Our publication by Kim et al.  is not purposed to criticize others but we try to deliver the problems that the current sensor has. In fact, I think that we have studied all types of sensor structure for Al melt since 2002 and experienced many problems mentioned in the paper. I hope that this discussion will be a good chance to widen the technical understanding in proton conductor type hydrogen sensor, which leads to a completeness of sensor performance. (C) 2017 Published by Elsevier B.V.