Abstract: A pressure equalizing cabinet for instruments intended to be submerged in water is described. The device includes a source of compressed air and a piston in a cylinder. One side of the piston is in communication with the interior of the cabinet and the other side is in communication with the ambient. The piston also mounts a needle valve at a port in a conduit between this source of compressed air and the cabinet interior. When the pressure in the ambient exceeds the internal pressure, the piston is displaced to admit a flow of compressed air into the cabinet. When the pressure in the cabinet exceeds the ambient the piston closes the valve. A bleed valve is also provided for venting excess pressure within the cabinet during ascent.