What is Hydrostatic Plumbing test?

Hydrostatic is testing piping or a pressure vessel with water rather then air as it is much safer.

Picture two balloons one filled with air one filled with water then puncture them and the one with air reacts violently and the water balloon just plops harmlessly open.

For testing a riser one will fill the piping and knowing that 1 ft of water will exert .434 PSI (figure a half pound per ft)

A 100ft pipe would have a static pressure of 43 PSI to prove this take the pressure and times it by 2.31 close enough for government work