A pressure compensation device with pressure relief detection and automatic oil making-up and pressure maintaining functions for an
underwater tool comprises a cylinder body, a
piston, a sealed leather bag, a
piston rod, a proximity unit sleeve, a proximity switch, a proximity switch support, an extension spring and an
oil storage cavity. The upper end of the cylinder body is fastened to the lowerend of the proximity switch support, the proximity switch is arranged on the proximity switch support, the lower end of the
piston rod is in sealing connection with the piston, the upper end of the
piston rod extends into the proximity switch support after penetrating through the cylinder body, one end of the extension spring is connected with the piston, and the other end of the extension springis connected with the
tail end of the proximity switch support. The sealed leather bag is tightly connected to the cylinder body and the piston, and therefore the sealed leather bag, the piston, thecylinder body and the
piston rod form the
oil storage cavity. In this way, the pressure compensation device is simple in structure, easy and convenient to control and stable in work, can effectively balance the
internal pressure and
external pressure of a transmission device, can conduct automatic compensation timely after oil permeating or oil leakage of a to-be-compensated device is detected, and thus
external liquid is prevented from permeating into the transmission device to damage the transmission device.