Error in using the incorrect thermocouple wire?

Does anyone know how to estimate the amount of error that would be created when mismatching a thermocouple and it's extension wire? I've got numerous installations where we have the DCS configured for Type K, have Type J extension wire, and Type K thermocouples. We're looking at the cost to replace the extension wire and I need to estimate the amount of inaccuracy in the current setup.

Thanks,

Brad

Reply to
sss
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Yes. If you take wire a thermocouple up with incorrect extension wire, the error will be roughly proportional to the difference in temperature at the two points. For example, in the case of J extension wire used on K, the difference in uV/degree is about 20%. Suppose the first junction is in a head on the outside of a furnace running at

120°C and the other end is on a subpanel running at 23°C. The error will be roughly 20% of the difference, or 41°C. It will change as the temperatures change, so it's not just a simple calibration error. The sign of the error depends on the relative temperature at the two sets of extra junctions (if the DCS is one set of junctions then the terminal strip of that instrument). OTOH, swapping K and T is not a big deal unless you've got a really fussy application, but there are not many such lucky accidents.

Generally this is really bad business. It might be cheaper to put a transmitter at the far side and use the thermocouple wire for 4-20mA if you have a long run already installed and no intrinsic safety issues.

Best regards, Spehro Pefhany

Reply to
Spehro Pefhany

There are two sources of error. One is caused by a differnce in temperature between the two junctions formed by connecting your thermocouple to your extension wire. The second is caused by a difference in temperature between the two junctions formed by connecting your extension wire to your readout. Ideally if you could keep both junctions at your thermocouple end at the same temperature and both junctions at your readout end at the same temperatuere there would be no error. Note I am not saying the opposite ends of your extension wire must be at the same temperature. A differnce in temperature between the opposite ends of your extension wire does not cause a problem.

Howard.

sss wrote:

Reply to
hrh1818

argh... ^^^^^ make that 19°C

Best regards, Spehro Pefhany

Reply to
Spehro Pefhany

Because Spehro and I have two different opinions on the amount of error that is created when you use themocouple extension wire that does not match your thermocouple I suggest you take a look at the following Web Site.

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particular attention to the Law of Intermediate Metals.

Howard

sss wrote:

Reply to
hrh1818

Pefhany is about right. Note too that the error is positive, the J pair adds to the signal.

More important: his example of 120 deg C at say, the T/C head, is typical; being close to the hot process.

That point can be down at say 23 deg C (causing no error) at a cold startup. When it warms up to to the 120 deg C in his example, the error climbs to 19 deg C.

Unpredictable hour by hour.

Arthur Holland

Reply to
aholland51

Pefhany is about right. Note too that the error is positive, the J pair adds to the signal.

More important: his example of 120 deg C at say, the T/C head, is typical; being close to the hot process.

That point can be down at say 23 deg C (causing no error) at a cold startup. When it warms up to to the 120 deg C in his example, the error climbs to 19 deg C.

Unpredictable hour by hour.

Arthur Holland

Reply to
aholland51

One work around, if the extension leads are in multi pair bundles, is to use one alloy from 2 pairs for one TC and the other alloy for a nearby TC. Ugly, but possibly worth it, to save pulling all that new wire, right away.

Reply to
John Popelish

If we recomend any of the workarounds, we destroy our reputations as competant practitioners of this trade.

Either: change the extension wire.

Or :change the T/Cs and rerange the instrument to match

Arthur Holland

Reply to
aholland51

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