On line measurement of chemical parameters in multiphase flows under high pressure and high temperature by using electrochemical sensors requires special devices for sensor positioning in the flow. These devices offer facilities for calibration, cleaning or replacement of sensor without pressure reduction or flow interruption. A new kind of automatic measuring and calibration unit with four different sensors for measurement in pipe flows is introduced. Miniature hydraulics ensure fast reliable sensor insulation from the pipe flow in order to calibrate or replace sensors. The unit was tested successfully at pressure up to 2.5 MPa and at temperatures up to 110 degrees C. Sensor response during calibration and sensor stability in aggressive two-phase flows were investigated. The construction is easily adaptable to different sensors and pipe diameters and suitable for a variety of pipe and sealing materials.