# Thermistor tables

I am having trouble finding a good basic description on how to make my own thermistor table for the thermistor I have. Its an NTC 3950 thermistor. I know that repetier firmware has a table for it, but no matter what # of table I choose, even the one I am supposed to I am like 110 C off when at high temperatures. For example, I want 240 C for my abs filament, however, I have to set my extruder to 350 in order to get that at least measured from my IR handheld thermometer at a very close range to the extruder.

So my question is, does someone have a very good step by step udnerstanding of how I go about chaning something in either my electronics, or making my own table so that I can get accurate temperature readings in repetier host because it drives me nuts to have to figure out what I am actually at each time.

• edited April 2016
Try this
• Thanks! I will give that a shot and see!
• Nope, wait, that is the type I use .. and I am still way off so it has to be something else in the electronics maybe?

• Thermistor measurement involves a 4.7k resistor on the board. Some board batches have wrong resistors making tables not work. So maybe you measure the resistance of yours. They are normally close to the pins and easy to find.
• Will do, its a ramps 1.4 board if that helps.
• Right now am waiting for the extruder to reach room temperature so I can measure the resistor in the extruder too. I did measure the pins in the board, I got 5.36K, so with that knowledge what do I change now to make to correct?
• If I wanted to just reduce that using ohms law, I am coming out to a around 40K resistor in parralell with th eboard to make the total resistance 4.7K, does that seems right? 4.7k = 1/(1/5.36K+1/R) then solve for R?
• edited April 2016
lol, that didn't work, made it way off, I did find however just adding resistors in series that it matches within 2-5 C between 170 and 235 when I have a 220 ohm resistor in series with the thermistor. That's much better than 100 C off, but still, would liek to understand how I could tweak a table in order to make it work better.

Edit: More like 5-10 C off, it starts 10 off at 170 C then is 5 C off at 200 to 235, but then after that the temperature goes way up while I am still reading like 250 or so, but the ext temp is actually 260.
• i think it is not possible to measure extruder temperature with handheld ir thermometer.
Measuring area depends on distance , also eta-factor is strongly dependent on surface structure and material .
metal surfaces are quite critical because depending on oxidation/pollution the emission factor changes drastically.
if you want to see difference just paint the nozzle with a black marker .

refer manual for correct settings of your thermometer, also see:

https://en.wikipedia.org/wiki/Infrared_thermometer

i personally prefer thermoelements
• The online config tool allows you to enter your temperature curves from thermistor datasheet. There you can also set your resistor value. So if you know it is wrong use that as a fix.

And yes, as RAyWB said, IR thermometer can not be trusted, also I think with that resistance you are surely not at correct temperature as it is very critical in that temp. region anyway.
• Hmm I will have to give that a shot, thanks for the information guys.