Abstract: In a fluid communication device such as a measure-while-drilling well-logging tool of the type in which the information concerning the downstream parameters that are measured is transmitted upstream, e.g. to the surface of the earth, in the form of positive pressure pulses in the drilling fluid, a pressure modulator for generating the pulses is driven by the drilling fluid passing through the tool. The pressure modulator is controlled by a solenoid actuated latching means which has low power requirements as compared to known measure-while-drilling logging tools.