I hate thermistor config....

peirof

Hi,

As I have commented in a previous post, I am having problems to configure the temperature of the Hotend. I think I have found the root of the problem.

Where I think the problem is in the temperature sensor, a 100k thermistor I think. In the configuration that I am using it looks like this:

M308 S1 P"temp1" Y"thermistor" T100000 B4725 C7.060000e-8 A"Hotend"

And when I heat up the Hotend, the temperature read by the installed thermistor, and the one I measure with a thermometer, are very different.

Where can I find the formula that transforms the thermistor readings into temperature?
I have read a method to obtain the value B, measuring the resistance at different temperatures... but it is quite entertaining... so I would like to try it first, if with the formula and a bit of excel, I can obtain more approximate values

What mathematical formula does M308 use to transform the resistance it reads into temperature?

what method do you use to configure a thermistor in Duet?

Thx

chrishamm

@peirof You may find this one useful: https://www.thinksrs.com/downloads/programs/therm calc/ntccalibrator/ntccalculator.html Or go to the configtool -> Heaters -> Any R25 field -> Select "Custom".

See also https://en.wikipedia.org/wiki/Thermistor#B_or_β_parameter_equation and https://en.wikipedia.org/wiki/Steinhart–Hart_equation.

engikeneer

@peirof

And when I heat up the Hotend, the temperature read by the installed thermistor, and the one I measure with a thermometer, are very different.

Be careful here. It is very hard to get an accurate measure with a hand held thermometer or a thermocouple to compare to the hotend thermistor reading. Your hotend thermistor is mounted in your heater block, measuring the temperature in the centre of the metal and had lots of contact area to give a good reading. A hard held thermometer will only be touching part of the surface so will always read lower (like 50-100C lower in my experience!)

what method do you use to configure a thermistor in Duet?

Once you've got roughly the right thermistor config (which it looks like you have!), the next important thing is that you find the right temperature for your filament. If you can extrude PLA happily in the range of 180-200C, your thermistor must be about right. After that, just print some temperature towers to tune it in for your filament.

zapta

@peirof, have you considered switching to PT1000 sensors? My understanding is that they are more accurate than thermistors and easier to hook up than PT100 sensors.

peirof

@zapta can be connected direct to Duet 3 mini? Or its necessary some intermediate board?

gloomyandy

@peirof Looking at: https://duet3d.dozuki.com/Wiki/Connecting_thermistors_or_PT1000_temperature_sensors it would seem they can be connected directly.

droftarts

@peirof PT1000 can be connected directly. PT100 and thermocouples need a temperature daughter board.

What is the specification of your thermistor? There are many different values of thermistor, and selecting a random value is not going to give you accurate results If you don’t know, you can get a rough idea by measuring resistance of the thermistor at room temperature, then in the freezer (around 0C) and then in boiling water (100C), and put the values into the thermistor calculator. Or buy a thermistor or PT1000 from a reputable source, so you actually know the values.

Ultimately, all you need to know is the temperature it melts plastic at in your machine. Heat up the hot end in 10C increments, and push filament by hand into the hot end. With force, PLA will start to extrude at around 150C, and extrude easily at around 180-190C.

Ian

peirof

@droftarts said in I hate thermistor config....:

PT1000

thx, i will give a try

zapta

@peirof , I am using the E3D version directly with my Duet 5 mini https://e3d-online.com/products/pt1000-temperature-sensor?_pos=1&_sid=d2ef5a77a&_ss=r

However, I plan to switch to the new Revo hotend so will have to regress to thermistors

peirof

I have find the root of the problem...

It was not a mistake in the configuration of thermistor.... It was simpler.... From what it seems, when mounting the thermistor in the heatblock the screw that held it in position, pressed the cable, affected the readings.... When I adjusted it, outside the Heatblock it gave some temperature values and when it was in the heatblock, it gave others