Abstract: The invention relates to a measuring device for a portable ultrasonic flow measuring system, which measuring device comprises two sensor heads. Each of the sensor heads comprises a sensor housing on which there is provided a sensing surface intended to be placed on or against a measuring tube. An ultrasonic transceiver is provided in each sensor housing for transmitting and receiving ultrasonic waves through the sensing surface. Additionally, the measuring device comprises a control and evaluation device for measuring a transit time difference for ascertaining the velocity of flow of a medium flowing through a measuring tube. Provision is made for each ultrasonic transceiver in the sensor head to be disposed therein parallel to the sensing surface in the sensor housing. The invention further relates to a measuring tube adapted for use with the measuring device of the invention and an information carrier that contains information concerning the measuring tube.