|replacing a bead-style thermo-couple on a paragon sc2||or look at larger kilns at paragonkilns.co.uk|
The oldest thermocouple is a bead-style PY70 K-type: it looks like two wires joined by a dot-weld. The wires pass through three ceramic sleeves and are long enough to reach the programmer board. The exposed bead can fail eventually because of everyday oxidation, corrosive gases, or prolonged high-temperature firing.
This was replaced by a stainless-steel sheathed-style PY54 K-type mounted on a ceramic back-plate held in place by two screws. The sheathed bead can fail eventually because of heat degradation, corrosive gases, or epoxy breakdown inside the sheath.
The new thermocouple is a heavy-guage welded PY84 K-type mounted on a ceramic back-plate held in place by two screws. It's a long-life design that responds more quickly to temperature changes and minimises the risk of corrosion and subsequent failure of the bi-metallic tip.
It's a universal design so it fits all the Paragon kilns. If the one you're replacing is shorter, carefully break off the last ceramic sleeve with some pliers. Partly straighten the wires, without breaking another sleeve, and fit the thermocouple. The wires will now be too long so nip off the excess so they don't touch the back of the kiln when it's all re-assembled. Don't do this before you fit it or you won't know which is the red connection.
To replace the newer styles, the ceramic back plate, inside the kiln, needs to be removed. It very rarely needs replacing, so a new thermocouple doesn't need a new back plate. When refitting the back plate, take great care not to over-tighten the retaining screws as the ceramic might crack.
The two wires, red and yellow, fit into two colour-coded push-down connectors on the programmer board. Make sure that the wires are gripped well, don't touch each other, and can't touch the metal case.
The K-type thermocouple, used in most of the Paragon kilns, has a nickel-chromium and nickel-aluminium bead. As an example, this pairing generates 12.2mV at 300°C.
If accuracy, long-life, reliability, anmd stability are vital, especially at temperatures above 1100°C, you can upgrade to an S-type platinum-rhodium thermocouple. This has a porcelain tube.