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Author Topic: Measuing Cylinder Head Temperatures (Thermocouple Ground Issue)  (Read 920 times)
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Portland, OR
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Hi all,

I'm trying to measure the cylinder head temperatures (CHT) on an engine. So far, I've experimented with the MCL Eng's Quad Thermocouple Interface (http://ryanjmclaughlin.com/wiki/Quad_Thermocouple_Interface) and Adafruit's MAX31855 breakout board (http://www.adafruit.com/products/269). Both work until the (K-type) thermocouples are connected to a common ground - which they are when installed (they look like this: http://www.aircraft-spruce.com/catalog/inpages/smicroprobes1.php and are like washers under the spark plug). As soon as more than 1 probe touches the engine block, all readings go to zero. This is with the engine off, and the battery disconnected and no other "hot" electric wires close by.

The same probes have worked with Auber Instrument's pyrometers (http://www.auberins.com/index.php?main_page=product_info&cPath=5_21&products_id=139), so I know it's possible to read multiple, grounded CHT probes.

Any ideas what to try next? Thanks!

Holger



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The chip on the Adafruit board detects if one the the thermocouple pins is connected to GND or Vcc.
I don't know if it can be disabled, and I don't know if that is the cause of a reading of 0.
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I'll check that out, thanks for pointing me to it, Erdin. I now also posted a question to the Adafruit forum, let's see if something comes back there.

Holger
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OK, got an answer from Bill@Adafruit. Evidently the MAX31855 can't handle grounded TCs. Time to look for alternatives, and Bill suggested to check out MAX6675.

Holger
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The picture shows the (-) grounded:
http://www.maximintegrated.com/datasheet/index.mvp/id/3149
But Adafruit replaced it with the newer one, so they don't sell it anymore.

The chip itself costs (less than) 5 dollars on Ebay. I can't find a nice breakout board.
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Sweet, this could work. Thanks, Erdin! I still have an unused MAX6675 breakout board here. I'll try it out tomorrow. Here are some more: http://www.goodluckbuy.com/max6675-arduino-temperature-sensor-k-type-thermocouple-amplifier.html, although who knows if they are still in stock. I'm surprised there isn't more information out there about measuring CHT/EGT with grounded probes. Maybe it's really only an aviation thing.

Holger
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I had a similar problem trying to read the temperature of a motor housing. I had to switch to a thermocouple that didn't make an electrical connection to the motor housing. I was using Max6675 breakout board. Maybe you'll have better luck with the max6675.
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Thanks for your comment, Scott. Ungrounded is certainly better, but I don't know if there's any in the washer style.

In the meanwhile I have received some very helpful advice from the UK here: http://forum.arduino.cc/index.php?topic=106975.0 . First tests are promising with all probes returning valid temperatures, even grounded. I'll experiment more and report back.

Thanks,

Holger
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