Talk:Thermocouple

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Contents

[edit] A how-to?

The article as it stands now is not an encyclopedia entry, but rather a how-to-use guide for electronics. Some pruning and editing must be done to make it good.

MattH 05:27, 15 Sep 2003 (UTC)


I agree with MattH. Also, there is no mention of thermocouples being used for power generation (e.g. in RTGs). Bobbis 00:37, 27 Oct 2004 (UTC)

Indeed; after following a link from the Radioisotope thermoelectric generator article, I was surprised that there was no further information on how the power is actually generated. -Belross
Unfortunately I don't know enough about them to change this - might do some reading. Bobbis 00:15, 15 Jun 2005 (UTC)
Peltier-Seebeck effect - Omegatron 03:36, Jun 2, 2005 (UTC)

I also agree, original text by Pico tech with alot of references to Pico TC-08. Should be edited to make more sense without references to TC-8 Psanderson 11:22, 15 May 2005 (GMT+1)

I have chopped the entire section on Precautions and considerations for use, because it is very unencyclopediac, and it is available on the external links anyway. Revert if you feel otherwise, but I think my previous edits are less controversial so should be kept. Bobbis 00:15, 15 Jun 2005 (UTC)
I think the precautions section was useful, but I'm not sure my feeling rises to the level of a revert yet :-). Let's see what others have to say.
Atlant 11:07, 15 Jun 2005 (UTC)

[edit] Thermocouple vs. Thermistor

The explanation of cold junction compensation says you need a secondary thermistor to correct errors in thermocouple reading. This begs the question, "why not just use a themistor?". I assume the answer has to do with dynamic range (and maybe precision, although the article says thermocouples are difficult to get better than 1 degree celsius resolution...). But it would be nice if there were some explanation in the article. --User:Chinasaur

The primary reason is "dynamic range" (a good thermistor usually has a narrow temperature range), with other reasons being self-heating (unlike a thermocouple, you have to apply current to it) and the very non-linear resistance vs. temperature curve. Hmmm, the real way to answer this is with the various electronic temperature measurement pages having the pros and cons of each style of device. I'll put that on my (long) todo list. -- Kaszeta 20:04, 5 Oct 2004 (UTC)
How would you program the Voltage-Tempature relationship?
24.206.230.2 02:51, 6 September 2006 (UTC)t59

[edit] Deleted 'Dawson' Theory

deleted the following:

"Recently discovered by MA Stephen Dawson, thermocouples may be used in conjunction to calculate circuit Q factors as well as measure Impedance. (XL, XC): The formulas: 2π x F x L OR 1 / (2π x F x C) may be used to calculate such things. Stephen Dawson's discovery states that these can be calculated by a thermocouple device, to gain accurate results an orifice plate may be used within a DP Cell. "

As written, it conveys no useful information, and makes confusing assertions without references, and sounds moreover like self-advertising. There may be a relation between thermocouples and electrical circuit Q, but none that I can find in any textbook, and none that I have encountered in a decade of working with thermocouples.

Sounds good. I don't even understand what they're implying, now that I think of it. Maybe using thermocouples inside an oscillator or something? - Omegatron 18:09, Apr 4, 2005 (UTC)

[edit] Two junctions

Why are two juctions needed, as I currently understand it one juction will produce a EMF, so why are two juctions needed? mickpc

Without a second junction, how could the electric circuit ever be closed? Think about it: We have the voltage generated at the (hot) junction between, say, the copper and iron wires, but now those electrons have to get back home. Somewhere, there must be a second place where the copper and iron touch (well, unless you're simply measuring the electrostatic field that is generated).
Atlant 12:19, 2 August 2005 (UTC)

Ok, well technically you can simply use the perant metal as a wire so u can use one junction. But my question revolves around; if the actual junction causes the EMF or the perant metal does, reading the article again I am tending to think that the EMF is devolped across the perant metal. mickpc

I think you need to draw yourself a diagram. Draw the iron wire. Draw the copper wire. Connect them at one end of your diagram to create the hot-side thermocouple. Now figure out how you connect the wires at the other side of your diagram to create a complete circuit.
Atlant 11:09, 3 August 2005 (UTC)

I asked my lecturer and he said that only one junction is needed and this is generated from the junction itself, now either he is wrong or the article is wrong (and I doubt he is entirly wrong) mickpc

Like I said, you figure out how to get the electrons back around the circuit without another metal-metal junction and you post it here.
Atlant 23:20, 21 August 2005 (UTC)

Quite simply, just use the parent metal as the lead wire (or use a lead that is the same composition as the perant metal). This is tyically done to extend the distance between the two junctions or to the output transducer. mickpc

I'm having trouble understanding why you don't get this. Draw the picture! Assuming you have an actual circuit (a "circle"), you are *ASSURED* of having at least two junctions (where A meets B and then where B meets back up with A), and if you have binding posts and other hardware in the circuit, you may have far more than just two junctions.
Atlant 23:28, 27 August 2005 (UTC)

Please dont get me wrong here, I am only trying to clarify (deepen) my understanding, so I am open to constructive critsisim. You can have a single junction for a thermocouple, just use two dissimlar metals (wires) and join them at the junction. This will produce an EMF. Why you typically use a reference junction I am not entirly sure on, I am thinking that it is most likely the linearisation and stabilty. mickpc

Great; you've got an EMF, but what will you *DO* with it? Unless you have an electrometer handy, there's no way to read that EMF that doesn't draw at least a little DC current from the thermocouple. And the moment you start to draw DC current, you have to close the circuit. And then you're back to the problem I've been trying to raise to your attention: a complete circuit must have at least two junctions.
Atlant 12:11, 30 August 2005 (UTC)

Point taken, well I should have thought more about the this before hand, I read that the voltages are produced at the junctions but the article Peltier-Seebeck effect it is said that it is due to the diffusion mickpc

Just wanted to point out (as my physics mentor told me long ago) that there are always at least 2 junctions in a thermocouple sense circuit. The other junction, is created when the thermocouple is connected to the EMF sense device (ie the thermocouple meter, or sensing circuitry). The volt meter probes will create the other junctions when one connects the iron-copper wires to read the TC junction voltage.

[edit] Re: Compensating/Extension cables

I agree of course, but was just trying to keep it a bit simpler. However, I have re-added the bit about adding the compensating voltage to the thermocouple voltage to get accurate result. Dave 21:59, 24 March 2006 (UTC)

[edit] Other kinds of thermocouples?

In a book by Patrick Moore thermocouples is said to be used to measure temperatures at Venus before the space age. This makes no sense to me as thermocouples are used in situ. Can anyone explain how thermocouples work in this case? Gunnar Larsson 20:12, 25 March 2006 (UTC)

I'm not an astronomer but I suspect the instrument used to measure Venus's temperature was a bolometer which is a specialized form of radiation thermometer that converts infrared energy into a tiny temperature rise, which then warms a thermocouple. So, it is an indirect use of the thermocouple principle. ( And now Wikipedia has got to the point where if you just blindly link a technical term like bolometer, chances are very good you come up with a blue link not a red link!) --Wtshymanski 17:26, 26 March 2006 (UTC)
That sounds exactly right; I couldn't remember the term last night.
Atlant 21:19, 26 March 2006 (UTC)
Ah, this makes more sense to me. Thanks! :-) Gunnar Larsson 18:40, 27 March 2006 (UTC)

[edit] Thermopile

A bit more information about thermopiles would be appreciated as I was redirected here from "thermopile". My understanding is that a thermopile is a number of "stacked" thermocouples to generate useful amount of power, where as a thermocouple can only output enough power to be useful as a sensor. I have no references for this piece of information other than sales information from an unvented propane heater.

Great listing of thermocouple types. Thanks. John H.

I use thermopiles on a daily basis to measure the power of laser beams. I added a section about this. I assume that the function in the propane heater is similar to that in heating appliances: the thermocurrent is used to keep a valve open. Putting several thermocouples in series increases the voltage. Han-Kwang 23:05, 7 July 2006 (UTC)
Right, but lots of convection heaters also use the thermopile voltage (through the thermostat) to also operate the main gas valve; the article discusses this.
Atlant 23:25, 7 July 2006 (UTC)
Yes, I can imagine so if it has to regulate a thermostat, but you have modified the section about guard flames. Search the net for "guard flame thermocouple millivolt" and you'll see plenty of service instructions explaining that its voltage should be in the 10--30 mV range, which corresponds to 200-600 K temperature difference over a single thermocouple. You don't need much to power a solenoid that is just there to keep a spring-loaded valve in the open position. Han-Kwang 11:02, 8 July 2006 (UTC)

[edit] losers

yeah you heard —Preceding unsigned comment added by 212.219.94.37 (talk) 14:28, 28 April 2008 (UTC)