How should thermal inertia be specified for industrial heating elements?

Specify thermal inertia from the real duty rather than a catalogue figure. Under agreed operating conditions, record heat-up, power-off and cool-down curves together with the furnace lining, sheath and workload, then relate them to wattage, diameter, material, surface load, protection, mounting and the control loop. Where fast setpoint changes matter, consider zoned elements or staged power switching, and account for thermocouple position and response lag. Verify the agreed response on site, because inertia depends on furnace type, load and insulation and cannot be transferred between installations.