Thermisors showing wrong temperatures

Dougal1957

@mendelevium said in Thermisors showing wrong temperatures:

@dc42
the problem here seems to be complex because i have built numerous 3d printers and never had this kind of issue. I have the duet fitted in a metal enclosure which has two large harting type metal clad plugs x 24 pin each. The connections from the board run to the harting around 30 cm long. The machine has two harnesses plugged into the control. The harnesses are around 1 meter long and there is around 1 to 1.5 meters of cables inside the machine. The issue is that they run along with limit switches and motor controls. So my solution is to substitute the cables of the thermocouples and with screened cables unable to pick up interference. Plus possibly also change the type of thermocouples I am using... Its a massive job to rewire and I do not want to do this in vain... What do you suggest?

What sort of wire are you using to extend the thermocouples?

dc42

@mendelevium said in Thermisors showing wrong temperatures:

So my solution is to substitute the cables of the thermocouples and with screened cables unable to pick up interference.

Thermocouple wires carry very low voltages at low impedance. This makes them susceptible to inductive interference pickup. Screened cables are effective at shielding the cable from capacitive interference pickup, but much less effective at shielding from inductive pickup unless they are shielded twisted pair.

Thermocouple wires MUST always be twisted pair, and should be kept away from stepper motor cables. If it's impossible to keep them away from stepper motor cables, use twisted pairs for the stepper motor cables too, one pair per phase. To get accurate temperature readings, the thermocouple extension cables must be made for the same type of thermocouple wire as the thermocouple. See https://www.eurotherm.com/en/support-articles-en/traps-and-colour-confusion-in-thermocouple-wiring/.

mendelevium

@dougal1957
ordinary wires.... Never encountered this problem even when running S**T electronics which I would expect far more issues....

Dougal1957

@mendelevium See DC42's reply above I was thinking the same as he was but wasn't 100% sure.

mendelevium

@dc42
interesting... are shielded and twisted wires found on the shelf?
do they have to be of a particular guage?

dc42

@mendelevium said in Thermisors showing wrong temperatures:

@dc42
interesting... are shielded and twisted wires found on the shelf?
do they have to be of a particular guage?

If the cable isn't the proper thermocouple extension wire, then that moves the cold junction from the daughter board to where the join is, resulting in inaccurate temperature readings. The reading will be off by approximately the difference between the temperature at the join and the temperature at the daughter board. See the link in my previous post.

Twisted pair is more important than shielding, however if the thermocouple wires run close to stepper motors wires then you may need both. The simplest option would be to use shielded twisted pair for the stepper motor cable (with the shield connected to Duet ground) and twisted thermocouple wire for the thermocouple.

Shielded twisted pair cable is readily available, for example Cat6a Ethernet cable.

mendelevium

@dc42
I have tried something. I have taken the loose thermistor and twisted the cable. Its just about 1 meter long. Connected this to the daughter board and pulled the wire straight up in a way that it is neither near nor in contact with anything. There is no change.. mostly spiking up to 2000...and touching it seems to invoke no temperature rise....
what do you suggest? Changing the thermistor to another brand.... i have around 100 pieces and its all the same behaviour
.

dc42

@mendelevium, if it's a loose thermocouple then it should have been supplied with either a twisted cable, or a shielded twisted cable.

Is it definitely a thermocouple?

mendelevium

@dc42 this is tge thermocouple and the cable. Worked great with ramps/arduino.

mendelevium

@dc42 Normally i sleeve them in a brass micro canister 3mm dia with thermal paste

dc42

@mendelevium, I don't think that's a thermocouple. I think it is a thermistor. Can you measure its resistance using a multimeter?

mendelevium

@dc42
you were absolutely right in this as the sensors were thermistors. The resistance was 118 Ohms. Thermocouples do not have any or infinity. I was really misguided. I connected everything and tested and the temperatures showing are actual off by 1-2 degrees.

My profound apologies for the din I created but I am sure I have learnt something else.
many thanks to everybody who lent a hand.