As I recall from my a-level physics lessons, the nature of any connections/interwiring doesn't actually matter as long as they are all at (practically) the same temperature as the cold junction.

For the dummies like me out there can you please explain why it matters ? I don't have a degree of any kind but like to learn so any info you have about this would be a worthy addition to the filing cabinet that resides between my ears !

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This is a good introduction: **LINK**

http://enacademic.com/dic.nsf/enwiki/19021/Thermocouple

and the 'why it matters' is pretty much covered by the Measuring Circuit diagram and the 'Principle of operation' text.

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If the thermocouple lead is extended with [say] twin copper wires, then the conector block will become the cold reference junction; regardless of the design intention of the measuring instrument.

In the Environmental Test House, we made our own thermocouples, simply by welding a junction on the end of a [sometimes very long] piece of 'extension wire' … The result is much more reliable.

MichaelG.

I wonder what actual difference use of the 'proper plug' and cable would have made in this project. The device appears to do everything its designer wanted except that the achieved temperatures may or may not be exactly as he thinks he measured, but different by how much and will it make a difference. Could be the guy who built it knew full well that an XLR plug and thin stranded copper wire would work well enough for his application.

So with a temp of 1000deg on a furnace does 10deg error really matter in this instance, probably not but may be an issue if heating a pendant

Also I expect the controller could be programmed to compensate for any errors if you had an accurate way to measure what that temp was.

Jason,

Why on earth bother trying to programme the PID controller to correct the error? I seriously doubt that one could anyway.

10 degrees is 10 degrees error that can be eliminated by using the correct connector, which is in fact cheaper than the specified connector! OK if you are measuring 1000 degrees it doesn't much matter, but if you are measuring a100 degrees it sure as hell does!! Once you have a PID controller, then who knows what you will be controlling in future?

Andrew

Yes I had to check a few facts using Google but I'm amazed how much I remember from 38 years ago!

A very timely article for me as I'm just upgrading the control of an enameling kiln.
I bought much the same gear but as a kit, which came complete with a heatsink for the SSR. That raised the question of how much heat is being dissipated by the SSR. Reference to the web soon indicated that it was typically 1.5 watts per amp of current through the device so your 2Kw kettle sends about 15w out of the SSR.
I think that is a bit too much for a sealed plastic box to withstand for longer periods such as with kilns etc.
There is no mention of a heatsink in the article so am I being over cautious mounting mine with heatsink in a ventilated steel enclosure?

Bob D

I use XLR for my homebrew astro kit. robust, designed to be repeatedly plugged and unplugged and most of all, positively locking.

As Neil says, it is simply necessary to ensure that the pairs of connector pins at each change of wiring are at the same temperature. That way, any thermocouple voltage induced across each set of contacts between dissimilar metals is automatically nulled by an equal and opposite one on the complementary connection. That goes for the contact between the back of the connector and any wiring, as well as the contacts between mating connector contacts themselves.

If you look at the "proper" thermocouple connectors, you my be otherwise puzzled to discover that both contacts are made of the same material and plating, despite the thermocouple itself being composed of different metals. And if you look beyond the thermocouple probes themselves where they are integrated into "proper" industrial systems, you may be puzzled to see they have just used "normal" copper wiring. Nothing wrong with that at all.

You can solder t/c junctions (with care) instead of spot welding them. And of course they work "correctly" from a thermoelectric point of view.

As for whether XLR connectors are appropriate, it's mainly a question of whether they are able to produce a consistent, low resistance contact over the lifetime of the application. I don't know the answer to that but I'd be surprised if they weren't OK. Assuming they are from a reputable manufacturer.

Murray