Physical examination and appearance: Use a magnifying glass to examine the solder joint. A cold solder joint will exhibit a dark grey ring-shaped black circle, tiny cracks, and insufficient solder wetting of the thermocouple wire surface, whereas a qualifying solder joint is smooth and full.
The thermocouple wire should be gently shaken close to the solder connection. A qualified solder joint won't feel loose at all, but a cold solder joint will feel visibly loose.
Testing for Electrical Performance: To measure the thermocouple circuit, use a multimeter set to continuity/resistance mode. In comparison to a qualified thermocouple of the same standard, a cold solder junction will exhibit high impedance, with a resistance value significantly higher. It may even exhibit sporadic open circuits.
Gently tap the area close to the solder joint while it is powered on. A qualifying solder junction will have an entirely steady signal, but a cold solder joint will have a surge in thermoelectric potential and brief variations in the temperature display value.
Verification of the Hot State and Operating Conditions: Use a hot air gun or soldering iron to warm the solder junction locally. It can be identified as a cold solder junction if the temperature signal abruptly returns to normal after heating but then drifts once further after cooling.
A poorly soldered thermocouple's temperature display will exhibit erratic leaps and not rise consistently with the hot runner's actual temperature during the heating operation.
