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I'm having problems with documentation and what appears to be conflicting use of the word grounded. The way I understand it, a thermocouple can be [grounded, ungrounded, or unsheathed] (http://www.omega.com/techref/themointro.html). But neither of the two wires are actually connected to ground, they're still floating when they go into the thermocouple amplifier.

However, I've been reading a lot about thermocouples and they talk about grounding them by connecting the T- side to ground through a 1M resistor. This is a completely different type of grounding to me (http://www.analog.com/library/analogdialogue/archives/44-10/thermocouple.pdf)

I'm trying to use a Max31855 breakout board, and it specifically says it's not compatible with grounded thermocouples. So which grounding do they mean? I've not had either wire connected to ground (but I'm using a K-type grounded thermocouple), and occasionally a static shock when I touch it will reboot my Arduino and send out all kinds of garbage over the serial.

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Omega's guide is focusing more on the thermocouple itself. Analog Devices' article focuses more on the sugnal conditioning for thermocouples. I'll explain what is implied by Grounded and Ungrounded in the Omega's guide. In the context of the Omega's guide, the thermocouples are a part of thermocouple assemblies or thermocouple probes.

ungrounded thermocouple
Ungrounded thermocouple. Thermocouple wires are not connected to the sheath of the thermocouple probe.

grounded thermocouple
Grounded thermocouple. Thermocouple is deliberately connected to the sheath to improve the response time.

Here's an example of how a thermocouple probe can be used. It measures temperature of water in a tank. The tank and water are grounded. As a result, the sheath of the probe is grounded. If the thermocouple itself is connected to the probe sheath, then the thermocouple is grounded to the tank, no matter how you connect the thermocouple wires. If thermocouple wires are not connected to the sheath, you can connect the output ends as you see fit.

enter image description here

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  • \$\begingroup\$ In this scenario with your picture, a grounded thermocouple connects the leads of the thermocouple to the sheath, which then touches water that's connected to a bin that is connected to ground. Doesn't that mean a ground loop is guaranteed? How is that desirable? If my grounded tip thermocouple is not connected to anything grounded, is it considered grounded for the purpose of the MAX datasheet? \$\endgroup\$ – Bob Baddeley Mar 4 '14 at 18:14
  • \$\begingroup\$ The ground loop in undesirable, of course. Grounded TC usually uses a floating front end. For instance, if you connect your MAX31855 through opto-couplers and isolate its power, you can make it floating. If your thermocouple probe sheath is not connected to anything except the TC inside, then it's not grounded for the purpose of MAX31855 datasheet. \$\endgroup\$ – Nick Alexeev Mar 4 '14 at 18:19
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The 1M resistor is to provide a path for bias currents when using an instrumentation amplifier with an ungrounded junction thermocouple. It does not affect a grounded junction thermocouple because it will be shunted with a much lower leadwire resistance. The 1M resistor does not make the thermocouple a grounded type.

The MAX chip will not work with conventional grounded-junction thermocouples where the two wires are welded into the end of the metal tube. Typically the tube is then shoved into a hole in a block of material or immersed in a liquid with the tube connected electrically to a grounded tank through a compression fitting. This type is the fastest and most robust type of thermocouple (and also somewhat more economical and more reliable), so it is unfortunate, but it is what it is.

You may be able to use shielding on the thermocouple wires to avoid the ESD events you describe or some kind of RC filtering to ground.

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  • \$\begingroup\$ Thank you. So the word grounded is indeed used two different independent ways, right? And my current configuration is incompatible. Is there an alternative thermocouple amp that DOES work with these very common thermocouples? \$\endgroup\$ – Bob Baddeley Mar 4 '14 at 17:17
  • \$\begingroup\$ You said neither conductor was connected to ground, so it should work in your case. I know of nothing cheap/easy that does what you're asking. \$\endgroup\$ – Spehro Pefhany Mar 4 '14 at 17:21

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