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Old 10-20-2004, 12:23 PM   #1
ride5000
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Default roll your own egt interface... (ie tuner pro)

take a look at the following chart:

notice how nice and linear the type K thermocouple voltage is, especially once you get above 0 degrees C. a type K nets 51 V/C at 20C. 1600F = 871C, which would give us ~ 44421uV, or 44.4mV from the hot junction.

the ability to scale the output voltage to correspond to a certain actual temperature is inherent to the tuner pro interface. all that is needed is a simple way to bump up the extremely low voltage output of the TC to a more useable range. it looks like an x200 voltage gain would net you approximately 8.88vdc at the highest EGT you'd want to see. extrapolate and input the scaling factors into the tuner pro and you would be able to read out the EGT from the tuner's log.

in other words, you'd need a tuner pro, an EGT probe (~$40), the molex breakout connector (free from an atx p4 computer power supply), and about $5 in electronic parts (op amp, couple of resistors) to make this work. the beauty of measuring such high temperatures is that the offset introduced by the non-zero "cold junction" temperature is proportionally tiny and can essentially be ignored.

i will be doing this myself to my car, and i will also be doing it to the intake air temps. i have an actual temperature compensated TC to DMM adapter that i can use to measure just how large the offset is at lower temperatures.

ken
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Old 10-20-2004, 12:43 PM   #2
Jon [in CT]
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Too bad the Tuner Pro didn't include a terminal connected to something like this Analog Devices AD595 Thermocouple Amplifier.
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Old 10-20-2004, 01:54 PM   #3
ride5000
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yeah, i pored over those datasheets for a while. obviously you're talking greater accuracy with the cold junction compensation. it would have been REALLY nice to have those chips already on board.

BUT

Quote:
Because a thermocouple output voltage is nonlinear with respect
to temperature, and the AD594/AD595 linearly amplifies the
compensated signal, the following transfer functions should be
used to determine the actual output voltages:

AD595 output = (Type K Voltage + 11 mV) 247.3

or conversely:

Type K voltage = (AD595 output/247.3) 11 mV
so while the offset due to cold junction is taken care of, the error due to non-linearity of TC characteristics is not. at 870C the output of the chip would indicate ~8.9v

the tuner, however, CAN be scaled appropriately to offset this proportional error.
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Old 10-22-2004, 05:45 PM   #4
Jon [in CT]
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Quote:
so while the offset due to cold junction is taken care of, the error due to non-linearity of TC characteristics is not. at 870C the output of the chip would indicate ~8.9v

the tuner, however, CAN be scaled appropriately to offset this proportional error.
It's easy to re-scale inside a spreadsheet.

Regarding the AD595 thermocouple amplifier chip, there's a very interesting discussion (with circuit diagrams) about using it to interface an EGT to the LM-1 wideband controller. It's at http://www.innovatemotorsports.com/f...=&threadid=371
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Old 10-22-2004, 09:42 PM   #5
ride5000
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wow..nice find. thanks jon!
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Old 10-27-2004, 04:54 PM   #6
ride5000
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Default

based on some info from the web, i settled on this ic:

http://www.analog.com/en/prod/0,,764_811_AD597,00.html

coming in the mail on the 2nd. looks to be a simple hookup.
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Old 05-08-2005, 11:23 PM   #7
snwman
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I am already working on a project based on that LM1 circuit. However I will be usings it with my aem ems and running 4 probes to allow me to watch individual cylider temps. you can get all the parts for this from www.digikey.com get the GOOD thermocouples from www.teamrip.com for $45 each plus shipping. they seem to be very fast responding and accurate compared to many other type k probes out there. I do not work for teamprip.com or digikey.com and I am not trying to promote those sites just trying to help others out.
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