Thermocouple to measure the temperature of solder iron tips

When soldering electronics the temperature of the solder iron tip is very important. It is also not easy to measure with a K-type thermocouple, since the K-type wires do not wet out with liquid solder, but rejects it and does not make a good contact with the liquid metal. The standard to measure the tips of solder irons is the Hakko FG-100B tester. But it has s pretty large thermal mass and 3 wires that lead off the heat from the tip. In my eyes not ideal.

For a long time I wanted to make a simple wire thermocouple that could be used with solder irons and that would work with common multimeters that have a thermocouple setting.

The idea was to overcome the solder rejection of the thermocouple wire by wrapping it with a little copper wire.  Many people believe that a 3rd metal at the thermocouple tip would introduce an error, but that is not true. The electrical connection between the two thermocouple leads can be made by any metal, in stainless steel probes this connection is actually the stainless steel cover. 

So when I received a call from a customer this morning, I finally set this into practice. I used a very thin AWG 30 thermocouple with PFA insulation and wrapped the tip 10 times with thin AWG35 copper wire. This thin thermocouple will limit the additional thermal mass and the heat loss through the wire.

I set the solder iron to 330 C and applied a bit of fresh rosin core solder, it did wet out the copper perfectly

I then plugged the thermocouple into the thermometer and held the probe to the tip, it read about 327 deg Celsius

I then took the probe with the copper and the solder at the tip and inserted it into my precision calibration block, which was set to 250.0 degree C.  Even with the additional metals at the tip, the meter read 251.7 C, which is inside the thermocouple specification of +/- 2.2 C. This proves that the readings at the solder iron tip will be accurate with this method.

I will probably launch this as a product shortly, including the adapter that will enable the probe with many multimeters.

29th Jan 2024 Carsten Franke

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