Thermocouples in automotive and aviation applications

automotive thermocouples

K-type thermocouples are commonly used for exhaust gas (EGT) and cylinder head (CHT) temperature monitoring in automotive and aviation applications. The sensors are commonly "grounded" at the thermocouple junction, through the screw-in plug (EGT) and spark plug ring (CHT), directly to the engine block and exhaust pipe.

Intuitively, I would think that this would introduce serious ground loops, and I would expect that the ignition and spark plug noise energy would wreak havoc on the micro-volt signal at the junction.

I have not found any discussion on this topic on the web. Can anyone direct me to a source for these topics?

automotive thermocouple

Hi,
the thermocouple isn't actually grounded as such, Commonly the thermocouple sits in a stainless steel thermal well for good heat transfer, the actual thermocouple isn't connected.
So the only the thermal well becomes grounded, not the thermocouple itself.

thermocouples

Oh, they're grounded all right. About 2 ohms between either wire and the threads of the stainless steel plug.

I wouldv'e thought they'd be electrically insulated as you describe, until I purchased and measured some.

how thermocouples work aviation

Of course, grounded TC may be used in some applications. But the industry standard is isolation, as said. With grounded TCs, the amplifier must have sufficient common mode rejection. Not a problem generally, but not provided by all instrumenta.

thermocouple automotive

Update:

The thermocouples described above all worked OK with two sets of circuits, in my embedded engine monitoring system, despite the sensor being grounded at the tip.

Four thermocouples were connected to a PIC processor via discrete amplifiers, and four were connected via the MAX6675 interface chips. I designed in both circuits not knowing which might perform better, but they both seem to work equally well, with the exception of some (not unexpected) scaling error on the discrete amp circuits.

I expected noise and a lot of "dithering" on the LCD readout but neither happened, happily.

To tweak accuracy I'm considering the purchase of a thermocouple simulator/calibrator. Can anyone advise what these instruments can do for me that I can't do with a low voltage source, a good voltmeter and a copy of the NIST tables?

Thanks in advance.