Actually, I think your strain gauge can be attached to the other side of the pressure vessel at the point you want to measure. For example, if I want to measure the strain at a certain point on the front of a steel plate, but because there are other parts or structures at this point, it is inconvenient to attach a strain gauge, then I can stick the strain gauge at this point on the back of the steel plate. The measured strain values ??are very close, but the directions of the strains are different.