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Interference on thermocouples

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Speedy

Mechanical
Jun 5, 2001
229
Folks,

I have a problem with interference on thermocouples.
It goes like this;

We are using K-type thermocouples to measure temp in a block of aluminium. There are a number of single phase cartridge resistance heaters (230v AC with heaters on different phases of 3), a number of solenoid operated water valves (24V DC) and water-cooling channels in the near vicinity of the thermocouple. To make matters worse there are high tension electric mains running directly over the factory roof.

The thermocouple are duplex, one side goes to a CAL temperature controller and the other to a PLC card. The screened extension cables are maybe 2~3metres with both round pin and flat pin connectors along the extension cables. All cables and connectors are K-type.

There are a number of identical machines side by side and on the same metal frame, but there is only the interference with the 1 machine. We replaced all the components with ‘good’ parts from the other machines but the error is still there.

I did a search on this site and found the following helpful thread.
Thread248-91067
Thread239-75319
Thread698-75509
Thread340-75326
Thread240-75320

When we move the round pin plug and socket around the vicinity of the thermocouple the error comes and goes depending on location. I twisted the exposed (unscreened) 1” of cable at the connector and this did correct the Controller reading side but the PLC still reads 60C under the actual temp.

I would appreciate any comments on this as we (including the people that built the machine) have already spent a few days on this.

Q’s as follows;
I gather from the above threads that I should ground all the screening on the cables. It this is the case why don’t the connectors facilitate the connection of the screens?
If there is a problem with electromagnetic fields, why doesn’t it cause problems on the other machines nearby?
If it is the overhead high tension cables, could it be possible to get nodes or localised areas of high field activity?

Speedy
[sadeyes][sadeyes]
 
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This may be silly stuff you’ve ruled out long ago, but is the connector contact resistance and mechanical integrity in good shape? Any chance that the “K” bi-metal connector and associated lead wires are reversed on both sides at one junction/connector?

For this type of instrumentation, front-end signal conditioning typically is designed with relatively long time constants, creating inherent fast-transient noise immunity. [Say—“bandwidth” below a Hertz, for a faster response to temperature changes/electrical levels is not often needed.]

Any chance of common-mode noise from a ground loop? That is, normally floating thermocouple leads {out on the ‘factory floor’ or ‘cold cruel world’} external to the signal-processing electronics, with only one intentional ground reference in the signal conditioning? This would best be confidently determined with ground-continuity tests in the {temporarily isolated} thermocouple loop.
 
Busbar,

Thanks for your help.

The problem was with the batch of thermocouples. These were duplex K-type but with only one thermocouple junction for the 4 leads.
These were ordered with 2 junctions. We believe the difference in voltage between the PLC and Temperature Controller caused the noise.

Annoying but at least we got to the bottom of it.

Speedy !!!!!!



 
I was wondering what was a "duplex thermocouple". Thanks for your description. For my education can anyone tell what can be the use of a thermocouple with one junction and four leads? (Given the low millivolts output of this device?)
Felix
 
Speedy, you are welcoming ANY comment? Aren't you?

One comment that I think is valid in this case - and most cases is this: Do not blame overhead high voltage power lines when you have interference in industrial electronics.

They are often the first to be blamed and I have yet to see any case where an overhead high voltage line actually has influenced any industrial or indoor laboratory electronic equipment.

Does anyone have a well documented case here?

(I exclude radio reception problems. Corona is known to emit RF that can disturb weak AM stations)
 
See my answer in the follwoing thread:
thread240-75320

We were able to use thermocouples in induction billet heaters up to several MW that way (directly on the billet!)
 
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