Abstract: A continuous obstruction monitor for well logging drill strings including a well logging instrument at the forward end thereof transports the logging instrument through an earth borehole while continuously monitoring for obstructions. An outer tubular housing is connected at its upper end to the drill string and a piston therein is connected to the upper portion of the logging instrument. The housing has a variable volume greater than the volume of an equal length of the drill string. The piston is electrically connected to the surface. A latch retains the piston in the lower portion of the housing, and releases it when an obstruction is encountered. The outer housing moves downward relative to the piston to a point where a sealing surface at the mid portion of the housing shuts off fluid flow below the piston and causing the drilling fluid to rise within drill string to be detected at the surface.