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I was looking to purchase a K-type thermocouple and noticed something peculiar. Wikipedia says it can withstand temperatures up to +1350 °C, but Amazon sells some K-type thermocouples that only go up to 200 °C, 250 °C, or 400 °C. What going on here?

What would happen if I tried to measure 400 °C with one that is advertised to only go up to 200 °C? Is it the insulation? Could I replace the insulation to get a wider range? Is it the build quality?

For example, what's the difference between:

and

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    \$\begingroup\$ Other materials in the thermocouple probe will fail before the thermocouple itself. If you're looking for serious thermocouples, look at Omega Engineering. \$\endgroup\$ Jul 3, 2013 at 3:12
  • \$\begingroup\$ The highest temperature thermocouples I have seen have no wire insulation - just ceramic spacers to keep the conductors separated. \$\endgroup\$
    – W5VO
    Jul 3, 2013 at 4:33

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Thermocouples create a measurable voltage difference given a thermal energy difference*. This is the Seebeck effect. A "K-Type" thermocouple uses a particular alloy combination which is, as the Wikipedia article states, a general purpose design that varies widely because technology has advanced since its inception.

Specifically:

"...a wide variety of probes are available in its −200 °C to +1350 °C ... range."

(emphasis mine)

Therefore, K-type alloys can measure up to 1350°C but not all products are designed with this upper limit in mind.

As Nick pointed out in a comment, you can find thermocouples designed for high temperature. For example, the Super OMEGACLAD® K-type have nearly such tolerances.

What makes this different from what you found on Amazon is the build quality and materials for the probe sheathing (high temperature probes use mineral insulation), conductors (not the chromel/alumel alloy, but the electrical connections to it), and probe length.


Consider an analogy. Let's say you found a very basic definition of automobile:

"Automobiles are transportation machines which can travel up to 429 km/h."

While this may be true, an inexpensive Honda Civic tops out at 209 km/h (ref). You'd need a Bugatti Veyron Super Sport to reach that upper limit (ref). What's the difference between a Honda Civic and a Bugatti Veyron Super Sport (other than about 2.4 million US dollars)? They both use combustion engine technology, but with radically different quality components.

* EEVBlog has a basic video tutorial on how thermocouples work.

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