thermistor calibration

ok I understand that this topic is as ancient as 3d printing itself, in fact a thread surfaced years back

[forum.prusa3d.com]

Now here is the motivations:
If you simply go to the "online flea markets" e.g. a simple search for a thermistor returns all of them claiming '3950' thermistors with *no specs*
[www.aliexpress.com]
no doubt they are cheap, but my own experience suggest that I've once got a (hotend) thermistor that measures 50k at room temperature (25 deg C) for what is a 100k thermistor, even with all that 'beta' calculations, I can't get a 50k value for a 100k 3950 thermistor at that temperature, it is possibly off by a scale of 50% against freezing to boiling

[www.ametherm.com]

Then comes the idea that if I can get an accurate reference, e.g. a semiconductor temperature sensor (some of them are pretty accurate) e.g. but not limited to
[www.ti.com]

I can probably calibrate those unspecified thermistors against the Steinhart–Hart equation

[en.wikipedia.org]

Then comes the 'million dollar question':

supposing that calibrating room-100 deg C is feasible, and hence I can try to fit the values against Steinhart–Hart equation or Beta equation up to boiling point.

Now for the hotend thermistor, can we do *without* calibrating *hotend temperatures* which is more like 200 C - 250 C?
i.e. fit the Steinhart–Hart equation between room to boiling and use that equation for *hotend temperatures* that is *double* and higher than boiling point?

... I've done some temp. sensor development with PT100, PT1000 and Ni200, where the sensing target range was between 0°C to 1000°C -- is pretty easy, when the curve is nearly linear, as with the PT sensors ...

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Viktor
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Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

@VDX thanks much for your response,

I'd think PT100 is probably a good idea for a reference at least.
A difficulty I still confront is that it isn't easy to find a suitable liquid to do high temperature calibration from 100-250 deg C.
from room temperature to boiling is 'relatively easy' to do in a lab

... you can try with fluid metal with a low melting temp.

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

just found out that a lot of 'cheap' PT100 are probably not platinium
[www.aliexpress.com]

more likely those are nickel
[datasheet.octopart.com]
[www.isabellenhuette.de]
those can probably still be used as some form of PTC thermistor
but that the coefficients and graphs are different, some are probably an attempt to replace the platinum PT100, not likely to be the same thing

... so more like hiding the fact that they're knock-offs?

With my knowledge nickel-sensors are named "NI(xxx)" for nickel ... and "PT" is meant for platinum

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

Quote
VDX
... you can try with fluid metal with a low melting temp.

I like [www.dow.com]. A bit expensive, but clean, transparent, good to 400C or so, and doesn't amalgamate to electrical leads like liquid metal.

Also see [reprap.org] for more than you want to know on platinum RTDs.

Hope this helps.

Even if you do find a technique to calibrate thermistors or PT100 sensors, how are you planning to actually achieve the calibration in the printer?

My heatbed is drastically incorrect, but works fine otherwise, and when I asked on the Marlin forums, apparently there's no way even to calibrate the thermistors, at least in Marlin firmware. This is crazy to me because in industry, any temperature sensor always has a way to be calibrated, but apparently aside from choosing a different thermistor from the thermistor table, there's no way to apply a calibration besides just entering an incorrect temperature in the slicer, which is what I'm still doing--I use 70C in the slicer and my heatbed is actually 50C.

... there are differen thermistor tables for specific sensors - to "calibrate" another sensor you have to measure the values at the specific temps an generate a new table ...

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]