Thermocouple readings lower than expected

[Ante Lausic]

I've looked through all the posts I can on thermocouple issues but most are about the 1024 error. What I have is a bit more weird and fear the motherboard may be shot :(

When I try to heat up my Replicator 1 Dual running 7.5, the target of 230C is never reached. The machine times out as the thermocouple reaches 215-220C but never higher.

If I set the temperature to 245C, it stops at 230C. Seems to always be off. So I took my own thermocouple device and stuck its lead right next to the extruders thermocouple. While the machine still reads 215-220C, my thermocouple reads 251C. Basically, the heater works and can get quite hot but the thermocouple is always off that value. I tried new thermocouples but always the same issue. Happens to both extruders. Because of this, I can never print.

Anyone have ideas on what's wrong? Thanks

[Ryan Carlyle]

Just ruling out the basics...

Have you spliced any copper wiring into the thermocouple line? Is there anything but type-k TC wire from the hot end to the mainboard?

Is your thermocouple electrically insulated from the hot block? (Check for continuity with a multimeter.)

What version of RepG are you using to connect to the bot?

Have you tried resetting the mainboard to factory defaults?

[Dan Newman]

Sounds like your thermocouple is coming loose. On Rep 1s (and Thing-o-Matics), the t/c is
wrapped in a layer or two of kapton tape and then loosely bolted to the outside of the heater
block under the head of a M3 bolt, possibly under a washer. They were bolted loosely so as
to not cause the t/c bead/joint to puncture through the Kapton tape. Overtime, the t/c's can
become looser and looser causing it to read lower and lower owing to the poor thermal contact
with the heater block.

Just another thing to check.

Dan

[Ante Lausic]

Thanks for the feedback, guys. 

I just finished a complete reset to factory settings. It is using RepG 40r33. Sailfish 7 I believe it was. 

I never spliced in any cable. The wire was cut a few years back so I took the two ends and twisted them into a rope-ish bit and that worked fine. 

I was not aware of the need for Kapton tape on the end. It had come off and I assumed wasn't necessary since the ends of the handheld thermocoupler I used also just were put on metal and gave a 'correct' reading. I will try to find some leftover tape and use that. 

I ran some tests. I asked the machine for 100C and it gave it perfectly. But my thermocoupler said the actual temperature there was 114C. Then I asked for 150C and again it got to 150C but thermocoupler said it was 175C. Then I asked for 200C and now it would get to 196-197C but never higher, like it just couldn't get hot enough. But when I checked the real thermocoupler, it said it was 232C and indeed I could put some ABS filament on it and it would immediately melt. I'll try the Kapton tape and ensuring it isn't too tight that metal touches the block. 

Ante

[Ante Lausic]

So wrapped the end in Kapton tape and its better :) Now the 'real' thermocouple and the Makerbot's measurement match perfectly. When I asked for 230C though, it still wasn't able to get there. It would bounce between 226 and 227C but never quite get there. Asking 220C gets 218-219C. Asking 240C gets 235C. Maybe my taping job wasn't good enough?

Ante

[tramalot]

bad cABLE

[Ante Lausic]

New thermocouples just installed, kapton tape over the ends, screwed in nice and snug. 

Set the extruder to 230C and the Replicator, the control panel on RepG, and my separate thermocouple all read 226-227C. It just doesn't want to get hotter. If I set heat to 240C it gets to 236C so the heater has the power to get to 230C but still doesn't when asking for 230C. 

Tried the second extruder (as this is a dual system) and when I ask for 230C, it only gets to 222-223C. Issue is on both extruders. 

Ideas? Thanks.

[TobyCWood]

Have you tested the PS?

[Ante Lausic]

Apologies for my lack of knowledge on it, but what is PS? 

[Jetguy]

Power Supply

[Ryan Carlyle]

What are your PID values? (Access via RepG onboard preferences.)
Do you have the hot block insulated?
Is there any significant airflow over the hot block?

[Ante Lausic]

Heating block is insulated as per delivery. Don't really see that big a gap to cause cooling. The whole makerbot is in a warm room with no airflow. The PID values for the P=7.02745104, I=0.32549021, and D=36.14117813.

I don't really know how to 'check' the power supply...

I did notice something... might be nothing... but when I left it try to get to 230C, it stops at around 226-227C. If I put my finger on the fan middle part to make it stop spinning, the temperature goes to 238C and printing starts. Once I let go, the fan resumes spinning, the temp drops to 223C before returning to 226C but the print still goes on and completes ok. 

Ante

[Ryan Carlyle]

Ok, what's happening is that the PID is maxing out at a power level too low to hit your target temp. That causes an offset: the hot end is always a few degrees below the target. This usually only happens when 

1) you have a low power supply voltage (which will usually be worsened while the heated build plate is on), or 

2) you have a lot of airflow PLUS not much insulation on the hot block. Note that the extruder fans blow onto the hot blocks through the bottom of the heatsinks! For example, it's quite common with the Replicator 2x, which doesn't have insulation covering the top of the hot block... the airflow from the extruder fans blows down through the carriage and cools the hot blocks by a surprisingly large amount. They see this same temp offset. 

There are a few things you can try to fix it:

• Verify you have no scorched/melted/damaged wiring connectors on the mainboard that could be reducing the heater power
• Use a multimeter and measure the heater cartridge resistance (with the bot off), it should be around 14.4 ohms
• Use a multimeter and check your power supply voltage to make sure it's a stable 24v while heating
• Add more insulation to the top and sides of the hot block
• Once you have run out of physical fixes to try, raise the PID "I" value in 0.05 increments (up to max of 0.50) until the problem goes away

Yes, raising the I value should fix the problem, but you really don't want to use that to mask a physical problem that could potentially cause a fire or printer damage. Start ruling out possible hardware causes first. 

[Ante Lausic]

Thank you so much Ryan! Totally the most informative and helpful post ever. I went through all the hardware issues and upon touching up the extruding and insulation, settings, it works! No need to touch the software. Thanks so much again as it was perfect! 

Ante