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. 

temperature_control.hotend.thermistor        EPCOS100K

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
EPCOS100K 4066 4193 0.000722378300319346F 0.000216301852054578F 9.2641025635702e-08F B57540G0104F000
Honeywell100K 3974 4385 0.000596153185928425F 0.000231333192738335F 6.19534004306738e-08F 135-104LAG-J01
Semitec 4267 4375 0.000811290160145459F 0.000211355789144265F 7.17614730463848e-08F 104GT-2
Honeywell-QAD 0.000827339299500986F 0.000208786427208899F 8.05595282332277e-08F 135-104QAD-J01
Semitec-104NT4 0.000797110609710217F 0.000213433144381270F 6.5338987554e-08F 104NT-4R025H42G
RRRF100K 3960
RRRF10K 3964
HT100K 3990

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 : 

temperature_control.hotend.coefficients 0.000722376862540841,0.000216302098124288,0.000000092640163984

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 : 

temperature_control.hotend.rt_curve          20.0,126800,150,1360,240,206.5