The essential
Standard rosin flux is not effective on iron and nickel alloys. Remove the oxide mechanically, tin the wire with Rectorseal Aqua Flux, then solder it to the terminal. Clean the flux off and measure the insulation resistance between the leads and the slip ring housing: several hundred megohms at least, above 2 000 megohms expected.
Which wires need a special flux?
The iron, Chromel and Alumel legs need it; copper and Constantan solder with rosin flux.
- Type J: the iron leg (white) needs it; the Constantan leg (red) solders with rosin flux.
- Type K: both legs need it, Chromel (yellow) and Alumel (red).
- Type T: neither leg; copper (blue) and Constantan (red) solder with rosin flux.
- Type E: the Chromel leg (violet) needs it; the Constantan leg (red) does not.
- To identify a wire: iron and Alumel are magnetic; Constantan is silver colored.
How is the wire prepared?
Use new thermocouple wire, 24 AWG or smaller, and remove the oxide mechanically. Wire several years old can carry an oxide thick enough to make soldering almost impossible.
- Cut the insulation and scrape the oxide off with a razor blade, sandpaper or a magnet wire stripper with two rotating abrasive fiberglass wheels.
- Do not use thermal wire strippers: they can create an oxide layer.
- Type K is especially difficult to tin unless the oxide is removed mechanically first.
- For stranded wire, use grit blasting.
How is it tinned and soldered?
Tin the wire with Rectorseal Aqua Flux first, then solder it to the terminal. Michigan Scientific uses this flux for iron, Chromel and Alumel.
- Stir the flux, dip the end of the wire in it and tin the wire with the soldering iron, at 260 to 315 °C (500 to 600 °F).
- Michigan Scientific tins with Kester SN96.5 AG03 CU0.5 solder, 3.3 % #48 rosin core; the rosin core does not seem to interfere.
- Repeat the dip and tin three or four times if the plating is incomplete.
- Once tinned, wrap the wire around the solder terminal and solder it with Aqua Flux or with rosin flux.
How is the joint cleaned and checked?
Brush Aqua Flux off with water, and rosin flux off with a rosin flux remover (50 % alcohol, 50 % toluene). Then measure the insulation.
- Between the leads and the slip ring housing: several hundred megohms at least, above 2 000 megohms expected.
- The thermocouples must also be insulated from each other.
- Kester 2331ZX water soluble organic flux can replace Aqua Flux but is more corrosive: wash it off with water soon after soldering, never leave it overnight.
- Zinc chloride fluxes are quite corrosive: neutralize the joint by brushing with ammonia, then water.
The technical note
To remember
- Aqua Flux for iron, Chromel and Alumel; rosin flux for copper and Constantan.
- Use new wire and remove the oxide mechanically, never with a thermal stripper.
- Clean the flux, then check the insulation to the housing: above 2 000 megohms expected.