Ultrasonic flow measuring principle

This method is used for measuring liquids and gases and utilizes the fact that the propagation velocity of sound waves within a fluid is directly influenced by the velocity of that fluid. Expressed in simple terms, swimming against the flow requires more power and time than swimming with the flow. Ultrasonic flow measurement utilizing the transit time effect is based on this simple physical fact (see Fig. 1).

In this method, the sensors work in pairs. One pair of sensors or more are mounted on the pipe, and they emit and receive ultrasonic pulses in sequence (denoted by signal t₁ and t₂ in the below figure). At “zero flow”, signal t₁ is similar to t₂, i.e. without transit time delay. But with a flowing fluid, the ultrasonic waves require differing lengths of time (flow dependent) to reach the other sensor. This results in a transit time (t₂ – t₁). If the distance between the two sensors is known, then the measured transit time difference is directly proportional to the flow velocity. Both sensors are connected to a transmitter. The transmitter excites the sensors to generate sound waves and measures the transit time of these waves propagating from one sensor to the other:

Q: Flow rate
K: K = f (length of acoustic path, ratio between radial and axial sensor distance, velocity distribution (flow profile), cross-section)
t₁: Transit time t₁ (with the flow)
t₂: Transit time t₂ (against the flow)

A Doppler flowmeter uses the Doppler effect (sometimes referred to as the Doppler shift) to measure flow. This phenomenon of physics is familiar to all of us in our everyday lives. It involves the reflection of waves from moving objects. The audio frequency of an ambulance siren, for example, falls distinctly once it has passed. Thus, a Doppler shift is an increase (or decrease) in the frequency of sound waves as the distance between audio source and receptor increases or decreases.

Doppler meters work only if the fluid contains particles, gas bubbles or similar, which will reflect the injected sound waves. A Doppler meter requires one sensor which acts as both transmitter and receiver. (Fig. 2).

Next to the doppler and transit time differential method, flow can also be measured with the help of a cross-correlation method. This method measures flow by sensing the passage of disturbance or flow profile pattern in the flow at one measuring point and detects how much time (transit time) that disturbance needs to travel to the next measuring point. The distance of the two measuring points (Δx) is known, and the travel time is measured. Based on that, the flow velocity and the volume flow can be calculated.

This method can be used for measurements with very high flow velocities and/or high contents of entrained gas bubbles or particles. It needs a powerful signal processing and hardware to process the many needed calculations and comparisons of disturbances and/or patterns.