Copper cable K type thermocouple connectors Disaster?

Is your application so critical that it requires precise temperature measurement, in turn requiring that the t/cs be connected via compensating cable rather than simple copper cable? I don't know exactly what difference it makes, but I doubt it's more than a few degrees, BIMBW. To be really precise, you should have a temperature-controlled cold junction in the circuit (not a lot of people know that).

V-------------------------------EMF-------------------------V

-----------------\ /------------------\ /------------ metal A \ / metal B \ / metal A \ / \ / V V hot cold junction junction

My experience was with studio pottery kilns using Type R thermocouples (Pt/Pt-13%Rh), and I did connect them with the appropriate compensating cable (orange sheath; yours should be green if IEC or BS, see

My set-up was in a garden shed, unheated, and potentially damp in winter. I wasn't aware of any corrosion problems with the connectors. Perhaps wrap them in insulating or self-amalgamating tape to keep out the damp if it's of concern in your application.

Thanks, tape isn't an option this is a "quality job" :-)

There is not a single thermocouple used anywhwere to measure anything, so cold junctions do not matter unless there is an influence on the PT100 bridge current. This will become obvious though on first use.

I too have used copper wire to connect TC's, and not had a problem, the cold junction was always in the instrument providing the measurement incidentally. I would not used copper for a serious long term measurement strategy though, hence my lack of experience with this type of setup.

I do recollect having problems with damp connections on more conventional connectors in a similar application that seemed to be exacerbated by the use of aluminium bootlace ferrules, so the reactivity is the item of concern. Anything else can be quantified and rectified.

AB

Back when Ford built a brand new factory near Valencia for the new Fiesta, I went out there to commission the paintshop bake ovens. The electrical contractor had no idea about the importance of the correct use of Thermocouple compensating cable, and the long runs were connected randomly. The effect of this was that some zone burners were running flat out, while others were idling, and there was a total lack of control of oven temperature. Once this was sorted out, it worked properly. Until then, it was chaos.

Davey submitted this idea :

It must be an Italian thing...

When I was out there, a French guy whom I got friendly with, who was staying in our hotel in Brescia, had exactly the same problems commissioning a process oven, except they had been wired in copper.

Even though Valencia is in Spain?

on 02/09/2017, snipped-for-privacy@yahoo.co.uk supposed :

:/

Not quite the same approach as no actual thermocouples are in use anywhere in the system.

There may be problems with the PT100 signals though, any other analogues would be swamped by the 4mA min.

As I said, I am expecting anomolies and thanks to digital displays these will be all too obvious.

The only think I cannot factor in is the possibility of corrosion, this will take weeks to appear, as I mentioned I have no experience of terminating K type connectors with copper wire.

Regards

AB

Just a thought, but are you sure that the actual connectors (blades and terminals) are made of the same materials as K-type thermocouples? I ask, because this site doesn't specify which type of thermocouple to use them with:

It might be worth abrading one or both of the blades with a bit of sandpaper to see if they're actually just copper.

And if you scroll down, you'll see larger sized connectors for heavier cable.

Following the k type further

"Manufactured in accordance with standard EN 50212 ?High purity thermocouple alloys used for male pin and female socket constructions ??Bevel tipped pins for easy insertion Polarised flat pins Related Connectors"

Some indeed have obviously different colours, but K do look like a standard nickel plated connector

AB

RS do an uncompensated version with copper terminals. They're colour-coded white.

Thanks!

You solved a long standing conundrum that bothered me a little from time to time.

The RS 4-20mA current injector uses just such a connector.

I did wonder why on earth a company would produce a test instrument with a connector normally associated with tc,s.

Methinks I know why we went down the route of a thermocouple plug/ socket also.

The genius ordering the connectors would have had access to one of the RS calibrators.

He probably thought green was a nicer colour though.

It will be a nice job replacing a hundred or so when they do give problems :-(

AB

I thought by definition you got a cold junction, whether you wanted one or not? The trick being that you should put the cold junction compensation (ie the thermistor, or chip with a semiconductor temperature sensor) close to the cold junction (where you solder/crimp/whatever to the hot junction leads) so that it is at the same temperature as the inherent cold junction. The cold junction doesn't have to be in an ice bucket, just there be minimal difference between the cold junction and the sensor which is used to compensate.

The problem with a non-compensated cable is the cold junction then becomes a long way from the compensating sensor and so there can be temperature differential between the two. Particularly problematic if there are large heating appliances involved.

Theo

Yes, that's a fair summary, especially the point that you always have a cold junction whether you want it or not. That cold junction has to be at a constant, reference, temperature, because in essence a thermocouple measures the temperature difference between the two junctions. If the temperature of the cold junction wanders around, for example if it's positioned over a radiator or other heat source, the apparent temperature of the thing you're trying to observe will also seem to wander, resulting in poor temperature control in reality. In the past, a sophisticated 'ice bucket' device was used, but these days cold junction temperature control can be achieved electronically.