Thermocouple Questions

I’m looking at making a cool down timer for my truck. Basically what it involves is taking the exhaust temperature from a thermocouple in and cutting power to the ignition only after the turbo has reached a safe temperature. I already have a thermocouple (k type) attached to a gauge in the car and was hoping to use something such as http://www.adafruit.com/products/269 to allow my nano to read it. My question is will this IC screw with my gauge that is already hooked to my nano or is it effectively infinitely resistive when reading the thermocouple?

Do you want the same thermocouple to go to you gauge and Adafruit module ?
That seems not right to me.
First of all, how will you split the wires ? You can't just connect the thermocouple to other wires, since that would make it unusable.

A thermocouple input should be cold-junction compensated and is highly optimized to get the best accuracy out of it. Anything else would make it less accurate.

Why not use two thermocouples ?

You can't share a single TC for two instruments/consumers.

Holger

I worked with instrumentation at a oil refinery that used thousands of thermocouples. There were a very few cases where a single coupled 'branched' to two 'receiving' devices, but it can be tricky as has been mentioned that each 'receiver' needs to perform it's own cold reference junction at the point of converting from thermocouple wires to copper conductors in the receiver. Also some receiving systems 'test' the thermocouple before measurement by checking for high resistance (open thermocouple detector) and would probably not work well with two independent asynchronous operating receivers using one couple. But fundamentally it is possible to have more then one 'receiver' wired to a single TC as the TC just generates a small DC millivolt low impedance voltage that certainly can be read by more then one high impedance input circuit, so the TC just acts as a small DC battery with a small millivolt voltage output proportional to the temperature at it's 'hot junction' end.

I also saw there one installation with the opposite situation of many thermocouples wired in parallel to one electronic receiver. This was to read about a dozen thermocouples monitoring a high temperature turbine generator section. The purpose was to gain a good 'average' turbine inlet temperature value and it also prevented turbine shutdown in case a single couple was lost. Part of a monthly preventative maintenance procedure was to disconnect one couple at a time and test to be sure that one couple was reading properly (not burned open) then reattach and test the next one, it could all be done on-line with the turbine running but still one had to be very careful as shorting out the 'average' measurement value (say by a slipping screwdriver) even for a second could trigger a automatic safety shutdown.

Lefty

Interesting information, Lefty, thanks for sharing!

Holger

Lefty,
I'm not sure if I am right, so could you confirm this:

If one thermocouple is connected to two measuring devices, extra wires must be connected to the thermocouple.
They should be the same metal (the thermocouple wires) for each of the wires, and they must be connected without solder.

If normal copper wires are used, or if the wires are soldered, the thermocouple effect is completely gone.

Erdin:
Lefty,
I'm not sure if I am right, so could you confirm this:

If one thermocouple is connected to two measuring devices, extra wires must be connected to the thermocouple.
They should be the same metal (the thermocouple wires) for each of the wires, and they must be connected without solder.

If normal copper wires are used, or if the wires are soldered, the thermocouple effect is completely gone.

Not completely gone, but rather it develops an offset error caused by the difference of the hot junction temp and the cold junction temp where the transition between TC wire and copper wire is performed. That transition from TC wire to copper wire is where the ambient temperature much be measured to perform the cold reference junction correction calculation.

Lefty

I see. Thank you !