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Apparatus for sending pressure pulses through drilling fluid in a drill string in a well bore includes an assembly adapted to be lowered from the surface through the drill string to a position adjacent the lower end of the drill string. The external dimension of the assembly is substantially less th

Apparatus for sending pressure pulses through drilling fluid in a drill string in a well bore includes an assembly adapted to be lowered from the surface through the drill string to a position adjacent the lower end of the drill string. The external dimension of the assembly is substantially less than that of the internal diameter of the drill string, so there is always a substantial clearance between the assembly exterior and the drill string interior to permit a substantial flow of drilling fluid down the drill string, past the assembly, through a drill bit, and into an annular space between the drill string exterior and the well bore. The assembly includes an internal bore to permit drilling fluid to flow through the assembly. Means are provided for intermittently restricting flow of drilling fluid through the assembly bore to send the pressure pulses to the surface in response to the magnitude of a downhole condition to be measured. The assembly includes means to permit it to be retrieved by a wire line from the surface and without removing the drill string from the well bore.

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1. Apparatus for sending information to a surface pressure pulse detector through drilling fluid in a borehole drilled in the earth with a drill bit on the lower end of a drill string in the borehole and through which the drilling fluid is circulated to flow through the interior of the drill string,

1. Apparatus for sending information to a surface pressure pulse detector through drilling fluid in a borehole drilled in the earth with a drill bit on the lower end of a drill string in the borehole and through which the drilling fluid is circulated to flow through the interior of the drill string, past the drill bit, and into an annulus between the drill string and the borehole wall, the apparatus comprising: a retrievable assembly adapted to slide into and out of the drill string from the upper end of the drill string to a location near the drill bit, the assembly having a main bore with an inlet and an outlet through which a portion of the drilling fluid which flows through the drill string and past the drill bit may flow, the assembly being constructed and arranged so a first portion of the drilling fluid which flows through the drill string and past the drill bit may flow through the bore and a second portion of drilling fluid may flow through a substantially fixed flow cross sectional area between the assembly exterior and the drill string when the assembly is in the drill string; means for generating a control signal responsive to a downhole condition; and means responsive to the control signal to change the rate at which drilling fluid flows through the bore and without changing the effective cross sectional area for fluid flow between the assembly exterior and the drill string to send a pressure pulse through the drilling fluid to the surface pressure pulse detector. 2. Apparatus according to claim 1 in which the cross sectional area for fluid flow between the assembly exterior and the drill string is substantially greater than that of the assembly bore. 3. Apparatus according to claim 1 or 2 which includes means between the assembly bore inlet and outlet defining a flow restriction between the drill string interior and the assembly exterior to create a working pressure drop in the drilling fluid between the bore inlet and outlet, and means responsive to the pressure drop to change the rate at which drilling fluid flows through the assembly bore. 4. Apparatus according to claim 1 or 2 which includes latching means on the assembly to permit it to be retrieved from the drill string by a wire line from the surface without removing the drill string from the well bore. 5. Apparatus according to claim 4 in which the latching means is constructed and arranged to receive an overshot tool secured to a wire line. 6. Apparatus according to claim 1 or 2 which includes a valve for opening and closing the assembly bore in response to the signal. 7. Apparatus according to claim 1 which includes a main valve for changing drilling fluid flow rate through the bore, and a pilot valve responsive to the control signal for actuating the main valve. 8. Apparatus according to claim 7 in which the main valve is powered by the pressure drop in the drilling fluid flowing through the drill string. 9. Apparatus according to claim 7 or 8 which includes a rotary solenoid for operating the pilot valve. 10. Apparatus according to claim 7 or 8 in which the pilot valve is a spool valve, and which includes an electric motor for operating the spool valve. 11. Apparatus according to claim 10 in which the electric motor is reversible. 12. Apparatus according to claim 11 which includes stop means for limiting the travel of the electric motor when moving the spool valve between the open and the closed portions. 13. Apparatus according to claim 7 or 8 in which the pilot is opened and closed by linear movement of a valve plug, and a linear solenoid for moving the plug. 14. Apparatus according to claim 1 or 2 which includes an outwardly extending sleeve on the assembly between the main bore inlet and outlet. 15. Apparatus according to claim 14 in which the sleeve is removable from the assembly. 16. Apparatus according to claim 1 or 2 which includes a replaceable sleeve mounted in the drill string at a location between the assembly bore inlet and outlet when the assembly is positioned in the drill string to send pressure pulses through the drilling fluid. 17. Apparatus according to claim 16 in which the drill string is made up of a series of pipe sections threaded together at adjacent ends, and an outwardly extending flange on the sleeve clamped between two adjacent ends of pipe sections. 18. Apparatus according to claim 16 in which the sleeve includes an inwardly extending portion to create a flow restriction between the inlet and outlet of the main bore in the assembly. 19. Apparatus according to claim 1 or 2 which includes a replaceable interior sleeve in the assembly bore.