
Transittime Ultrasonic Flow Meter A typical transittime flow measurement system utilizes two ultrasonic transducers that function as both ultrasonic transmitter and receiver. The flow meter operates by alternately transmitting and receiving a burst of sound energy between the two transducers and measuring the transit time that it takes for sound to travel between the two transducers. The difference in the transit time measured is directly and exactly related to the velocity of the liquid in the pipe. To be more precise, let's assume that Tdown is the transittime (or timeofflight) of a sound pulse traveling from the upstream transducer A to the downstream transducer B, and Tup is the transittime from the opposite direction, B to A. The following equations hold: Tdown = ( D / sinq ) / ( c + V*cosq ), (1) Tup = ( D / sinq ) / ( c  V*cosq ), (2) where c is the sound speed in the liquid, D is the pipe diameter and V is the flow velocity averaged over the sound path. Solving the above equations leads to V = ( D / sin2q ) * ∆T / (Tup * Tdown), (3) where ∆T = Tup  Tdown. Therefore, by accurately measuring the upstream and downstream transittime Tup amd Tdown, we are able to obtain the flow velocity V. Subsequently, the flow rate is calculated as following, Q = K *A* V, (4) where A is the inner crosssection area of the pipe and K is the instrument coefficient. Usually, K is determined through calibration. From equations (3) and (4), we see that the measurement results, V and Q, are independent of fluid properties, pressure, temperature, pipe materials, etc. The sound speed term does not appear in the final equations. These characteristics, plus large turndown ratio, no pressure drop, no moving parts, no disturbance to the flow and many other features, make ultrasonic transittime flowmeter extremely attractive. The transducers come with two types, one is clampon type, the other is wetted type. The wetted type can be further categorized into insertion type and flow cell (or spool piece) type. A brief comparison among those types can be found here. The transducers can be mounted in three ways: Zmethod, Vmethod and Wmethod. With Zmethod, the two transducers are mounted on opposite sides of the pipe (see the figure on the top) and the sound pulse crosses the pipe flow once. This method is usually used for large pipe size, say above 12". With Vmethod, the two transducers are mounted on the same side of the pipe and the sound pulse crosses the pipe flow twice. This is the most commonly used installation method, which could apply to pipe size from 1" up to 12". With Wmethod (refer to the following drawing), the two transducers are still mounted on the same side of the pipe. However, the spacing between the two transducers is doubled comparing with Vmethod. The sound pulse is bounced twice from the other side of the pipe, thus it intercepts the flow four times. This method is used for small pipe, usually less than 1 1/2", for better accuracy.
It should be mentioned that the actual implementation of the above principle is much more complex than what it looks like. The challenges include:
Different manufacturers have different answers to the
above questions. As a result, there are many brands of ultrasonic flowmeters,
some of them may work well in a wide range of applications, some may not.
Some may be expensive, and some may be less expensive. Some may be easy to
use, some may be difficult. A lowcost, high accuracy, reliable and easy to
use ultrasonic flowmeter is always the pursuing goal of all ultrasonic
flowmeter manufacturers. Spire Metering
claims to be the top performer along this line. You may take a look at their liquid
ultrasonic flowmters together with their "guaranteed high
quality, guaranteed lowest price" policy. 

