From bigclivedotcom.

While I knew that these common soldering iron bits combined heating and thermal sensing onto two connections, I wasn’t sure of the internal construction – so I dremeled one open so we can look inside.

The primary advantage of these soldering iron bits is that the energy is converted to heat and also monitored accurately in the tip of the bit. So they have a fast recovery time when soldering.

I think the T12, T65 and T75 elements are all made in a similar way.

With a type N or type K thermocouple the voltage generated at a typical soldering temperature of 350C will be 10 to 15mV. However, when I applied a soldering iron to the thermocouple and measured the voltage it was only around 5mV. A search online found an article where someone had tested one in the same way, and had deduced that it is probably a Type-C thermocouple. Probably chosen to match a suitable heating element wire.

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