Measurement Principle

Introduction to Transit Time Ultrasonic Measurements

“Transit time difference” is the most accurate of the ultrasonic methods used for the measurement of fluid and gas velocity and the only one which provides true integration of the velocity profile along the acoustic path.


Ultrasound pulses are continuously sent in both directions between a pair of probes. The travel time in each direction is measured to calculate the difference in transit time. Extreme precision (typically to 0.2 ns) in the transit time difference (Δt) enables an accurate velocity measurement to be calculated from these times and the geometrical position of probes. The difference in transit time is slower against the flow and directly proportional to the velocity of the liquid or gas. With non-intrusive clamp on sensors, this technology enables precise measurements to be installed in challenging conditions with minimal effort and cost.


The hydraulic flow conditions and the desired accuracy determine how many measuring points (chords) are required. A chord comprises of 2 probes.

Single Path Installations

Multipath Installation

Flow measurement in open channels

With ultrasonic sensors on each bank of a river or open channel, a velocity measurement can be calculated using the transit time principle.   A flow rate can be calculated when the cross sectional area profile is known and the depth is measured.