A Comparison between Doppler and Transit Time Ultrasonic Flow Meters

Ultrasonic flow meters are non-invasive devices that use acoustic vibrations to determine the flow rate of a liquid. There are two types of ultrasonic flow meters – Doppler and transit time. Both meters are designed to attach onto the outside of a pipe without breaking or interrupting the fluidic flow. This process also eliminates any pressure losses and prevents leaking of the liquid – a problem which is commonplace with in-line flow meters. Additionally, the flow meter is never in contact with the liquid, thus preventing corrosion or degradation of the sensors. The Doppler and transit time flow meters operate on using similar principles but is varied in the technology utilized. To record accurate measurements, it is important to know which flow meter is required for your application.

Doppler Ultrasonic Flow Meters

Doppler ultrasonic flow meters operate using the Doppler Effect, which was a principle documented by the Austrian born physicist and mathematician Christian Johann Doppler in 1842. Christian stated that the frequencies of the sound waves received by an observer are dependent upon the motion of the source or observer in relation to the source of the sound. Doppler ultrasonic flow meters utilize a transducer to emit an ultrasonic beam into the flow stream of the pipe. To ensure a high operational efficiency, there must be solid particles or air bubbles in the flow stream to reflect the ultrasonic beam. The motion of the particles changes the frequency of the beam, which is received by another transducer.

Christian Johann Doppler

Christian Johann Doppler

The flow meter measures the shift in the frequency, which is linear to the flow rate. This value is multiplied by the internal diameter of the pipe to deduce the volumetric flow, as shown in the following equation:

Δf = 2fT sinΘ • VF/VS

By Snell’s Law (the law of refraction):

sinΘT/VT = sinΘ/VS

VF = Δf/fT • VT/sinΘT = KΔf

Where:

VT = Sonic velocity of transmitter material

ΘT = Angle of transmitter beam

K = Calibration factor

VF = Flow velocity

Δf = Doppler frequency shift

VS = Sonic velocity of fluid

fT = Transmitter frequency

Θ = Angle of fT entry into liquid

Volumetric flow rate = K • VF • D2

Where:

K = Constant

D = Inner diameter of the pipe

In contrast, the Doppler ultrasonic flow meter relies on particles flowing through the liquid to operate and consideration must be given to the lower concentration limits and to the size of solid particles or bubbles. In addition, the liquid must flow at a quick enough rate as to stop the solids from sedimenting.

Transit Time Ultrasonic Flow Meters

Transit time ultrasonic flow meters measure the time difference from the transmission of the ultrasonic signal until it crosses the pipe and is received at the second transducer. This is followed by a comparison of the upstream and downstream measurements. When there is no flow, the travel time will be the colloquial in both directions. When there is a flow present, the sound waves move faster if they are traveling in the same direction as the flow, but slower if moving against it. As the ultrasonic signal must navigate through the pipe to reach the sensor, the liquid cannot contain a high concentration of solid particles or bubbles, otherwise the high frequency sound will be reduced and too weak to travel across the length of the pipe.

The difference in the upstream and downstream measurements taken over the same path is used to calculate the flow through the pipe:

V = K • D/sin2Θ • 1/(T0 – t)2 ΔT

Where:

V = Mean velocity of flowing fluid

K = Constant

D = Inner diameter of the pipe

Δ = Incident angle of ultrasonic waves

T0 = Zero flow transit time

ΔT = T1 – T2

T1 = Transit time of waves from upstream transmitter to downstream receiver

T2 = Transit time of waves from downstream transmitter to upstream transmitter

t = Transit time of waves through pipe wall and lining

The above equation shows that the flow velocity of the liquid is directly proportional to the difference between the upstream and downstream measurements.

There are three usable configurations when using a transit time ultrasonic flow meter and are known as Z, V and W. All are recognized by a single measurement path, whereas the ultrasonic beam follows a single path. All three configurations produce an output from the transducer and is converted into a current, frequency or voltage signal. The preferred configuration is often determined by the following factors:

  • Pipe size
  • Space available for mounting the transducers
  • Condition of the internal walls of the pipe
  • Type of lining
  • The characteristics of the flowing liquid

The “Z” configuration utilizes transducers located on opposite sides of the pipe with one downstream from the other. Normally, the downstream distance is approximately D/2, where D equates to the diameter of the pipe. A converter calculates the optimal distance. This arrangement is only advisable when there is limited space, high turbidity, a mortar lining or a thick build-up of scale on the interior walls of the pipe. It should be avoided for small pipes, where there is a tendency for a degradation in the measurement accuracy.

The “V” configuration is common for most installations. This configuration incorporates both transducers on the same side of the pipe and roughly spaced a diameter of the pipe from each other. A rail attachment clamps onto the pipe and allows the transducers to be slid horizontally to position them to the required distance apart.

The “W” configuration is commonly used for installations on pipes with diameters of ½ inch to 1½ inches. In this configuration, the ultrasonic signal reverberates from the wall three times; therefore, it travels a greater distance. High turbidity liquids, and scale or deposit build-up on the inside of the pipe wall can reduce the accuracy.

Factors Influencing Accuracy of Ultrasonic Flow Meters

The accuracy of ultrasonic flow meter measurements is reliant upon proper mounting. Temperature changes within the pipe or a large amount of vibration can affect the transducers alignment and acoustic coupling to the pipe. These factors need to be accounted for during the installation process. To provide an accurate reading of the volumetric flow rate, all ultrasonic flow meters require a full pipe. A Doppler ultrasonic flow meter on an unfilled pipe will continue to generate measurements if both transducers are mounted below the fluid level in the pipe.

Conclusion

Ultrasonic flow meters are a non-invasive way of measuring flow velocity in a pipe. They are attachable devices that clamp onto the exterior of the pipe to measure corrosive liquids without causing any damage to the sensors. There are two types of ultrasonic flow meters – Doppler and transit time, and each function using two different technologies. Understanding how each of the flow meters operates, enables an appropriate choice to be made. The Doppler ultrasonic flow meter must have particles or bubbles present in the flow to reflect the ultrasonic signals. It is best utilized for dirty or aerated liquids, such as wastewater and slurries. On the other hand, a significant number of solid particles or bubbles in the liquid will reduce the signal produced by a transit time ultrasonic flow meter. Therefore, it is best utilized with clean liquids such as water or oil.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

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