The hydrostatic test shows whether a cylinder’s wall still stretches and springs back, or has started to yield. With a water jacket the expansion is read straight off the burette or scale. Cylinders too big for a jacket are tested by measuring the water pumped in and let out again, but water compresses at test pressure, and a good part of what you pump in never reaches the cylinder wall.
The calculator corrects for the compressed water, gives the permanent expansion as a share of the total and checks it against the limit of the standard the cylinder was made to.