We usually have a huge corrosion in the wall on the top section of main tower, above LGOk. Is there any technique that is possible to measure the wall thickness of the main tower running ? In our case we stopped the tower, cooled down and measured the wall thickness by ultrasom. We also we are using the technique Pulsed eddy current testing with the main tower in operation. Are this technique reliable to measure the wall thickness of the main tower in operation ? It seems that the corrosion on the tower wall is from HCl formed on the top . How do we detect this before the tower damage occurred ?