thermocouple wiring? (1 Viewer)

[Christ Reynolds]

i am taking industrial process control now, and we covered thermocouples. as you may or may not know, the red wire on a thermocouple is actually the negative, and the black is the positive. as you probably know, this is usually reversed for the vast majority of electronics in north america. our professor told us we could get an extra lab grade if we could find out why this is, so i'm asking here if anyone knows, or could point me in the right direction. our professor isnt sure why this is either, so even if i come up with something that sounds good, i'm sure it would work. i would appreciate it very much, an extra lab grade in this class certainly couldnt hurt my grade. thanks guys

CJ

 

[BrianW]

I know that the positive lead of thermocouples is color coded according to type (mixes of metals):

Type E - Purple
Type J - Black
Type K - Yellow
Type T - Blue

My guess (and it's only a guess) is that the color black was already taken as a thermocouple type and became an accepted standard, so another color had to be selected for the universal negative lead. So why not pick the most confusing color you can? Red it is!

 

[Chris Tsutsui]

I did a quick search and didn't come up with much: :frowning:

United States and International Color Coding Standard ANSI color coding (United States) is used on all insulated thermocouple wire and extension wire when type of insulation permits. Thermocouple grade wire normally has a brown overall jacket. For Types B, R and S the color codes relate to the compensating cable normally used.

Additionally, various national and international standard agencies have adapted color codes for the identification of thermocouple wire and products. These generally differ from those used by ANSI.

Any two dissimilar metals may be used to make a thermocouple. Of the infinite number of thermocouple combinations which can be made, the world has standardized seven types which exhibit a range of desirable features. These thermocouple types are known by a single letter designation: J, K, T, E, R, S or B. While the composition of these thermocouples are international standards, the color codes of the wires are not. In the USA, the negative lead is always red, while the rest of the world uses red to designate the positive lead.

I say it's the same reason USA uses feet and inches while the rest of the world uses metric. Because our relatively "new" country adopted bad habits we have no intention of changing any time soon.

 

[Glenn Overholt]

What country(ies) are the thermocouples made in? Maybe they use red for negative.

Slightly off-topic - I found it funny that the +5v line in computers is red, and the positive lead on computer fans is also red, but they are 12 volts. Who was the idiot that did that?

Glenn

 

[Todd Hochard]

In the USA, the negative lead is always red, while the rest of the world uses red to designate the positive lead.

This was done to purposely make guys like me, who work with this stuff daily in the US, and occasionally in Europe, look stupid. No other reason.

Todd

 

[Daren Welsh]

An EE buddy of mine says to check here.

The negative wire is the wire used as the measuring junction. Now because the measuring junction can vary in temperature i.e. get hotter, it stands to reason that it would be coded in the color red to signify hot as in temperature hot not electrical "hot". It's the common sense approach for coding temperature hot objects. It's the same reason why most radiator caps on cars have some type of red marking on them.

 

[Todd Hochard]

Interesting. I know about the reference junction and measuring junction, but it still didn't click with me. I look at it as an electrical device.

 

[BrianW]

Well, that's just silly. I didn't consider the possibility that any wire color standard would look at a thermocouple as anything but an electrical device. Of course it's an electrical device, and its electrical properties are your main, if not only, concern when you're messing with the wires.

So when we see a red wire, we should assume that it is either positive, or that it requires oven mits to handle, but not both? Who came up with that?

 

[Christ Reynolds]

ive been searching around, no luck. hey darren, thats a good link, but i cant seem to find the text that you copied and pasted within it. is it somewhere else on the page? anyway, thanks for the replies.

CJ

 

[Daren Welsh]

The quote is my buddy's response.