Re: Copper-constantan thermocouple.

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From	"Carlos E.R." <robin_listas@es.invalid>
Newsgroups	sci.electronics.repair
Subject	Re: Copper-constantan thermocouple.
Date	Thu, 5 Mar 2026 10:51:39 +0100
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On 2026-03-04 21:59, peter@easthope.ca wrote:
> Anyone worked with copper-constantan thermouples?  How are the two
> wires joined?

Good thermocouples use special cables that have wires made of copper and 
constantan, to avoid having extra joints in the way to the electronics. 
You don't need to weld them, even twisting together the wires of the 
cable creates a sensor, and it works.

> 
> A dry bath heats a block of aluminum which has holes to receive test
> tubes.  Temperature is controlled by a Cal Controls # 3200.  The
> heating pad heats when power is applied but the relay in the 3200
> never switches.  So I wonder whether the fault is in the 3200 or the
> thermocouple.
> 
> Can heat the thermocouple and measure voltage across the two wires.
> https://its90.nist.gov/downloadFiles/type_t.tab.txt
> Not sure a common multimeter has the precision to detect a faulty
> thermocouple.

Some multimeters have a scale to measure temps with a thermocouple, and 
the proper socket.

Notice that the setup actually has two joints, one at the sensor and 
another (same loop) at the electronics board. The thing measures 
temperature differences between the two joints, so there is another type 
of sensor (ex.: thermistor) near the second joint. Think that the 
constantan wire has to join the electronic board metals at some point.

> 
> Ideas?

My experience (old) is with sensors and measurements. The setup used in 
heaters is different.

Sometimes we had failures and the answer was to replace sensor or 
cables. We did not study why they failed. We just tried a spare sensor, 
and if it worked, problem solved. We had a bunch of them. Once we had 
the vendor give us a lecture :-)

> 
> Thx,                              ... P.
> 
> 
> 

-- 
Cheers, Carlos.
ES🇪🇸, EU🇪🇺;

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Thread

Copper-constantan thermocouple. peter@easthope.ca - 2026-03-04 13:59 -0700
  Re: Copper-constantan thermocouple. Marte Schwarz <marte.schwarz@gmx.de> - 2026-03-05 09:49 +0100
  Re: Copper-constantan thermocouple. "Carlos E.R." <robin_listas@es.invalid> - 2026-03-05 10:51 +0100
    Re (2): Copper-constantan thermocouple. peter@easthope.ca - 2026-03-06 11:09 -0700

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