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Temperature not reading right

Posted by jpotts 
Temperature not reading right
April 17, 2014 10:03PM
Hopefully someone can help me out here...

I have a Sanguinololu 1.3a board with 4.7k pullup resistors installed.
I have a J-Head hotend MK V-BV (the real deal, not a knockoff)

The hotend has a 104GT-2 thermistor installed.

The problem that I have is that it reads values, just not correctly.

For instance...
repetier host is set to: 255C
The Thermistor reads: 235C in Repetier
My meter says the temperature is 172C

I think my meter thermocouple is the most accurate considering it seems to follow the melt point of the plastic.

Any ideas why this could be so far off?

Re: Temperature not reading right
April 18, 2014 03:57AM
First forget the temp set in the host. It is what you want and what firmware tries to reach. If it only goes to 235 your heater has not enough power or you have limited power. You can see this in the host temperature display. If it is at constant 100% your heater is to weak. If it is limiting to something below, you configured the firmware to limit power artificially.

The difference to your meter can have different reasons:
- Temp sensor type not set to 8 = ATC Semitec 104GT-2
- It is a different thermistor then you think
- 4,7k is not 4,7k
- There is always a difference since you measure different points. Temp. spreads from heater barrel = hottest and goes down by distance. But I would agree that 60° difference is quite a lot.

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