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I'm using a 65W heater cartridge and T-D500 type NTC thermistor. This setup has been working fine since I installed it sometime last year. Suddenly today, the temperature reading started jumping around 5-25° below the set temperature, but only while printing. It holds temperature solid at idle, and PID autotune produces numbers like normal.

How should I go about diagnosing what component is bad? Is it necessarily the cartridge or thermistor (or rather their wiring) or could it be noise or voltage instability or something else?

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This isn't a complete diagnostic procedure, but what worked for me.

After asking around and thinking about the magnitude of the fluctuations, a bad heater cartridge or bad connection to the heater cartridge didn't make a lot of sense, even at 25 mm³/s flow and over 300 °C where I was experiencing the problems. To try to confirm this, I went back and tried a print at 250 °C. I also measured the cartridge resistance and found it about 1 ohm high (underperforming) but this was nowhere near enough to explain any problems.

Since the thermistor seemed the most likely culprit, I then disassembled the toolhead to check its mounting and possibly replace it. It was still firmly mounted, and had no visible damage to the wiring, which is protected by a spring coil. Since my block has an attachment point for the glass bead style thermistor too, I found my original Creality-issue one of those and attached it too. After putting things back together and switching configuration to accept the old thermistor, I was able to make test prints without the rapidly fluctuating temperature readings. Over 300, there's still some fluctuation, but that seems to be accounted for by the very low resistance (and thereby low measurement precision) the 100k has at these temperatures, where 1 ohm is more than a 5 °C difference and the ADC resolution is only something like 1/16 of an ohm.

So, it seems to be a bad thermistor.

Update: After limping along for a good while on the 100k NTC thermistor bead, I finally disassembled everything to replace the T-D500, and immediately found the culprit. The wires to thermistor are not a single piece, but were sloppily solder-joined and heat-shrink-wrapped by the vendor(/reseller?) a few centimeters from the thermistor. This exact point in my wire harness was somewhat strained at the point where it exited between the hotend enclosure and extruder body, and apparently suffered a hairline crack that fell apart completely during disassembly:

photo of disassembled toolhead with broken thermistor wires

To make it easier to switch thermistors in the future if I need to, and since I was needing to learn crimping, I put JST-XH connectors on both sides where it was broken, and shifted the junction to sit inside the enclosure but away from the heater block. It's now fully operational again, and the replacement I purchased is still sitting in its bag as a spare.

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Well, it could be a bad thermosensor...

Bad Thermosensors do show themselves in multiple ways, among them fluctuating their readings before fully failing.

Thermosensors have ranges

Every thermosensor comes with a range for which it actually can operate reliably. For example, the TS2289 is rated for -40 to 90 °C, and thus would be not usable for a 3D printer at all, while TS2452 is rated for -100 to 600 °C.

The glass bead sensor on a Creality Ender3 is a ‎ntc 100k thermistor. That is only a class, and it contains multiple entries. This is not actually a specific sensor but a class that contains multiple sensors. For example, NTC 3950 is listed as -40 to 300 °C, which means it might be barely useable in that area, yet there might be larger fluctuations than normal as the resolution at the ends of the measuring range gets higher. In about the same dimensions, you might however also get a very similar sensor like thos from the Datasheet of the NTC Type BR Series. This document lists BR11/14/16/25 with a ceiling of 200 °C and B32/42/55 with a ceiling of 300 °C, all of which are in a very similar size range.

Not all sensors are glass-bead style, like the standard e3D v6 Thermosensor cartridge, which is listed as a Semitec 104NT thermistor the ceiling listed as 300 °C, but the same form factor also fits other cartridges. The sensor listed by e3D actually does come as a glass bead style too, listed as -50 to 300 °C

Such other sensors might be PT100B or PT1000 are claimed to accurately work up to 450 °C and might operate reliably even at 500 °C. A similar E3D Thermosensor is the PT100, and would also operate fine in that range.

Simply put: to reliably print at those high temperatures, you might want to change the thermosensor for a type that actually does operate reliably in those high-temperature areas.

P.S.

I do not use the word thermistor (or thermocouple) here, because those describe specific types of sensor. The more generic term Thermosensor describes anything from a simple thermometer, over thermistors and thermocouples to an IR sensor.

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  • $\begingroup$ As written in the question, "and T-D500 type NTC thermistor". This is rated for up to 500 °C. $\endgroup$ May 14, 2023 at 11:42
  • $\begingroup$ @R..GitHubSTOPHELPINGICE have you compared the outcome of the two thermosensors? that's how you notice if oue is bad $\endgroup$
    – Trish
    May 14, 2023 at 14:39
  • $\begingroup$ When not printing, they match pretty much exactly. In motion, the 100k works fine up to ~290 and as well as can be expected above that (too little ADC precision for the PID loop to track it well) but the T-D500's resistance jumps all over the place. $\endgroup$ May 14, 2023 at 19:12
  • $\begingroup$ @R..GitHubSTOPHELPINGICE sounds like that particular sensor failed... $\endgroup$
    – Trish
    May 14, 2023 at 19:59
  • $\begingroup$ Yes, that's what my question and self-answer seem to indicate. I have a replacement unit now but still haven't installed it because installation is a bit invasive. I'll follow up when I do as to whether things are 100% back to normal. $\endgroup$ May 14, 2023 at 21:40

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