Pitot anti icing
Don't you think that the problem might be both more subtle and more complex than just the amount of heat applied?
Why did it seem to be almost limited to one particular design of probe? (well by about 10:1 anyway)
What was peculiar about that probe? It must I think be critically dependent on the shape and finish of the probe internal geometry which is a closed book to all except the probe manufacturers.
It wasn't blockage of the drainhole by ice because the sensed pressure dropped to ambient static; so the blockage must have been upstream of that. Did the crystals melt and refreeze? Are the internal diameters critical? Or the radius at the corner of the 'L'? Or the actual location of the drainhole? Would the distribution of the heating along the length of the 'horizontal' bit of tube be important? When they went to the AA probe Thales also made some changes to the surface finish to improve corrosion resistance. Was this important?
I don't know - does anybody else?