According to at least one aspect, a fluid flow tube can define a first cavity for receiving a first ultrasonic sensor. The first cavity can have a first longitudinal axis intersecting a longitudinal axis of the fluid flow tube. The fluid flow tube can define a second cavity for receiving a second ultrasonic sensor. The second cavity can have a second longitudinal axis aligned with the first longitudinal axis. The fluid flow tube can define a third cavity having a first transverse cross sectional area larger than a second transverse cross sectional area of the fluid flow tube and sized to receive a shut-off valve.