There are different models of thermistors. Each hotend or heated bed has one, and it's a specific given type. Different thermistor models translate a given temperature into different resistances. This means you must tell Smoothie which exact thermistor model you have, to be sure Smoothie can read it correctly.
This is done using the thermistor option in the configuration file. You pass it the name of your thermistor, and it will configure the math correctly accordingly.
Smoothie does not know the name of all the thermistors in existence. At the moment these are the ones it knows about:
|Name||Beta for 0-80°C||Beta for 185-230°C||I for Steinhart Hart||J for Steinhart Hart||K for Steinhart Hart||Part number|
In case you have a thermistor that is not known to Smoothie you can simply define the parameters in config. There are two methods, using the beta value ( good but not perfect ), and using the Steinhart Hart algorythm ( perfect ).
Using beta values :
Set the beta value in the configuration file :
temperature_control.hotend.beta 4066 # set beta for thermistor
About beta values
The betas published by most manufacturers, are for the 0-80°C range.
For the 185-230°C this results in readings being about 7-10°C too high.
This means beta values are usually good for a heated bed, but not for a hotend.
If the thermistor is 100K ohms at 25°C then this is usually enough.
You can also set r0,t0,r1,r2 but this is not usually needed as the defaults are sufficient.
If you do not know what thermistor model you have, contact the designer or seller of your 3D printer, hotend or heated bed and get the spec which will tell you what the beta value is for the thermistor.
Using the Steinhart Hart algorythm :
This is the recommended method. Set the parameters in the configuration file :
To find what the Steinhart Hart coefficients for a given thermistor are, please read the SteinhartHart page.
If you have the temperature curve for your thermistor, you can also define three points on that curve, and let Smoothie calculate everything :