detecting instruments next in the string is illustrated power supply sub 14 and above that valve subassembly 16 which is the subject of the present invention instrument sub 17 houses associated electronics for encoding information indicative of detected data into a format which in turn drives valve subassembly 16 to impart data to the drilling fluid for telemetry to the surface the drilling fluid or mud is circulated from storage pit 18 or the like at the surface by means of pump 19 which moves the mud through stand pipe 21 into the drill string the mud is carried through the string of hollow pipe comprising the drill string to the bottom of the borehole where it exits through drill bit 12 carried on the bottom of the drill string as the mud passes through bit 12 it experiences a substantial drop in pressure as it moves into the enlarged space of borehole annulus 22 surrounding the drill string the mud then carries cuttings from the bottom of the borehole to the surface where they are removed and the mud is returned to pit 18 by conduit 20 valve assembly 16 includes a bypass passageway which serves to connect the interior of the drill pipe fluid flow path with borehole annulus 22 a sufficient volume of mud can be vented through valve 16 to cause a.