In well bores with included tubulars, it is often desirable to segregate a portion of the wellbore's pressure-driven fluid flowpath in the conduit portion formed by included tubulars to permit work to be completed in a pressure-controlled portion of that flowpath. The subassembly and included frangible device provided here does so with minimal exposure of seals to well fluid environments, and provides a field-adjustable variability to the pressure differentials required to open the conduit by removal of the frangible sealing device from the flowpath into which the subassembly is installed or included.