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!