From Newsgroup: sci.electronics.repair

Anyone worked with copper-constantan thermouples? How are the two
wires joined?

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.

Ideas?

Thx, ... P.



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From Newsgroup: sci.electronics.repair

Hi Peter,

Anyone worked with copper-constantan thermouples? How are the two
wires joined?

understandung the Seebeckeffect helps to understand that the junction
only have to be electric conductive. all other doesn't matter.

Marte
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From Newsgroup: sci.electronics.repair

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.
ESEfc-Efc+, EUEfc-Efc|;
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From Newsgroup: sci.electronics.repair

Carlos & all,

In article <ba0o7mxlse.ln2@Telcontar.valinor>, "Carlos E.R." <robin_listas@es.invalid> wrote:

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

Seen in marketing publications but don't have one.

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.

In the setup here, the second connection would be inside the Cal
Controls 3200 which I haven't opened. Wikipedia refers to the
"reference junction". https://en.wikipedia.org/wiki/Thermocouple#Reference_junction

A salvaged 3200 isn't expensive. I think of buying one, comparing observations and re-evaluating.

Thanks, ... P.

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