PID Controller - MEW 269 - wrong connector

An interesting article, but anyone considering building something similar should note that the author has used an XLR connector for the thermocouple instead of a proper thermocouple connector that should have been used.

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.

Sorry Neil, but I think it does matter, I haven't time to dig out my Physics/ElecEng degree text books to calculate it though.

The article talks about "affordable precision" and "with great accuracy" - but then spoils it.

Proper thermocouple connectors are probably no more expensive than the XLR connector used. No excuse not to use them.

Also, re-reading the article, it describes using "thin gauge stranded wire for the thermocouple connections" - it should be the appropriate thermocouple material wires that are used to extend the thermocouple appropriately, not generic copper wire for both cores.

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 !

Agree with Dave that any cables extending thermocouple's should be as specified by the manufacturer and they certainly won't be standard copper cables, used to buy mine from RS but that was many years ago.

Emgee

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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.

Very educational this site, it would never have occurred to me that plugs & sockets might matter on a thermocouple. Nothing but good news because the proper plugs are cheaper than XLR.

Dave

Edited By SillyOldDuffer on 16/06/2018 09:13:12

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.

You're right Nige, it will work, but not as well. The thermocouple effect - in which the genuine signal is very small - arises from dis-similar metals at a different temperature to the 'reference junction'. Every change of material, such as using the wrong wire, introduces at least one extra thermocouple which will bring an error. Using connectors, there will be several changes of material, and with plated pins, there may be a number of transitions. Wrong wire and wrong connector errors can introduce errors of maybe up to 10 degrees or more; and they won't be just fixed errors - each transition is a separate thermocouple, so depending on ambient and temperature inside the enclosure, the errors won't even be fixed.

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.

You would have to buy the female facia mount too.

Might still be cheaper though. Haven't checked. 😉

Steve.

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

Any other voltages generated by thermocouple effects will be:

1 symmetrical if the wiring for the routes in and out are the same, having no impact on the situation (Kirchoff's law.

The voltages around a circuit sum to zero).

2 The voltage across the thermocouple is generated across the portion of the sensor wires where a temperature gradient exists. It is negligible for any parts of the system at a uniform temperature.

Certainly for ultimate accuracy you need consistency and continuity of the connections.

However, it is not the temperature difference between the cold junction and the hot junction that creates the voltage, it's the temperature difference between the hot and cold parts of the thermocouple wires.

As variations in temperature beyond the kiln are likely to be negligible compared to the gradient inside and outside the kiln the impact on overall accuracy will be negligible as well.

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

Irrespective of the thermal rights and wrongs of connector choice the use of XLR seems a bit odd. They're large and expensive. One of the few places they get used is in professional audio and broadcast equipment. Bizzarely on some OB equipment they're a "standard" for connecting external battery packs.

Even if the use of an XLR has little effect I think a proper thermocouple connector would be better. They're cheaper, and a sensible design goal would be to eliminate the errors that are easy to do, and concentrate on those that aren't.

Andrew

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

Well the heat produced by the SSR will still be inside the enclosure whether there is a heatsink or not. In practice though the simplest is to mount the SSR on an internal surface of the enclosure. Sttel is not the best heatsink material but the losses in the SSR that cause it to heat up are only really when it is in the 'ON' state. Depending on the duty cycle the final heating effect will probably be insignificant. I have used hundreds of controllers and SSRs, rarely if ever done any calculations and usually just mounted them on a convenient surface. I dont recall any ever getting noticeably hot or failing.

Ian P

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

How many of us go to the junk box first to find components we need.. Want a two wire connection, Mmmm let me see, what have we here, a bit of chocolate block, Nah not easy to plug/unplug, 5 pin DIN plug, damn, would be ok if I had a socket for it, oh here we go, a 3 pin XLR plug AND a panel mount socket, jobs a good un

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