An instrument designed for studying harbor surging. A strain gage unit, used in connection with pressure sensitive bellows, comprises the transducer of the pressure head. The four strain gages in the unit are connected to form a bridge circuit that is linked to the recorder by an electrical cable. A direct current voltage is applied to the bridge, and the record is obtained by recording the unbalanced current from a magnetic oscillograph. Any standard strain gage recorder can be used for the recording system.