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Transit Time Ultrasonic Flowmeter: A pair (or pairs) of transducers, each having its own transmitter and receiver, are placed on the pipe wall, one (set) on the upstream and the other (set) on the downstream. The time for acoustic waves to travel from the upstream transducer to the downstream transducer td is shorter than the time it reqires for the same waves to travel from the downstream to the upstream tu. The larger the difference, the higher the flow velocity.

td and tu can be expressed in the following forms:

where c is the speed of sound in the fluid, V is the flow velocity, L is the distance between the transducers and theta is the angle between the flow direction and the line formed by the transducers.

The difference of td and tu is

where X is the projected length of the path along the pipe direction (X = L costheta).

To simplify, we assume that the flow velocity V is much smaller than the speed of sound c, that is, . We then have


Note that the speed of sound c in the fluid is affected by many factors such as temperature and density. It is desirable to express c in terms of the transit times td and tu to avoid frequent calibrations:

The speed of sound c becomes

The flow velocity is now only a function of the transducer layout (L, X) and the measured transit times tu and td.

The above formula can be further simplified by utilizing the following approximation:

The flow velocity can therefore be written as

Common Specifications

Common specifications for commercially available transit time ultrasonic flowmeterss are listed below:

  Fluid Phase:

Score Phase Condition
 Gas  Clean 
 Liquid  Clean 
 Gas  Dirty 
 Liquid  Open Channel 
: Recommended
: Limited applicability
  Line Size: Inline model: 10 ~ 1200 mm (0.4 ~ 48 inch)
Clamped-on model: 75 mm (3 in) and up
  Turndown Ratio: 100: 1
Pros and Cons

  • Pros:
  - No obstruction in the flow path, no pressure drop
  - No moving parts, low maintenance cost
  - Multi-path models have higher accuracy for wider ranges of Reynolds number
  - Can be used in corrosive or slurry fluid flow
  - Portable models available for field analysis and diagnosis
  • Cons:
  - Higher initial set up cost
  - Single path (one-beam) models may not be suitable for flow velocities that vary over a wide range of Reynolds number