouple seems to be out by a factor of around 4. So if the duet interface is telling me that the temperature is 200C then it's actually 800C give or take. Obviously this is a bit of an issue so I was wondering if anyone had any pointers?

The thermocouple is a ceramic tube type thermocouple that has a terminal block at the end. From here I have around 4ft of compensated wire to the daughter board. The thermocouple is 290mm long. I have set the thermocouple as a type R in the config file.

I had considered moving the daughter board to the rear of the thermocouple but would an extension wire between it and the duet 3 board cause any issues or work at all? Is there something else I'm missing?