How should insulation aging be specified for industrial heating elements?

Judge insulation aging by measured heat loss, not by heater appearance. Log the empty-furnace heat-up curve, time to setpoint and holding power against baseline data; a clear rise in loss points to degraded insulation. Before replacing elements, check shell temperature and surface heat flux to separate insulation loss from element degradation. If compensation is needed, size the extra wattage from the time gap and recheck diameter, sheath material, surface load, protection class and mounting fit. Validate the new rating by control deviation and over-temperature tests, and file the heat-up data with acceptance.