Hydraulic High Pressure Calibration Pump – Additel 938. The 938 Hydraulic High Pressure Test Pump is specially designed for calibrating oil-free pressure measuring instruments in the range of 85% vacuum to 15,000 psi (1,000 bar). All parts used in the pump have been carefully cleaned to remove oil. The pump uses water as the media. With the patented screw press technology, high pressures can be easily generated, as well as increased and decreased smoothly. A specially designed shut-off valve makes the pressure as stable as possible during calibration. With no check valve (non-returning valve), the 938 avoids the troublesome leakage issues that is usually experienced with hand pumps and allows for minimal maintenance.