Formerly the first in the world I have calculated the correction to be applied to the test pressure of large diameter pipes due to the back pressure of the hydraulic test machine. I used the equations provided by Von Mises (criterion of relative strength in ductile materials based on the maximum energy of distortion). My algorithm was first used to calculate the test pressure pipe API X60 grade OD 30" pipeline intended to Michigan in 1974. This criterion was accepted by all the testers for over 10 years, but in 1988 it was replaced by a formula derived from the theory that the 'tau max' ( maximum shear stress ), which was then included in the standard API. 
This method provides ​​discontinuous values with the diameter, and generally values more favorable to the manufacturers in case of large diameter pipes, when the shear stress is negligible; instead is better in case of pipes with lower ratio between diameter and thickness.

My method, best for large diameter pipes, it is with high ratio between diameter and thickness, gives instead continuous results.
On this occasion (and also in others similar) I was struck by the low mind elasticity and insulting attitude of sufficiency shown by the API Technician who in 1988 couldn't understand why I had reduced the formula to a step by step algorithm. Note that my management called me to explain formulas to this fellow, when I was working from years in the hot strip mills.
In fact, because in '74 we did not have PCs, I made a simple step table to facilitate other employees who had to calculate manually the testing pressure.