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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 t_d is shorter than the time it reqires for the same waves to travel from the downstream to the upstream t_u. The larger the difference, the higher the flow velocity.

t_d and t_u 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 is the angle between the flow direction and the line formed by the transducers.

The difference of t_d and t_u is

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

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

or,

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 t_d and t_u to avoid frequent calibrations:

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

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

The flow velocity can therefore be written as

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

	Fluid Phase:	Score Phase Condition  Gas   Clean   Liquid   Clean     Corrosive      Dirty   Gas   Dirty   Liquid   Open Channel     Viscous  : 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