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Applying Ultrasonic Clamp-on Transit Time Flow Meters

Ultrasonic clamp-on transit time flow meters offer a non-intrusive method of flow measurement of liquid filled pipes and are a useful tool in many temporary and some permanent full pipe flow applications.

Because the sensors are installed on the outside of the pipe, these type of meters are easy to install, making them a good choice for a quick check of existing in-line flow meters, and for temporary installations when doing flow balance studies.

For permanent applications they can be the lowest cost solution when installation factors are considered.

Ultrasonic clamp-on transit time flow meters use sensors that are mounted on the outside of the pipe creating a diagonal measuring path across the pipe. Acoustic signals are transmitted and received along the measuring path. A sound wave going downstream with the flow travels faster than a sound wave going upstream against the flow. The difference in transit time, corrected for the angle of the path, is directly proportional to the mean flow velocity of the medium.


Figure 7-1: Measuring principle

1 Transducer A
2 Transducer B
3 Flow velocity
4 Transit time from transducer A to B
5 Transit time from transducer B to A

For proper operation in both temporary and permanent applications several factors need to be considered:

  • The sensors need to be mounted at least 10 pipe diameters downstream of any flow disturbance that can be created by an elbow, valve, header or pump.
  • The pipe must be full and the liquid should be relatively homogenous with a consistent viscosity and specific gravity.
  • The pipe type, diameter, wall thickness and presence of a liner need to be determined.
  • Pipes with a grease or mineral build-up on the inside of the pipe should be avoided as this can cause degradation of the acoustic signal and cause errors associated with the reduced cross sectional area in the calculation of flow.
  • Entrained air or excessive solids can degrade or block the acoustic signal.

At CC Lynch we have experience with a number of makes and models of clamp-on transit time flow meters in both permanent and temporary applications. Our service department is equipped to provide portable meters for verification of existing in-line meters and perform temporary flow studies. We also sell, service, and install permanent and portable ultrasonic clamp-on transit time flowmeters.

KROHNE manufactures two clamp-on ultrasonic flow meters with a user friendly rail system to aid in proper transducer spacing. The Optisonic 6300 is a robust flow meter, made for continuity and long term reliability. The OPTISONIC 6400 Portable is a powerful portable flow meter, great for temporary and long term flow measurements with the ability to set up the device quickly using a single or multi part rail mounting system with sliding transducers. These sliding rails reduce setup time by eliminating guesswork of transducer mounting locations.

For help in determining if an ultrasonic clamp-on transit time flow meter is appropriate for your application please contact your nearest representative.

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