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Topic: Dual lead (4 wire) type K thermocouple (Read 770 times) previous topic - next topic

transcendtient

I'm installing a few extra thermocouples into a Dedrich IR-5 coffee roaster and so I contacted the manufacturer for any information I could get from them. They do not have an exploded view to help with placement within the machine (problem for another time) and the representative also informed me they were using a dual lead, 4 wire, type K thermocouple. I've been trolling the internet for information regarding this and haven't come across a wiring diagram showing a thermocouple with 4 wires for the Arduino.

I'm simply asking if anyone has experience with 4 wire thermocouples and if anyone has any ideas as to how to wire it into a board with access to 4 thermocouple connects with 2 connects apiece (8 terminal block connections) as I'd ideally like to use manufacturer provided parts to match the resolution and response time of those already in use.

jremington

#1
Jan 17, 2014, 10:38 pm Last Edit: Jan 17, 2014, 10:50 pm by jremington Reason: 1
The only four wire thermocouples that I know of are special purpose, for fault detection like broken leads. Here is some general information, which unfortunately doesn't provide a unique connection diagram: http://www.omega.com/temperature/z/pdf/z021-032.pdf

Edit: here is some material on using a four-wire thermocouple to detect induced electrical noise: http://www.electronics-cooling.com/1997/01/notes-on-using-thermocouples/

transcendtient

All those references say it's not for fault protection, but for noise. I tried to find the elusive "REF. 8" Omega says goes in more depth, but I can't find anything at all other than these descriptions. I haven't seen an application, but Omega says... on Z-32

"One pair is used as a thermocouple to determine the temperature level, and one pair is used to determine the temperature difference between two points."

So if I have the tip-branched type I'll be reading temperatures from both sides of the probe basically, and I've read to put the "hot end" as the compensation line.

I'm wondering what the implications of this being the alternative leg-branched type as I can't find their "respective merits" in REF. 8.

jremington

You need to read a bit more carefully. The Omega guide very clearly says (bold face added):

Quote
Tip-branched and leg-branched thermocouples are four-wire thermocouple circuits that allow redundant measurement of temperature, noise, voltage and resistance for checking wire integrity.


It is can be extremely important to know if your thermocouple leads are intact, otherwise you could melt down some very expensive equipment.

michinyon

I think that I have seen four-wire thermocouples ,   which have the measurement couple  and also a reference couple   and some sort of bridge circuit incorporating both of them.

transcendtient


You need to read a bit more carefully. The Omega guide very clearly says (bold face added):

Quote
Tip-branched and leg-branched thermocouples are four-wire thermocouple circuits that allow redundant measurement of temperature, noise, voltage and resistance for checking wire integrity.


It is can be extremely important to know if your thermocouple leads are intact, otherwise you could melt down some very expensive equipment.


I guess I should say then, for my purposes, this is a closed circuit used only for logging data.
There isn't any operational behavior based on these measurements.

But I do appreciate the slap in the face, what type of measurement from the reference would I be looking for? An increase in resistance and decrease in voltage?

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