PID Controller – MEW 269 – wrong connector

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 18/06/2018 22:17:55:

Actually the 'cold junction' is almost a myth.

usually it's drawn as the inner part of the special connector.

If you connect the two wires at both ends, you won't have any voltage to measure…

.

Perhaps this is a more helpful diagram:

**LINK**

http://enacademic.com/pictures/enwiki/84/Thermocouple_circuit.svg

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by DrDave on 18/06/2018 22:39:15:

… The Wikipedia entry on thermocouples (or that referenced byMichael Gillian on p1) gives a good overview.

Ducks and awaits incoming criticism!

.

We are, I'm sure, in agreement !!

MichaelG.

Edited By Michael Gilligan on 18/06/2018 23:00:55

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 18/06/2018 22:41:17:

Posted by Neil Wyatt on 18/06/2018 22:17:55:

Actually the 'cold junction' is almost a myth.

usually it's drawn as the inner part of the special connector.

If you connect the two wires at both ends, you won't have any voltage to measure…

.

Perhaps this is a more helpful diagram:

**LINK**

http://enacademic.com/pictures/enwiki/84/Thermocouple_circuit.svg

MichaelG.

Indeed, it proves my point exactly.

Neil

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 19/06/2018 08:21:30:

Indeed, it proves my point exactly.

Neil

.

Then could you please explain your point [either here or privately], because we appear to be having different conversations.

MichaelG.

.

Edit: I am particularly keen to understand why you made this obvious but irrelevant statement:

"If you connect the two wires at both ends, you won't have any voltage to measure…"

Edited By Michael Gilligan on 19/06/2018 09:08:17

[Michael GilliganParticipant @michaelgilligan61133]

As we are now well into p3 and still struggling … I would just like to re-state the issue:

• The Schematic Fig.2 [not Fig.1] shows the Thermocouple correctly connected to terminals 9 & 10 of the PID Controller.
• The controller will treat those terminals as the reference junction[s], and will presumably have a local correction device to normalise it to a 'cold junction'. …. Please see the Wikipedia page for a picture of how this is done on a Fluke meter.
• Chris Gabel has, for convenience, added an extension lead of 'thin gauge stranded wire' and an XLR connector, which moves the actual cold reference junction 'outside' his ABS box.
• The temperature at the controller's 'cold junction' correction point is therefore not tied to that of the actual cold junction.
• If the extension lead had simply been made of thermocouple extension wire and a matched plug and socket, this invitation to error would not exist.

MichaelG.

.

https://en.wikipedia.org/wiki/Thermocouple#/media/File:Cold_Junction_Compensation_with_Thermistor_to_measure_the_junction_temperature..jpg

Edited By Michael Gilligan on 19/06/2018 09:47:53

[Michael GilliganParticipant @michaelgilligan61133]

Apologies if people are bored with this discussion:

This thread has just dropped off the first page, so I'm bumping it in the hope that Neil will see it again, and respond.

[ I simply don't understand your point, Neil; and that leaves me uncomfortable ]

MichaelG.

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 19/06/2018 08:45:52:

"If you connect the two wires at both ends, you won't have any voltage to measure…"

Reductio ad absurdum refutation of the idea that you have to use the thermocouple materials the whole way. The volatge has to be measured across the inputs of a high impedance amplifier which means the 'loop' has to involve non-thermocouple materials which will introduce errors in even the most perfect of setups.

The diagram you linked to makes it clear that what is usually lazily called the 'cold junction' is actually TWO cold junctions, at the points where the thermocouple materials transition to normal materials at the cold temperature.

In that case they are shown as being held at zero degrees by melting ice, but for ordinary purposes simply having the two junctions close to the (known) ambient temperature will be perfectly adequate.

Which is then point I have been making – the thermocouple wires only need to be consistent materials until they reach the 'cold junction' temperature and in the case of a furnace controller which is unlikely to use an ice bath the interposition of an XLR connector in unlikely to make any meaningful difference.

Sorry to labour the point, but to me it is very clear.

Neil

[Ian PParticipant @ianp]

Posted by Neil Wyatt on 20/06/2018 19:28:21:

Posted by Michael Gilligan on 19/06/2018 08:45:52:

"If you connect the two wires at both ends, you won't have any voltage to measure…"

Reductio ad absurdum refutation of the idea that you have to use the thermocouple materials the whole way. The volatge has to be measured across the inputs of a high impedance amplifier which means the 'loop' has to involve non-thermocouple materials which will introduce errors in even the most perfect of setups.

Neil

True you dont have to use thermocouple materials 'the whole way' but then the question is, what is the whole way, or in other words, where does the whole way end?

As I see it the thermocouple materials matter all the way to the terminal block because after that point the signal can be treated as pure voltage. Its a very low voltage so good circuit design has to be applied to ensure the accurate measurement signals of only a few millivolts. One significant thing about this final connector is that both its terminals are at identical temperature so any error introduced by the transition to copper is cancelled out.

I imagine that in the instrument engineering discipline the 'loop' only covers everything that connected to the input terminals of the controller or indicator, the internal circuitry is not part of the loop.

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 20/06/2018 19:28:21:

[ … ]

Which is then point I have been making – the thermocouple wires only need to be consistent materials until they reach the 'cold junction' temperature and in the case of a furnace controller which is unlikely to use an ice bath the interposition of an XLR connector in unlikely to make any meaningful difference.

Sorry to labour the point, but to me it is very clear.

.

Thanks for replying, Neil

I am now happy that the problem is as I suspected : We were having two different conversations!

The real objection to the XLR connector and its lead is as I [hopefully] clarified in my post of 09:45:02 yesterday. … It inevitably moves the cold junction to a point which is not monitored/normalised by the instrument.

MichaelG.

[Author]

Posts