4-wire Thermocouple - Page 1
Forum section: electronics / beginners

author: 16bitanalogue
author_rank: Regular Contributor
timestamp: October 04, 2024, 02:58:22 pm
content: Has anyone ever seen/used a 4-wire Thermocouple?And no, I am not talking about an RTD. I have an SA3-K-120-SRTC from Omega Engineering. It has an adhesive pad on the end and I accidentally folded the damn thing and broke the wires. No problem I will attempt to fix! But when I stripped back the wires I found 4 of them. There are the 2 'main' wires each with a smaller gauge wire wrapped around them.I have no experience with 4-wire thermocouples and frankly if I someone were to ask me I would also default to "You must be talking about an RTD." I have attached some pictures of the Omega Engineering thermocouple in question.I only found 2 other references about 4-wire TC circuits:https://forum.arduino.cc/t/dual-lead-4-wire-type-k-thermocouple/205914/3which also references Omega Engineerings super hidden technote on their French website variant:https://www.omega.fr/prodinfo/pdf/Practical-Thermocouple-Measurement-Z-93-to-Z-98.pdfThe "special thermocouples" paragraph abruptly ends unfinished.I contacted Omega Engineering tech support over the phone and via email. The gentlemen on the phone had no idea, and the email I received this morning from someone else said if it has 4 wires then it is an RTD.So presuming we are talking past one another, what are the 2 smaller gauge wires supposed to be?

author: Stray Electron
author_rank: Super Contributor
timestamp: October 04, 2024, 10:52:35 pm
content: I've never heard of a four wire thermocouple and I only see two contacts on that connector so my guess is that it's a standard (two wire) type K thermocouple and, possible with a shielded cable. I would try a regular type K thermocouple and see if it works in that circuit, If it does then find a bead type K and make a pad/mount for it. You may need to find a way to shield the cable but since I don't see a Ground or Shield connection, I don't think that you'll need to do that.FYI, RTD is an entirely different type of thermocouple and uses different materials and an entirely different output curve than a type K so it will not work for you.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2024, 11:16:06 pm
content: An RTD is a resistor, not a thermocouple, so a four-wire connection is often used for better accuracy.The device exploits the known temperature variation of a well-defined metal or other conductor.A popular type uses a platinum element that is 100at 0oC.

author: showman
author_rank: Contributor
timestamp: October 05, 2024, 03:18:33 am
content: Doesn't Omega mostly make thermocouples to order? I have ordered from them couple of times and I'm fairly certain it took way too long for those to arrive to be stock items. Maybe they just grabbed a wrong wire or had too much 4-wire in stock or something?

author: edpalmer42
author_rank: Super Contributor
timestamp: October 05, 2024, 03:39:30 am
content: Are the 4 wires insulated with, perhaps, a layer of enamel or are they bare? And are all 4 wires actually connected?That model of thermocouple is listed as fast response. That means minimizing the mass and size involved, so small wires make sense. But that would also make things very fragile, so maybe the larger wires are only there for mechanical strength but don't actually connect to the thermocouple.The data sheet specifies that the wires are #30 AWG but doesn't mention anything about 4 leads.

author: Poroit
author_rank: Frequent Contributor
timestamp: October 05, 2024, 07:53:11 am
content: G'day Ed,The Yellow plug indicates that it is a type K Thermocouple.The Negative wire should be magnetic so with a few simple tests you can determine which wire is which.They use Duplex Thermocouples in Industry so the two thinner wires would be the fast response T/C if they prove also to be Type K.

author: gamalot
author_rank: Super Contributor
timestamp: October 05, 2024, 08:13:29 am
content: Here is some information on four-wire thermocouples:https://www.electronics-cooling.com/1997/01/notes-on-using-thermocouples/

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 05, 2024, 09:10:33 am
content: Can you post a picture of everything you have?The plug is definitely from a thermocouple, not from a RTD.If both wires are not isolated to each other then it is a fast-response thermocouple. The thick wire is there to keep the resistance of long lines low, the thin wires are joined where the temperature is to be measured.If the thin wire is insulated from the thick wire then it may be just strain relief.It´d help a lot if you could show the internal terminals of the plug (usually you just have to open up one screw on the plug to expose them).

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 05, 2024, 10:31:13 am
content: Quote from: TimFox on October 04, 2024, 11:16:06 pmBut what the OP has is a thermocouple and not an RTD.

author: 16bitanalogue
author_rank: Regular Contributor
timestamp: October 05, 2024, 10:43:32 pm
content: Quote from: edpalmer42 on October 05, 2024, 03:39:30 amThere are two layers of insulation. The chromel wire has a smaller gauge wire twisted around it and there is an insulating sheath around both. This is also true for the alumel wires. Then there is an outer sheath.And you are correct the datasheet is completely void of any information, The tech support engineer looked at the datasheet with me and there is nothing in there.To be clear:The Omega Engineering part number is for a type K thermocoupleThere are only two prong contacts which is what I would expect for a thermocoupleThere is literally no information on what the smaller wires are there forQuote from: Poroit on October 05, 2024, 07:53:11 amI think this is the answer. But damn, Omega couldn't help. I sent the same pic to them to see if they could add more infomation.