Rotary pulser for transmitting information to the surface from a drill string down hole in a well

A rotary pulser for transmitting information to the surface from down hole in a well by generating pressure pulses encoded to contain information. The pressure pulses travel to the surface where they are decoded so as to decipher the information. The pulser includes housing containing a stator forming passages through which drilling fluid flows on its way to the drill bit, a rotor, and a replaceable wear sleeve enclosing the rotor. The rotor has blades that are capable of imparting a varying obstruction to the flow of drilling fluid through the stator passages depending on the circumferential orientation of the rotor, so that rotation of the rotor by a motor generates the encoded pressure pulses. The rotor is located downstream of the stator and the rotor blades are shaped so that when the motor is not in operation, a hydrodynamic opening torque is imparted to the rotor that tends to rotate the rotor blades away from the circumferential orientation that results in the maximum obstruction and toward the circumferential orientation that results in the minimum obstruction. A torsion spring provides a mechanical force that also tends to rotate the rotor into the orientation that provides the minimum flow obstruction.