A method for monitoring the pressure difference across an ESP comprises:—connecting the ESP (7) to a production tubing (6)—providing the production tubing (6) with a side pocket (9) which comprises an opening (13)—inserting a pressure sensor assembly (10) into the side packet (9) such that the opening (13) is located between a pair of annular seals (14, 15)—monitoring the pressure difference across the ESP (7) by inducing the sensor assembly to measure a pressure difference between an upper section of the side pocket which is in communication with the interior of the tubing and a middle section (16) of the interior of the side packet (9) which is located between the annular seals (14, 15).