Sciencemadness Discussion Board

K type thermocouple

vmelkon - 16-10-2017 at 12:53

I'm a newbie when it comes to these things for now.

If I want to measure températures up to 1200 C, would a k type thermocouple work? I guess it needs to be in some kind of casing. What about oxidation of the thermocouple.

It is made of chromium-nickel and aluminium junction, right?

How long do these things last? Is it a consumable or it goes for 20 years?

I bought a multimeter that comes with this function and a simple thermocouple that can handle 250 C, since most of the conductor wire is in plastic.

Sulaiman - 16-10-2017 at 13:26

The wires that make a type-k thermocouple should be ok to 1350 oC
an oxidising atmosphere (typically the outer part of a flame) is ok,
but not a reducing one (e.g. the blue part in the middle of a flame..

Melgar - 16-10-2017 at 14:24

To be fair, the sheathing on the thermocouples will tend to fail before the wires do. The probes are about a dollar each or less when ordered from China. You can get the ones with a ceramic housing for the end. They tend to be a few bucks more, but they seem to be a lot closer to what you'd want. Here's one for $10:

http://www.ebay.com/itm/352168951729

wg48 - 16-10-2017 at 15:25

Quote: Originally posted by Melgar  
To be fair, the sheathing on the thermocouples will tend to fail before the wires do. The probes are about a dollar each or less when ordered from China. You can get the ones with a ceramic housing for the end. They tend to be a few bucks more, but they seem to be a lot closer to what you'd want. Here's one for $10:

http://www.ebay.com/itm/352168951729


That looks like an insert. Its should be put in a sheath when being used which will put the cost up.

JJay - 16-10-2017 at 15:33

This type has a thermocouple inside a steel tube: http://www.ebay.com/itm/161320380555

The thermocouple, steel tube, and wires can withstand high temperatures, but the section of the thermocouple behind the screw threads is not intended to be placed under intense heat. Both the spring and the wire casing will burn at around 900 C.

vmelkon - 16-10-2017 at 16:46

Quote: Originally posted by JJay  
This type has a thermocouple inside a steel tube: http://www.ebay.com/itm/161320380555

The thermocouple, steel tube, and wires can withstand high temperatures, but the section of the thermocouple behind the screw threads is not intended to be placed under intense heat. Both the spring and the wire casing will burn at around 900 C.


I saw that. I'm assuming the surface will get oxidized. What's inside the tube? Anyone have one of these?

And I'm just curious, what temperature would it show if you put the tip in the flame of a propane torch? 500 oC? 600 oC. Yes, I realize that would not be the true temperature of the flame because the steel radiates away quickly.

BTW, this the multimeter
http://www.canadiantire.ca/en/pdp/mastercraft-deluxe-digital...
The price is wrong. It isn't 60$. It is 35$ and I got it for 25$.

JJay - 16-10-2017 at 17:06

It's a standard type K thermocouple... they are mostly nickel but also contain iron, chromium, aluminum, and silicon. The thermocouple itself is tiny piece of bimetallic (two nickel alloys) wire a few mm long. The temperature reading is the temperature at the tip of the steel tube. I assume the leads are probably nichrome.

Twospoons - 16-10-2017 at 17:58

No, the leads are the same material the thermocouple is made from - in this case one wire will be chromel, the other alumel. This way the 'cold' junction of the thermocouple will be at the measurement end of the system, where it can be compensated for.

Melgar - 16-10-2017 at 18:11

Quote: Originally posted by JJay  
It's a standard type K thermocouple... they are mostly nickel but also contain iron, chromium, aluminum, and silicon. The thermocouple itself is tiny piece of bimetallic (two nickel alloys) wire a few mm long. The temperature reading is the temperature at the tip of the steel tube. I assume the leads are probably nichrome.

Just a polite suggestion: it might help if you read up on the thermoelectric effect, which explains how thermocouples, peltier junctions (thermoelectric coolers), and thermoelectric generators work:

https://en.wikipedia.org/wiki/Thermocouple

Basically, the voltage across the two wires is related to the temperature difference of the two junctions (one at each end). If you know the temperature of the junction that's inside your measuring device (easily measured via an IC), you can derive the temperature of the other junction.

JJay - 16-10-2017 at 18:53

Well, technically chromel is a type of nichrome :) Either the alumel part of the thermocouple has to interface directly with a chromel lead in the cool zone or the chromel part has to interface directly with an alumel lead in the cool zone.

Melgar does raise a good point about the IC; without it, a thermocouple really can't measure absolute temperature.

Melgar - 16-10-2017 at 19:47

Quote: Originally posted by JJay  
Melgar does raise a good point about the IC; without it, a thermocouple really can't measure absolute temperature.


Yeah, it kind of bugs me when people explain something, but leave out a really important part that you need to know to understand it. Like, I was a junior in college before I actually understood how uranium/lead dating of rocks worked. I asked every single teacher "how do we know that the lead wasn't there when the rock formed?" But they'd write me off as a creationist or something instead of answering me. Even the recent Cosmos reboot attempt didn't answer that question! They explain everything BUT that, but if you don't know WHY there isn't any lead in freshly-solidified zircon crystals, then the rest doesn't make any sense.

Well, come to find uranium fits into zircon's crystal lattice quite nicely, and lead doesn't fit at all. So when the crystal is solidifying, it attracts uranium and repels lead. Of course, once it's solid, atoms can't move around in it anymore, so any uranium that turns into lead is stuck there. But that also means that any lead found in zircon would have come from uranium decaying. Since we know uranium's half-life, we can then figure out when the zircon solidified, which gives you the age of that particular rock at a minimum.

But to get back on topic, that's the nice thing about thermocouples, is that you can keep one end a convenient temperature for measuring with an IC, and then the other end that you measure can be whatever infernal temperature you like, as long as you can find two dissimilar metals that can withstand it.

highpower48 - 17-10-2017 at 08:58

I just closed the end of a 5mm glass tube and insert my thermocouple and then use, just like a normal thermometer. Works for me but not sure about really hot temps.

vmelkon - 17-10-2017 at 10:51

Quote: Originally posted by Melgar  
Yeah, it kind of bugs me when people explain something, but leave out a really important part that you need to know to understand it. Like, I was a junior in college before I actually understood how uranium/lead dating of rocks worked. I asked every single teacher "how do we know that the lead wasn't there when the rock formed?" But they'd write me off as a creationist or something instead of answering me. Even the recent Cosmos reboot attempt didn't answer that question! They explain everything BUT that, but if you don't know WHY there isn't any lead in freshly-solidified zircon crystals, then the rest doesn't make any sense.


Yes, I hate it as well when there is crucial information missing.

BTW, there are other dating methods. Even if your sample contained primordial lead, primordial lead has multiple isotopes. Example, if you have Pb206 (I don't really know, this is a rough example.) in your sample, then this is not a decay product of U238. So, with respect to that, you would also have some Pb208.
Let's say U238 also leads to Pb208. Well, now we know that a certain % of Pb208 is primordial and a certain % is a daughter product from the U238. The excess Pb208 is the daughter product. Yes, all this stuff destroys Young earth creationism. They claim that there is no way to know how much U238 you started with. In fact, look at all the radioactive isotopes on Earth. They all have super long half-lives except for C14.

What you were describing is fractional crystalization. Certain atoms or ions don't fit well in a lattice and get excluded.

Melgar - 19-10-2017 at 17:35

Quote: Originally posted by vmelkon  
Example, if you have Pb206 (I don't really know, this is a rough example.) in your sample, then this is not a decay product of U238. So, with respect to that, you would also have some Pb208.
Let's say U238 also leads to Pb208. Well, now we know that a certain % of Pb208 is primordial and a certain % is a daughter product from the U238. The excess Pb208 is the daughter product. Yes, all this stuff destroys Young earth creationism. They claim that there is no way to know how much U238 you started with. In fact, look at all the radioactive isotopes on Earth. They all have super long half-lives except for C14.

Lots of things disprove young-earth creationism, but according to them, words in the bible that have been retranslated three or four times are somehow more true than stuff that it's possible to actually observe. Also, self-delusion is actually called "faith", and is a virtue.
Quote:
What you were describing is fractional crystalization. Certain atoms or ions don't fit well in a lattice and get excluded.

I suspect that the concept of an impure crystal (incorporating other compounds into its lattice) is beyond the understanding of many teachers, since "isn't crystallization how you purify things?"
Quote: Originally posted by highpower48  
I just closed the end of a 5mm glass tube and insert my thermocouple and then use, just like a normal thermometer. Works for me but not sure about really hot temps.

So as to not derail this thread, it actually works better if you heat the center of the tube gradually, then pull the two ends apart slowly, to make the glass thinner. When it's about as narrow as you want it to get, let it cool, score it, break it, then seal the thinner ends. That way, the temperature response is a lot better, and is at least as good as a mercury thermometer, since it doesn't have to conduct heat through so much glass and stainless steel.

vmelkon - 21-10-2017 at 16:46

Quote: Originally posted by Melgar  

I suspect that the concept of an impure crystal (incorporating other compounds into its lattice) is beyond the understanding of many teachers, since "isn't crystallization how you purify things?"


It's a pretty neat technique. It can be used to separate near identical molecules, such as L- and D-.
Partial crystallization is another one we all use.
Quote: Originally posted by Melgar  

So as to not derail this thread, it actually works better if you heat the center of the tube gradually, then pull the two ends apart slowly, to make the glass thinner. When it's about as narrow as you want it to get, let it cool, score it, break it, then seal the thinner ends. That way, the temperature response is a lot better, and is at least as good as a mercury thermometer, since it doesn't have to conduct heat through so much glass and stainless steel.


That's a good idea if you need a fast response time.
For me, I would be dealing with mostly stable temperatures, I would be looking at the thermometer every 5 min or 10 min, not every second.
I can't believe I don't have a high temperature thermometer until now and that's what motivates me to go for a thermocouple solution.