which termistor do I chose in marlin ?

Hi everybody, I am new here and I am in the process of building my first Reprap

So far so good, all my electronic is tested and working, I am waiting for parts to finish the mecanical side.

I have just one question:

I bought these thermistor for my hotbed: [www.ebay.ca]

I am not sure which thermistor to chose in Marlin? (See farther below for thermistor specifications)

Thank's

Choices in Marlin:

//===========================================================================
//=============================Thermal Settings ============================
//===========================================================================
//
//--NORMAL IS 4.7kohm PULLUP!-- 1kohm pullup can be used on hotend sensor, using correct resistor and table
//
//// Temperature sensor settings:
// -2 is thermocouple with MAX6675 (only for sensor 0)
// -1 is thermocouple with AD595
// 0 is not used
// 1 is 100k thermistor - best choice for EPCOS 100k (4.7k pullup)
// 2 is 200k thermistor - ATC Semitec 204GT-2 (4.7k pullup)
// 3 is mendel-parts thermistor (4.7k pullup)
// 4 is 10k thermistor !! do not use it for a hotend. It gives bad resolution at high temp. !!
// 5 is 100K thermistor - ATC Semitec 104GT-2 (Used in ParCan) (4.7k pullup)
// 6 is 100k EPCOS - Not as accurate as table 1 (created using a fluke thermocouple) (4.7k pullup)
// 7 is 100k Honeywell thermistor 135-104LAG-J01 (4.7k pullup)
// 8 is 100k 0603 SMD Vishay NTCS0603E3104FXT (4.7k pullup)
// 9 is 100k GE Sensing AL03006-58.2K-97-G1 (4.7k pullup)
// 10 is 100k RS thermistor 198-961 (4.7k pullup)
//
// 1k ohm pullup tables - This is not normal, you would have to have changed out your 4.7k for 1k
// (but gives greater accuracy and more stable PID)
// 51 is 100k thermistor - EPCOS (1k pullup)
// 52 is 200k thermistor - ATC Semitec 204GT-2 (1k pullup)
// 55 is 100k thermistor - ATC Semitec 104GT-2 (Used in ParCan) (1k pullup)


THERMISTOR SPECIFICATIONS

R25℃

100KΩ±1%

B Value(R25/50℃)

3990K 1%

Dissipation Factor(mW/℃)

1.1~1.6 In still Air

Thermal Time Constant(S)

10~17 In Still Air

Operating temperature range

(-50~+260°C)

Dimension

2.0mm Dia X4.1mm

Lead Length

32mm

Using a Beta R25/50 value to extrapolate to the stated 260C range seems like range would be asking for big errors.

The beta(R25/50) of 3990 would give a resistance of 100000/exp(3990^(1/(273.15+25)+1/(273.15+50))))=34822 ohms, which , with a 4700 ohm pullup would generate an ADC reading of 1024*(34822/(34822+4700)=902 counts, so Marlin table 1 would match for the 25-50C range pretty well.

I'd suggest taking measurements at a few temperatures toward your operating temperature and build your own table with createTemperatureLookupMarlin.py or at least modifying the table to match your measurements.

Thank's for your answer.

Just to make a test I chose #5, it's the same setting required for my hot end thermistor (E3D)

I set the hotbed temp to 40C first and then 60C and checked the temp with my small infrared thermometer and was whithin 1C on both readings

So I guess I will go with this setting for now.



Thank's for the help!