Quote Originally Posted by mklotz View Post
Mount a DTI (Dial Test Indicator) on a height gage or surface gage and lay the diaphragm on the surface plate or, lacking that, any good flat surface, e.g., mill table, plate glass.
In the case of possible varying thickness and concave for formed shape diaphragms, mounting a ball bearing on a pedestal centered directly under the point of the DTI as mentioned above will allow for measuring any small imperfections which may exist