Thermocouple temperature fluctuations and bad soldering (solved)

[verminsup]

I'm using three old J type thermocuples in my Stratasys Sst 768 duet conversion using two Thermocouple daughterboards. These are the weird things I've seen:

1. When the chamber is heating up the two temperatures on the first board go down to between 1c and -3c.
2. When I open the door the temperature rises on the two temps under 0
3. Using a seperate temperature sensor the room temp is correct, this thermocouple is on the second stacked board while the extruder thermocouples are on the first
4. When I switch boards the temperature fluctuations stay the same and the chamber temp stays the same
5. After removing the board I noticed a weird almost wet patch on the board
   and one of the components was soldered in a bad way. (pics)

Any way to fix this or is this board just bad?

Config for the extruders (don't mind the weird chamber PID)
M308 S0 P"spi.cs0" Y"thermocouple-max31856" T"J" ; configure sensor 2 as thermocouple via CS pin spi.cs2
M950 H0 C"out1" T2 ; create chamber heater output on out0 and map it to sensor 0
M307 H0 A11 C99000 D2000 B1 ; Pid Tune heater 2
M140 H0 ; map chamber to heater 2
M143 H0 S100 ; set temperature limit for heater 2 to 100C
M570 H0 T15 S20000 ; Heater fault detection heater 2, 20000 seconds and 15c difference

M308 S1 P"spi.cs2" Y"thermocouple-max31856" T"J" A"Support" ; configure sensor 0 as thermocouple via CS pin spi.cs0
M950 H1 C"out2" T0 ; create nozzle heater output on out2 and map it to sensor 0
M307 H1 B0 R5.606 C78.0:75.6 D4.59 S1.00 V24.1 ; Pid tune heater 0
M143 H1 S350 ; set temperature limit for heater 1 to 350C
M570 H1 S10 T20 ; Heater fault detection heater 0, 10 seconds and 20c difference

M308 S2 P"spi.cs1" Y"thermocouple-max31856" T"J" A"Material" ; configure sensor 1 as thermocouple via CS pin spi.cs1
M950 H2 C"out3" T1 ; create nozzle heater output on out3 and map it to sensor 1
M307 H2 B0 R5.483 C90.7:90.7 D4.79 S1.00 V24.1 ; Pid Tune heater 1
M143 H2 S350 ; set temperature limit for heater 2 to 350C
M570 H2 S10 T20 ; Heater fault detection heater 0, 10 seconds and 20c difference

[dc42]

@verminsup said in Thermocouple temperature fluctuations and bad soldering:

When the chamber is heating up the two temperatures on the first board go down to between 1c and -3c.
When I open the door the temperature rises on the two temps under 0

Are you saying that the temperatures are reading in reverse, i.e. they are OK at room temperature but when you increase the thermocouple temperature, the reading drops below room temperature instead of increasing? If so, swap the two thermocouple wires where they connect to the daughter board.

[Dougal1957]

@dc42 said in Thermocouple temperature fluctuations and bad soldering:

@verminsup said in Thermocouple temperature fluctuations and bad soldering:

When the chamber is heating up the two temperatures on the first board go down to between 1c and -3c.
When I open the door the temperature rises on the two temps under 0

Are you saying that the temperatures are reading in reverse, i.e. they are OK at room temperature but when you increase the thermocouple temperature, the reading drops below room temperature instead of increasing? If so, swap the two thermocouple wires where they connect to the daughter board.

David

Does the thermocouple board work with J Type TC's

[verminsup]

@verminsup No it is reading in the correct direction. When I increase the nozzle temperature the reading also increases. I just have no way of knowing when it's hot if the temp is correct or even stable

[dc42]

@dougal1957 said in Thermocouple temperature fluctuations and bad soldering:

Does the thermocouple board work with J Type TC's

The newer thermocouple boards using the MAX31856 chips do, but you have to tell RRF in the M308 command that the thermocouple is type J.

[dc42]

@verminsup said in Thermocouple temperature fluctuations and bad soldering:

@verminsup No it is reading in the correct direction. When I increase the nozzle temperature the reading also increases. I just have no way of knowing when it's hot if the temp is correct or even stable

1. Does the reading depend on whether the heater is on or off, and on whether VIN power is applied or not? If so, that suggests there is electrical leakage between the heater and the thermocouple.
2. Does the reading depend on whether any of the stepper motors is activated or not? If so, that suggests electrical interference between the stepper motors wires an the thermocouple wires.

[verminsup]

@dc42 Nothing changes when motors are on or off when It does go down it goes down slowly as the chamber heats up. When this happens onlt the heaters for the chamber are on.

[dc42]

@verminsup can you confirm that you are using the proper thermocouple wire all the way from the thermocouple to the daughter board?

[verminsup]

@dc42 Just double checked the wiring and all seems to be in order. I truly wish I could convince my boss to just use some glass bead thermisters on the hotends because those would fit just fine.

[dc42]

@verminsup can you post photos of the wiring?

[verminsup]

@dc42 This uis the only part that is vissible without taking the machine further apart. The wires go through the cooling duct to the printhead (this is the original desing of the stratasys) The red light is from the config because I've turned that one off.

[rjenkinsgb]

@verminsup
Those wires do not all look to be thermocouple cable?
Examples of what it should be:
https://uk.rs-online.com/web/c/automation-control-gear/sensors/thermocouple-extension-wire/?applied-dimensions=4294876816

Also, even adding ferrules adds extra metal-to-metal junctions and can introduce errors.

There should be nothing other than purpose made, type-matched (eg. Type J, if that is what you are using) thermocouple cable and connectors until you reach that board.

A thermocouple is only a connection between two dissimilar metals, so add in any other dissimilar metal connections along the wiring and it messes things up in a big way!

(I've had a lot of experience with them in industrial machines over many years).

[verminsup]

@rjenkinsgb
That must be it, the whole wire is rated thermocouple wire until the last 50cm. My boss wired this machine and had no idea this was a thing with thermocouples, thanks for the reply this has finally convinced him to switch over to regular thermistors.

[dc42]

@verminsup if you use regular wire for the last section at the daughter board end, this will have two consequences:

1. The cold junction will be where the thermocouple wire joins the regular wire, instead of at the daughter board. This means that the temperature reading will be out by approximately the difference in temperature between that cold junction and the temperature of the daughter board.

2. If the regular wire is not tightly twisted pair (as thermocouple wire always is), then it will pick up inductive interference, for example from stepper motor cables.

In view of the wiring disadvantages of thermocouples, we recommend them only for use with temperatures that are too high for PT100 or PT1000 sensors.

HTH David.

[verminsup]

@dc42 The chamber and the cooling fans are 5 very large blower fans, could the temperature change when opening the door be explained by a lower pressure in the chamber causing the fans to spin faster and creating more interference?

[dc42]

@verminsup I guess that's possible if the fan PWM changed.

Is there any reason why opening the chamber door would change the temperature difference between the TC daughter board and the cold junction?

[ozgunkilicafsar]

@dc42 is the T"J" the correct command to define a J type thermocouple? In https://docs.duet3d.com/User_manual/Connecting_hardware/Temperature_connecting_thermocouples it says K"J" and I've seen both in various posts on the forum.

[ozgunkilicafsar]

@dc42 nvm found your reply on another thread from 2022 "...there was an error in the documentation. The thermistor type letter parameter is K not T. I have corrected the documentation."