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A transit time oil flow meter is similar to the Doppler flow meter in the sense that both of them use an ultrasonic signal for oil flow measurement. Even though the ultrasonic signal is transmitted into the pipe in a transit time oil flow meter as well, the working principal is still slightly different. In the Doppler flow meter, the velocity of the signal is measured, but for the transit time flow meter, the time taken for the wave to traverse the pipe is measured. Two transducers are mounted on the exterior wall of the pipe, which act as a transmitter and a receiver. The transmitter sends an ultrasonic signal into the pipe, but this is reflected off by the wall and the fluid in the pipe. This reflected signal is detected by the receiver. The total time taken by both the signals is measured, and then the time difference is calculated. This value is then used to estimate the frequency shift in the pipe. Once the frequency shift has been obtained, the flow rate can be determined from the Reynolds number and the internal pipe diameter. Reynolds number assesses the fluid as transitional, turbulent or laminar, and uses this to estimate the flow rate. The flow rate is significantly different for each of these three conditions. All these calculations are done with a software application, and the flow rate can be displayed in any unit. The flow rate, measured by this method, depends on the velocity of the fluid, the distance between the transducers, the angle between the ultrasonic signal and the surface of the fluid and the speed of sound in the fluid. The flow rate is directly proportional to the velocity and angle, but inversely proportional to the distance. The advantage of a transit time oil flow meter lies in the fact that it can measure the flow rate of ultrapure fluids as well, and so integrity of the fluids can easily be maintained. Another benefit is the accuracy of this device, which is in the range of +1%. Other than this, a transit time oil flow meter is easy to install, and does not wear off with time. It is also capable of detecting zero pressure drops or zero flow, which is not the case with most of the other types of oil flow meters. The most common application of a transit time flow meter is to measure the flow rate of light or medium crude oil in petroleum refineries.
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