OT: Cable increases resistance over time... (Mystery)

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Mar 2008
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Vanderbijlpark
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Slightly off topic. I'm just curious as to whether anyone else ever encountered this. Maybe one of you wizards have an explanation...

I was recently called to a fairly simple job on one of our compressors. The second stage suction thermocouple (type T) was reading O/C. I started checking the loop, and after a little fault-finding, saw the five meter run of cable to the thermocouple was reading 9 ohms. Thinking there might be damage to the cable, since this seemed too much for such a short run, I had the cable replaced. Thermocouple read again, and problem solved. A short while later the same couple started giving trouble again. Turns out that the terminal was slightly loose on the termination card and it would occasionally go O/C for a short period. There never was a problem with the cable, for when I measured another section, it turned out that that type of cable had a high resistance per meter (Silly mistake, I know). But here comes the mystery...

Upon measuring the same piece of cable I'd installed three months earlier, I saw the resistance over the same length had changed from 9 Ohms, to 21 Ohms. There was no damage to the cable, and it was not exposed to any extreme heat or cold which might effect it in any manner. Just an average piece of cable on a cable tray.

Anyone have an explanation why the resistance would change over time?
 
It's getting wet, and the copper is turning green. Change from cloth insulated to PVC or Teflon. Coat exposed areas with Silicon grease to keep out water and oxygen.
From Wiki:
Type T (copperconstantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes. As both conductors are non-magnetic, type T thermocouples are a popular choice for applications such as electrical generators which contain strong magnetic fields. Type T thermocouples have a sensitivity of about 43 µV/°C.
 
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I don't know how you are measuring the cable, but I usually find that a slight coating of corrosion or contaminants can cause widley varying resistances, especially if you are measuring with a multimeter and at the lower resistance levels.
 
The measurement was made by joining one end with a termination block and measuring ohms from the open end. There wasn't any corrosion though. If there was a Copper Oxide build-up, obviously it would have explained it, but there was no corrosion on the cable at all. It was PVC sheathed and wasn't anywhere near a water source. All connections were sealed off in junction and pratley boxes. This was what confused me.

Oh yeah, it was type-T compensating cable, approximately 5 meters in length.
 
Did you try to reverse the leads and see if the number changes... It may be possible, that the millivolt signal from the thermocouple is throwing off your meter.
 
Oh yeah, it was type-T compensating cable, approximately 5 meters in length

By joining the ends of your type T cable (Cu/CuNi) together you have created a thermocouple. If there is a difference in temperature between this and your meter then you will get a mV signal that will affect your meter reading.
 
I agree with John W - the measurement of the resistance of thermocouple lead wire with a VOM device that provides a current/voltage excitation (a resistance measurement of an VOM) to the device is not really valid unless the lead wires are disconnected from the thermocouple itself. The reason is that any thermal gradient across the T/C from the 'hot' end to the cold end will generate an emf that will affect the device making the resistance measurement. So resistance readings can be affected by the T/C mV signal

If both ends of the thermocouple are at the same temperature, so that it is 'isothermal' then a resistance measurement will show the resistance of the junction. Chemical pollution of a thermocouple junction due to ion exchange from surrounding metals at elevated temperatures increases the resistance of the thermocouple junction. The effect is a Seebeck voltage output that does not follow the standard thermocouple tables. This is commonly called 'drift' where the T/C output does not correspond to the junction temperature.

This junction resistance change is used by at least one vendor for extreme accuracy measurement, in order to alarm on 'drift'. Another vendor calculates junction resistance using the T/C break current.

So you could be seeing a deteriorating thermocouple junction.

Dan
 
Originally posted by danw:

Chemical pollution of a thermocouple junction due to ion exchange from surrounding metals at elevated temperatures increases the resistance of the thermocouple junction.

So how 'elevated' do your temperatures have to be before this becomes a real concern? That sounds like a pretty insidious issue (spoken by a guy who doesn't use thermocouples much).

Keith
 
I don't know if there's a critical temperature .

People I've talked to who acknowledge problems with thermocouple drift tend to be those working in excess of 1200°F.

The stock reply from the vendors is that the higher the temperature, the higher the ion migration activity and the faster thermocouples tend to 'drift'. It can be insidious, because the input sees' a mV value, and the input can't tell whether the mV value is good or bad. The only standard protection is detection for an open circuit, but the graphs the one vendor showed me show that serious drift can occur for days/weeks before the T/C breaks.

The resistance level where drift is probable/likely is on the order of 80 ohms for the junction itself (not counting the lead wire resistance).

I personally doubt the validity of a thermal oxidizer owner who hasn't changed out his thermocouple in 8 years with combustion temps >1800°F. But if the EPA doesn't care, he doesn't.

Dan
 
Don't expect an accuracy of better that 3 deg C (5.4 deg F) from a thermocuple and if they are an important part of your process control then either get them calibrted annually or replace them annually depending on which is the most cost effective. There is a load of good information on the web about thermocouples. And the NPL have good stuff on thermocouple calibration. If your process is in the range that you can use PT100 sensors then consider if you can use these instead.
 

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