in a borehole forming operation , the flow of gas through a downhole pneumatic motor is regulated to eliminate horsepower variations due to variations in bottom hole pressure . the pneumatic motor is connected to the lower end of a tubing string and an orifice is placed within the entrance and exit gas passageways leading to and from the pneumatic motor . the uphole source of gas pressure and gas flow rate is maintained as a constant value which assures that sonic flow conditions exist at the lower orifice . the employment of gauged orifices placed within the inlet and exit passageways of the gas motor assures that sonic flow conditions exist at the lower orifice and as the bottom hole pressure varies , there is no resultant change in the motor output horsepower since constant flow rate and pressure drop ratio through the motor is maintained . this is the case so long as the bottom hole pressure does not increase to the critical ratio .