On Sat, 02 Sep 2017 08:42:58 +0100, Chris Hogg wrote:

On Sat, 02 Sep 2017 03:47:01 +0100, Archibald Tarquin Blenkinsopp
wrote:

Hi
Some nameless cretin decided it would be a good idea to streamline
the selection of signal cable connectors on a current little device
I'm playing with.

We have K type two pin connectors for everything carrying digital or
analogue signals, apart from thermocouples of course because this is a
"quality job" using PT100,s, none of your cheapo thermocouples here!

Of course the fact that the K type connector cannot accomodate the
compensating cable is a different matter entirely :-)

There are a gross of possible problems, which will all become obvious
on first use, but my main concern is the fact that these connectors
are in a damp environment and the chrome/ nickel- Aluminium/ nickel
pins connected to copper wire will I'm sure corrode to a state that
will produce problems in weeks or months.

Has anyone come across an instance of copper cable being used with
these connectors?

I would guess that ist isn't a frequent configuration as I am still
waiting for a reply from the supplier.

AB

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
http://tmseurope.co.uk/applications/...es-tolerances),
but I always thought it was a bit overkill for my application. The
connectors come in different sizes IIRC, so you can size them to the
cable. But t/c cables don't carry significant current so don't have to
be very large.

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