Transit-time meters, as the name implies, measure the difference in travel time between pulses transmitted in the direction of, and against, the flow. This type of meter is also called time of flight and time of travel.
In the example shown in Figure 1, the sonic beam is at a 45º angle, with one transducer located upstream of the other. Each transducer alternately transmits and receives bursts of ultrasonic energy; the difference in the transit times in the upstream vs. the downstream directions (TU - TD) measured over the same path can be used to calculate the flow through the pipe: V=K*D/Sin2?*1/(To-t)²?T
where: V= Mean Velocity of flowing fluid K= Constant D= i.d. of pipe ?= incident angle of ultrasonic burst waves To = zero flow transit time ?T = T2-T1 T1 = transit time of burst waves from upstream transmitter to downstream receiver T2 = transit time of burst waves from downstream transmitter to upstream receiver t = transit time of burst waves through pipewall and lining
This equation shows that the liquid flow velocity is directly proportional to the measured difference between upstream and downstream transit times. Because the cross-sectional area of the pipe is known, the product of that area and the flow velocity will provide a measure of volumetric flow. Such calculations are easily performed by the microprocessor-based converter. With this type of meter, particles or air bubbles in the flow stream are undesirable because their reflecting qualities interfere with the transmission and receipt of the applied ultrasonic pulses. The liquid, however, must be a reasonable conductor of sonic energy. Figure 2 shows three placements that can be used for the two transducers. All are identified as single measuring path because the sonic beam follows a single path, and in all three the two transducers are connected by cable to a converter that can output a 4-20 mA DC signal. The selection of one configuration over another is dictated by several factors associated with the installation, including pipe size, space available for mounting the transducers, condition of the inside pipe walls, type of lining, and nature of the flowing liquid. The Z configuration places the transducers on opposite sides of the pipe, one downstream of the other. Generally, the distance downstream is ~D/2, where D = pipe diameter. The converter uses specific data on piping parameters to compute the optimum distance. The Z method is recommended for use only in adverse conditions such as where space is limited, the fluid has high turbidity (e.g., sewage), there is a mortar lining, and when the pipe is old and a thick scale has built up on the inside wall that tends to weaken the received signals. It is not recommended for smaller pipes, where its measuring accuracy tends to degrade. In most installations, the V method is recommended, with the two transducers on the same side of the pipe about a pipe diameter apart. The rail attachment that can be clamped on the pipe facilitates sliding the transducers horizontally along the pipe and positioning them the calculated distance apart. The W method should be considered on pipe 1½ in. down to ½ in. dia. Its main limitation is a possible deterioration in accuracy due to buildup of scale or deposits on the pipe wall-note that the sonic signal must bounce off the wall three times. Turbidity of the liquid also could be harmful since the signal has a longer distance to travel.