Rotary pulser and method 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 pulser includes a rotor having blades that are capable of imparting a varying obstruction to the flow of drilling fluid through stator passages, depending on the circumferential orientation of the rotor, so that rotation of the rotor by a motor generates the encoded pressure pulses. A spring biases the rotor toward the stator so as to reduce the axial gap between the rotor and stator. When the pressure drop across the rotor becomes excessive, such as when increasing drilling fluid flow rate or switching from a high data rate to a low data rate transmission mode, the spring bias is overcome so as to increase the axial gap and reduce the pressure drop across the rotor, thereby automatically reducing the thrust load on the bearings.