Error in using the incorrect thermocouple wire?

On Sat, 21 Jan 2006 17:57:33 -0500, the renowned sss <> wrote:


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
--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

On Sat, 21 Jan 2006 18:57:21 -0500, Spehro Pefhany


extension

argh...                                    ^^^^^ make that 19°C



Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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:

extension

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.
http://www.wici.com/technical_info/articles/temp_prm/tmprmch1.htm
Pay particular attention to the Law  of Intermediate Metals.

Howard


sss wrote:

extension

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

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

aholland51@cogeco.ca wrote:

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.

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