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K-type hightemp and corrosion resistant thermocouple-but how to attach it?
Leland_Gaunt:
Hello gentlemen
I need some help and hope i can find somebody who is familiar with k-type thermocouples (and extension wiring). Im not so much into electronics, but i wanted to build myself, for my chemistry reactions, a thermometer that is finally both able to handle at least 500 C and is also non reactive with bases or acids. Glassthermometers are first choice but they only reach 200-300C. I already have the regular, cheap "wire" k-type probes and also the 304 stainless steel probe rods. These can handle 500 C but they do react with certain acids. So i came over some probes that are made from Korund-Ceramics, Aluminiumoxid it is chemically.
So i just ordered one of these probes for 20 bucks and also a new K/J type Thermometer for 20 bucks from my trusted cheapo-brand Uni-T. Its not a boygroup, its a chinese brand that manufactures all kinds of testing equipment. While i was waiting for delivery, i realized i really dont even know much about K-types. The bottom of the probe has 6 screws, instead of 2, i asked the seller if its really a K-Type, he assured me it is. Theres also no cable coming with the probe, i dont know what i was thinking, probably i just wanted to cut of the end of one of these 2 dollar-wire thermocouples and screw it on the probe, using it as a cable. Now these cables are made out of different kinds of nickel-alloys, its probably not even possible or a good idea to use them as an extension cable. If i dont get a somewhat accurate reading, i could have stayed with my IR-thermometer.
So my questions are: is this indeed a probe that i can make work on my K/J type thermometer? And if, what kind of cable shall i use? I can spare one of my old probes to get a yellow K-Type Plug that fits iinto the thermometer. But what kind of cable? Just regular copper wire?
Kean:
A thermocouple uses two different types of metal to form a junction that creates a voltage potential that has a known relationship to temperature.
The K-Type thermocouple uses alloys called Chromel & Alumel (Nickel Chromium & Nickel Aluminium).
To avoid errors, you want to use the same type of alloys in your wiring all the way to your measuring device (DMM, etc). The connectors are also made with these alloys.
On the probe you show, I imagine the two centre screws are for mechanical assembly. Two outer screws connect to the wires from inside the tube. The other two outer screws are for your extension cable. Be careful with disassembly and reassembly as you cannot easily tell the type of alloy of the metal contacts, so you don't want to get the mixed up. I think I can see plus and minus symbols on those connection points.
You can buy the connectors and extension cable from usual suppliers. I bought some from element14 under the brand name Lab Facility.
Also note that there are different standards for colour coding of both connectors and cable insulation - IEC, ANSI, DIN, and JIS. Your yellow connectors for K-Type are actually the ANSI (US) standard, and you might want the green connectors and cables per IEC/DIN standards.
Here are some links to PDFs with further info
https://assets.omega.com/landing-pages/colorcodes/tc_colorcodes.pdf
https://www.labfacility.com/media/productattach/t/h/thermocouple_product_guide_labfacility.pdf
I've also one attached below
J-Type uses different alloys to K-Type, so also make sure you know what you have and don't mix them up.
Annoyingly the different standards do re-use some of the same colours for different thermocouple types. |O
You want to minimise the use of copper connections. If curious, you can learn more by reading up on isothermal blocks and cold junction compensation.
jpanhalt:
I am definitely not an expert on thermocouples. The only time I ever worked at temperatures like your are using was making ketene from acetone many years ago. My comment is made based only from what I see.
First, is it type K? I would trust the seller. You need to identify the two wires. Alumel is magnetic and chromel is not. In regular TC's, alumel usually has red insulation and is the negative. Chromel (+ lead) can be some other color, usually yellow. However, it's not surprising the wires from that probe don't have insulation based on its intended temperature.
What you show is a junction. I suspect the center two screws are probably to hold the assembly together. The TC wires are attached at one end, and the other end is for attachment to your instrument. Can you just connect under that same screws used for the TC wires? Probably, but that may not be as secure as using the other two screws. Can you use ordinary copper rather that chromel alumel? Yes, but then that junction becomes your cold junction and you will need to know its temperature (see attachment). Are the metal bars connecting the leads plated copper or chromel/alumel? If you use chromel and alumel, then your cold junction can be remote. Read about the "Law of Intermediate Metals" as applied to TC's.
Attached is a picture from Omega of various ways to connect TC's. Figure 1 seems most applicable to your situation, if you use copper.
Leland_Gaunt:
this probe is rated 1300C i think. Theres not the usual cable inside, they claim its Rhodium-Platin or something like that. I couldnt find any cable for it, the sellers also dont offer a cable.
I cant imagine the wire has to be the same stuff than inside. It wouldnt make much sense to me. because there are already different metals in the circle, like the connection bracket for example. (Allthough i think i would try to screw it under the same screw)..but also the K-type connector...and the wiring inside the Thermometer right?
i mean what about those long 304 stainless steel rod probes? Theyre like 20cm long. Is the complete wire made out of the same alumel/nickel alloys? I was under the impression that these 2 alloys are only used inside the rod and that the cable from the rod to the thermometer is just regular copper cable...?
well meanwhile ill just hook up a copper table and make some comparisons to see if i get real or fantasy readings.
nfmax:
You could use K-type compensating cable, rather than extension cable. Compensating cable matches the thermoelectric behaviour of the probe, but only over a limited temperature range about room temperature. It can be made more robust and flexible, at less cost, than extension cable. Use the ‘spare’ pair of screws to attach it. Provided the two connecting bars are at the same temperature, the intermediate metal should not introduce significant error.
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