The field of ultrasonic measurement models and covers many very different types of flowmeters. The term "ultrasonic" is an unmistakable characteristic of a type of flowmeters. "Ultrasonic" indicates only that the flow rate is measured using ultrasound. The flow is actually measured by any of the following methods:
- The Doppler method
- The transit time method of signal
The Doppler method
A Doppler flowmeter uses the Doppler effect (also known as Doppler shift) to measure flow. This physical phenomenon is familiar because it belongs to the realm of everyday experience. It is the effect that occurs when a wave front is reflected in a moving object. The frequency of the acoustic signals of, say, an ambulance is approaching us is significantly reduced once we exceeded. The Doppler effect is, therefore, an increase (or decrease) the frequency of sound waves as the distance between sound source and receiver increases or decreases.
A precondition for the functioning Doppler flowmeter is that the fluid contains particles, gas bubbles or other inhomogeneities similar, reflecting the sound waves. For this purpose, a Doppler flowmeter requires two sensors. The first fluid emitted by an ultrasonic wave at a particular frequency and receives the second reflected wave (Fig. below).
Figure: Flow measurement by Doppler ultrasound. The frequency of waves emitted (f1) and reflected (f2) varies depending on the flow velocity of the particles / bubbles transported
The variation in frequency of sound waves reflected beam is directly proportional to the speed of the particles or bubbles moving stream. It is assumed that the speed of the particles or gas bubbles is the same as the speed of the fluid. Then the flow calculation is given by the following expression:
Q = K * Af
Af: Variation of the frequency (f1 - f2)
K: K = constant = f (angle of incidence / reflection, reflective position of the particle cross section)
The Doppler method is simple and fairly accurate when measuring the speed of a single particle. Consider a police officer with his radar traffic flowing through the road. Every time you can determine the speed of a single vehicle, but can not measure the average speed of traffic flow.
The situation is similar in case you want to measure a fluid flow. In this case it is also necessary to measure the velocity of many particles. But the speed of each particle is different depending on their orientation and position in the fluid velocity profile. Calculate the volume required to make a weighted average of the results of each measurement based on the position of each particle in the fluid. Furthermore, it should be noted that a reflected signal can be affected by more particles / bubbles on their way back.
Ill.: Ultrasonic flow measurement from! transit time of the signal speed at which sound waves propagate varies with fluid velocity and direction
Transit time method of signal
This method is based on the fact that the fluid velocity directly influences the speed of propagation of sound waves in the fluid.
This phenomenon can be understood in simple terms from an analogy: swimming against the current requires more effort and time to swim in the direction of flow. The method of ultrasonic flow measurement from the transit time of the signal is based on this physical evidence (see above).
Two sensors on the pipe emit and receive ultrasound pulses simultaneously. A "zero flow", both sensors receive the sound waves transmitted at the same time, ie without any delay in transit times of the signal. But with a circulating fluid, the sound waves from each sensor need different time intervals (flow dependence) to reach the other sensor. If the distance between the two sensors is known, the difference in transit times of the signal is directly proportional to fluid velocity.
Both sensors are connected to a transmitter. The transmitter induces the sensors to generate sound waves and measuring the transit time of these waves that propagate from one to another sensor.
Q = K * (t1 - t2) / (t1 * t2)
t1: Time t1 traffic signal (in the direction of current)
t2: Transit time t2 signal (upstream)
K: K = constant = f (acoustic path length, ratio between the radial and axial distances from the sensors, velocity distribution (flow velocity profile), cross section)
