Inline Ultrasonic Flow Meter

A typical transit-time flow measurement system utilizes two ultrasonic transducers that function as both ultrasonic transmitter and receiver. The flow meter operates by alternately transmitting and receiving a burst of sound energy between the two transducers and measuring the transit time that it takes for sound to travel between the two transducers. The difference in the transit time measured is directly and exactly related to the velocity of the liquid in the pipe. To be more precise, let’s assume that T down is the transit-time (or time-of-flight) of a sound pulse travelling from the upstream transducer A to the downstream transducer B, and Tup is the transit-time from the opposite direction, B to A. The following equations hold:

T down = ( D / sin ø ) / ( c + V*cos ø ), (1)

Tup = ( D / sin ø ) / ( c – V*cos ø ), (2)

Where c is the sound speed in the liquid, D is the pipe diameter and V is the flow velocity averaged over the sound path. Solving the above equations leads to V = ( D / sin 2ø ) * ∆T / (Tup * T down), (3)

Where ∆T = Tup – T down. Therefore, by accurately measuring the upstream and downstream transit-time Tup and T down, we are able to obtain the flow velocity V. Subsequently, the flow rate is calculated as following, Q = K *A* V, (4)

Where A is the inner cross-section area of the pipe and K is the instrument coefficient. Usually, K is determined through calibration.

From equations (3) and (4), we see that the measurement results, V and Q, are independent of fluid properties, pressure, temperature, pipe materials, etc. The sound speed term does not appear in the final equations. These characteristics, plus large turn-down ratio, no pressure drop, no moving parts, no disturbance to the flow and many other features, make ultrasonic transit-time flow meter extremely attractive.