Measuring Principle

All KROHNE ultrasonic flowmeters operate using the transit-time differential method.

The Transit-time differential measurement is based on a simple physical fact.

Imagine two canoes crossing a river on the same diagonal line, one with the flow and the other against the flow. The canoe moving with the flow needs much less time to reach the opposite bank.

Ultrasonic waves behave exactly the same way. A sound wave travelling in the direction of flow of the product is propagated at a faster rate than one travelling against the flow (vAB > vBA).

Transit times tAB and tBA are measured continuously. The difference (tBA - tAB) in time travelled by the two ultrasonic waves is directly proportional to the mean flow velocity (vm) of the product.

The volumetric flowrate per unit time is the product of the mean flow velocity (v
A liquid product is identified by direct measurement of the transit time of ultrasonic waves. Assuming the same path length (L), the transit time in water is shorter than in crude oil, for example.

A (A’)   Sensor A, transmitter and receiver
B (B’)    Sensor B, transmitter and receiver
co         Sound velocity in the product
GK        A calibration constant
L           Length of measuring beam, distance between sensors A and B
tAB       Transit time of ultrasonic waves from sensor A to sensor B
tBA       Transit time of ultrasonic waves from sensor B to sensor A
vAB       Propagation rate of ultrasonic waves between sensor A and sensor B
vBA       Propagation rate of ultrasonic waves between sensor B and sensor A
vm         Mean flow velocity of liquid product
a;            Angle between pipe axis and measuring beam