how to measure more than 500°C on the electronics ?

Hello guy`s

I have to measure at more than 500°C but running into a issue.
My setup is:
K-Type Thermocouple up to 800 °C
Sanguinololu Board (also can test megatronics,gen6delux, tensyluu), connected the Thermo-board to Pin 4
Thermocouple Preamplifier board with AD597 from Reprapworld.com connected 12V to +
Marlin thermistor is set -1 for the Thermocouple, pin is also assigned correctly.

So now i can measure from 0°C to 499,5°C thats where the AD597 produces 5.0V output to analog Pin 4 of the Sanguino.
althouth when i heat the element more up there comes more voltage out of it (up to about 5.4 volts with a lighter)

So for my understanding, do all electronics only measure between 0 and 5volts ?
So if i want to measure higher do i have to put in a "offset" resistor (what value?) to lower the voltage and then change the firmware, but what would i have to change ?
I think this may work to get a reading at higher temperatures then so for eg. from 300 to 800°c but it`s not a very elegant solution.
Is there maybe a other way to solve this to measure the full range from 0 to 800°C ?

Chri

Edited 2 time(s). Last edit at 10/01/2014 01:09PM by Chri.

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[chrisu02.wordpress.com] Quadmax Intel Delid Tools

The AD597 IC does a job of interpreting the thermocouple signal, comparing and filtering it and outputting a signal of 10mV/degreeC.
You can not intervene on the thermocouple side because that is very sensitive. On the output side, it could output 8v if the ic is supplied with 8V+ (tho i guess its only getting 5v to start with). However it could still output more than 5v. But we can not read that with the uC analog pin direclty, and using a voltage divider or anything there is possible, but may and will slow down the signal. That could be an acceptable hack, but not a solid solution imo.

Could use another thermocouple IC which outputs like 5mV/degree, and get higher range but will be even more sensitive to noise.

Or could use a thermocouple IC that sends the readings without an analog pin, like max6675. For last case i think there may be some code somewhere, but have to check if the firmware still supports it.

Edited 2 time(s). Last edit at 10/01/2014 01:51PM by NoobMan.

... you can measure any temperature with a 0-5 Volts signal, when you select proper resistor bridges to 'fit' the signal in this range.

I've done this for temps from 0 to 800 degC with a PT100 as nearly linear sensor (different calculation for 0-300 and 300-800 degC) and an ADuC847 µC.

This µC has a 24 Bit ADC, but only up to 22 Bit really usefull - so the resolution was 'only' 0.0002 degC

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Use a different thermocouple amplifier. The MAX6675 mentioned in a previous post is obsolete, but the MAX31855 is nearly compatible with it. Search ebay, Sparkfun etc. for MAX31855 breakout boards.

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Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for the help, i ordered one from china now, couldn`t find a other source so may take some time ^^
i hope i get it running on the sanguino, seem`s that this MAX31855 has to be connected to the expansion port.

Chri

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