When making the Measuring (hot) junction of MI Thermocouples and RTD ’s it is advisable to keep the open end of the cable warm.
This avoids the slow ingress of moisture into the cable insulation, which would lower the Insulation Resistance. This parameter is important for high measurement integrity.
Keeping the cable warm also has a beneficial effect on the welding process.
- Moisture in the insulation lowers both the resistance and the capacitance. If these two parameters become too low the high frequency component used to start the weld becomes corrupted. In extreme cases the spark will travel across the surface of the insulation and create weld spots and melting of the cable sheath. This is more prevalent with smaller cables which, although they have the same capacitance as their larger family members have increased voltage stress across the powder surface.
- When making a sheath closure weld any moisture inside the cable begins to expand as the cable heats up. This affects the weld as the water vapour mixes in with the weld gas restricting the ionising process. In the extreme the expanding moisture creates a pressure differential within the weld causing the molten metal to rise up. (blown weld)
- For both conductor welds and sheath closure welds the starting process is improved if the cable sheath or conductors are hot. This seems to affect the initial breakdown of the argon gas allowing a smoother more rapid start and less spluttering. Most MI Thermocouple manufacturers properly store their cable prior to welding in large ovens. However when it comes to a warming facility close to the welding machine, there are no proprietary ovens on the market. Subsequently users have adopted a variety of ingenious and novel approaches, from hair dryers to domestic plate warmers.
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