An apparatus for measuring one or more characteristics of a fluid flowing within a pipe is provided. The apparatus includes a housing, sealant, and an array of at least two ultrasonic sensors. The housing is operable to be mounted on the exterior surface of the pipe. The housing has at least one pocket extending between a pipe-side surface and an outer surface. The pipe-side surface mates with the exterior surface of the pipe. The sealant is disposed between the pipe exterior surface and the housing pipe-side surface. The sealant forms a seal around the pocket when the housing is mounted on the exterior surface of the pipe. The array of ultrasonic sensor units are disposed within the at least one pocket. Each sensor unit includes a transmitter and a receiver. The transmitters are operable to transmit ultrasonic signals through the fluid flow within the pipe. The receivers are operable to receive signals from the transmitters.