PID Controller – MEW 269 – wrong connector

[Ian PParticipant @ianp]

If one is not to fussy about accuracy then using any old connector for thermocouples is not going to be a problem although using a bulkhead connector is definitely not a good idea as the internal and external parts of the connection will probably be at different temperatures.

Mention was made earlier in this thread about controlling a heater at 100 degrees C, the PID controller will have no problem but if accuracy is required then a thermocouple (whatever type it is) is a poor choice of sensor. A PRT (Platinum Resistance Thermometer) is a much more suitable and accurate device and is not fussy about connectors.

I suppose this whole thread revolves around what is 'fit for purpose'. If the authors project did what he wanted then that is what matters. If Neil wants to use XLR's because they are robust and latching thats fine, personally I put electrical integrity first.

Ian P

[Anonymous]

Posted by Nige on 16/06/2018 16:53:15:

How many of us go to the junk box first to find components we need.

Not me! I don't do much electronics at home now; having been a professional electronics engineer for 40 odd years I've got a bit bored with it. So I've binned or flogged most of my components. As and when I design a circuit I just order what I need from Farnell and it's here the next day. There's no point in wasting time hunting through boxes of components for something you may, or may not, have. And for SM components it's darn near impossible to tell what the value is anyway, without measuring it. Much simpler to buy new.

Mind you I have kept the better quality components such as Jackson Bros variable capacitors and Denco radio coils.

Andrew

[Anonymous]

Posted by Ian P on 16/06/2018 20:54:32:

If Neil wants to use XLR's because they are robust and latching thats fine, personally I put electrical integrity first.

If one really wants robust and latching connectors one could try Lemo. We used them extensively on high end racing cars, mostly Indy cars and a bit of F1. Mind you, if you have to ask the price you can't afford them!

Andrew

[not done it yetParticipant @notdoneityet]

Posted by Ian P on 16/06/2018 20:54:32:

… Mention was made earlier in this thread about controlling a heater at 100 degrees C, the PID controller will have no problem but if accuracy is required then a thermocouple (whatever type it is) is a poor choice of sensor. A PRT (Platinum Resistance Thermometer) is a much more suitable and accurate device and is not fussy about connectors….

Ian P

A bath of boiling water would suffice for maintaining 100 degrees C (373K)!

[Ian PParticipant @ianp]

Posted by not done it yet on 16/06/2018 23:20:27:

Posted by Ian P on 16/06/2018 20:54:32:

… Mention was made earlier in this thread about controlling a heater at 100 degrees C, the PID controller will have no problem but if accuracy is required then a thermocouple (whatever type it is) is a poor choice of sensor. A PRT (Platinum Resistance Thermometer) is a much more suitable and accurate device and is not fussy about connectors….

Ian P

A bath of boiling water would suffice for maintaining 100 degrees C (373K)!

My 100 degrees C was a typo, I meant to put 100.5 degrees C for which boiling water is unsuitable!

Many of the controller I installed were for cooling/refrigeration systems for which thermocouples just do not cut it.

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 16/06/2018 23:20:27:

A bath of boiling water would suffice for maintaining 100 degrees C (373K)!

.

^^^ Assuming that just the right barometric pressure prevails.

MichaelG.

[richardandtracyParticipant @richardandtracy]

After reading this thread, it seems less improbable that people died in the middle ages during arguments over whether it was 6 or 7 angels that could dance on a pinhead.

This really is like that argument, so abstruse as to be arguing for the sake of it. Just as pointless as the argument over some extra zeros appearing in the same article in MEW 269. Life happens. Just live with it. Or don't. Your choice, but is it really important enough to go round and round in circles about this sort of thing? There's a limit to how much pedantry is amusing and fun to read. I read most posts on this forum for the gems people come up with – and there are quite a few, but dilution of the gems still has to be waded through to find the gems., and, I dunno, this weekend has been a bad one for dilution.

Regards

Richard.

Edited By richardandtracy on 17/06/2018 21:19:57

[not done it yetParticipant @notdoneityet]

Posted by Ian P on 17/06/2018 09:05:46

…

My 100 degrees C was a typo, I meant to put 100.5 degrees C for which boiling water is unsuitable!

….

Ian P

Add the appropriate amount of common salt, or any other salt for that matter? As long as the bath were maintained at constant volume…. 58g of salt per litre of water would do fine.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by richardandtracy on 17/06/2018 21:14:14:

After reading this thread, it seems less improbable that people died in the middle ages during arguments over whether it was 6 or 7 angels that could dance on a pinhead.

This really is like that argument, so abstruse as to be arguing for the sake of it.

[ … ]

.

Richard,

If you really find this discussion so tedious, you might like to spend a few minutes reading this note by Prof. Peter Harrison instead: **LINK**

http://nq.oxfordjournals.org/content/63/1/45.full.pdf+html

It's one of several items linked from the Wikipedia page: **LINK**

https://en.wikipedia.org/wiki/How_many_angels_can_dance_on_the_head_of_a_pin%3F

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

Posted by richardandtracy on 17/06/2018 21:14:14:

…

whether it was 6 or 7 angels that could dance on a pinhead.

…

Your figures are seriously adrift Richard! Modern quantum analysis suggests an upper bound of 8.6766×10⁴⁹ angels per pinhead.

[Anonymous]

Posted by richardandtracy on 17/06/2018 21:14:14:

This really is like that argument, so abstruse as to be arguing for the sake of it. Just as pointless as the argument over some extra zeros appearing in the same article in MEW 269. Life happens. Just live with it. Or don't. Your choice, but is it really important enough to go round and round in circles about this sort of thing?

Well maybe, but professional engineering is all about attention to detail, and an extra zero can have unexpected consequences! While MEW isn't for professionals there's no reason why it shouldn't strive to get things right.

Andrew

[JasonBModerator @jasonb]

Can I ask a question, you will have to be gentle with me as I only did GCSE Electronics at school.

If it is so important to have the right connector materials so like materials come into contact at a joint then what happens when you come to connect the connector? Assuming it is soldered to a circuit board or uses screw terminals then they would not match and again if type of wire is critical what happens when the signal has to flow around the circuit board which would not be the same as the thermocouple wire?

[Michael GilliganParticipant @michaelgilligan61133]

That's an excellent question, Jason

The full answer really depends on how [and how well] the measuring instrument is designed, but; as I mentioned on p1:

"… the connector block will become the cold reference junction …"

If the instrument is designed accordingly, that's fine.

MichaelG.

.

Edited to put the second 'n' in connector 

Edited By Michael Gilligan on 18/06/2018 13:38:16

[Anonymous]

Posted by JasonB on 18/06/2018 13:22:10:

………..you will have to be gentle with me as I only did GCSE Electronics at school.

That's more than I did at school.

Andrew

[John RuddParticipant @johnrudd16576]

To answer Jasons's question,

Assuming the thermocouple is connected to some measuring/indicating/controlling device, it is calibrated using a millivolt source. Simply inject the corresponding mV that equate to a given temperature and adjust the electronics to ensure the device indicates that….Usual adjustments are Zero and Span……I have encountered devices that have a linearity adjustment too. The Zero adjustment doesnt necessarily correspond to the literal value 0, it may be the measuring/indicating device could measure between a negative and a positive temperature….

Of course, the mV ipnut signal injected is type dependant, this is where reference tables are used for alternative types of tc…and of course taking into account cold junction compensation…..in fact some of the more sophisticated test equipment has cjc built in…..

 

 

 

Edited By John Rudd on 18/06/2018 15:29:50

[JasonBModerator @jasonb]

So kind of comes back to what I said that the controller could be programmed to compensate for any error caused by the connector and that would also be what Michael said about correctly designing the instrument to allow for how it was connected.

Andrew, I seem to remember that there was no O Level Electronics offered by any of the boards so that is no doubt why us comprehensive kids got to take the GCSE

[Ian PParticipant @ianp]

It would be a clever controller that could compensate for errors it did not know about

If the error is created by the use of incorrect connector/s then the error will vary with the temperature gradient across those connectors, which are probably going to be at different temperature to the one doing the measuring.

Not particularly relevant to this particular thread, but an important point to bear in mind when controlling of temperature (with whatever sensor) is that the only place in the whole system where the temperature is actually accurately known, is at the actual sensor itself.

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 18/06/2018 16:48:56:

… and that would also be what Michael said about correctly designing the instrument to allow for how it was connected.

.

In the real world; I think it would be safer to turn that statement around, and say that the instrument needs to be connected in the way it was designed to be.

MichaelG.

[DrDaveParticipant @drdave]

It has been too many decades since I last used a thermocouple for me to remember how we wired them up. So I had to go back to basics (Wikipedia) to find out. What seems to have been missed in much of the above is that a thermocouple gives the temperature difference between its hot and cold junctions, not the hot temperature. The cold junction is typically within the instrumentation: the instrument measures the cold junction temperature by a different means (cold junction compensation) and uses this, with the thermocouple voltage, to calculate temperature. Hence the need to use “thermocouple material” all the way to the cold junction. After this, copper is fine.

Room for one more angel on this thermocouple?

[Michael GilliganParticipant @michaelgilligan61133]

Good to have your input, Dr Dave … But I like to think I made that reasonably clear in my reply to XD 351, on p1.

MichaelG.

[DrDaveParticipant @drdave]

Posted by Michael Gilligan on 18/06/2018 18:55:31:

Good to have your input, Dr Dave … But I like to think I made that reasonably clear in my reply to XD 351, on p1.

MichaelG.

True, but I didn’t recall anyone pointing out the importance of measuring the cold junction temperature. Too many post to remember the details?

[John RuddParticipant @johnrudd16576]

Posted by DrDave on 18/06/2018 19:07:25:

Posted by Michael Gilligan on 18/06/2018 18:55:31:

Good to have your input, Dr Dave … But I like to think I made that reasonably clear in my reply to XD 351, on p1.

MichaelG.

True, but I didn’t recall anyone pointing out the importance of measuring the cold junction temperature. Too many post to remember the details?

Not so much as measuring cj temp per se, but I did mention cjc when calibrating, some more advanced tc simulators have this incorporated saves having to take it into account when inputting the relevant millivolt input to the 'kit' being calibrated……Although we used to have a seperate thermometer measuring ambient temp when we calibrated transmitters/trip amps etc….

[niet lullenParticipant @nietlullen87612]

Hi, I would say that all temperatures in the article are wrong . 12600 Celcius, seriously ?

[Neil WyattModerator @neilwyatt]

Posted by DrDave on 18/06/2018 18:48:38:

. Hence the need to use “thermocouple material” all the way to the cold junction. After this, copper is fine

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…

The 'cold junction' is always going to include the measuring circuit which is probably going to include copper, tin and lead if old style solder), doped silicon and who knows what else.

This is from Maxim Integrated:

Somehow the makeup of the special connector is critical, but you can use a copper wires to take it to your carefully crafted differential amp!

What's magical about calling some point INSIDE the special connector the cold junction, instead of OUTSIDE it?

Lot's of smoke and mirrors around these precious connectors! As long as your connections are symmetrical in terms of materials and their temperatures are close (I'd love to see a 10 degree gradient sustained across an XLR connector that's carrying less than a milliamp of current…)

Neil

[DrDaveParticipant @drdave]

There is nothing magic about the cold junction or where it is physically located: it is at the transition from “thermocouple material” to copper. So long as you measure the temperature of the cold junction, all is well. The cold junction is inside something like a handheld instrument so that cold junction temperature measurement can be integrated in the instrument. I suspect for Model Engineering projects, changing to copper in a Molex connector and feeding into the instrumentation via XLR plug will give acceptable results, even using ambient temperature as the cold junction temperature. The Wikipedia entry on thermocouples (or that referenced byMichael Gillian on p1) gives a good overview.

Ducks and awaits incoming criticism!

[Author]

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