A transit time ultrasonic flowmeter uses a pair of ultrasonic transducers placed at $45^{\circ}$ angle, as shown in the figure.
The inner diameter of the pipe is $0.5\;m$. The differential transit time is directly measured using a clock of frequency $5\;MHz$. The velocity of the fluid is small compared to the velocity of sound in the static fluid, which is $1500\;m/s$ and the size of the crystals is negligible compared to the diameter of the pipe. The minimum change in fluid velocity $(m/s)$ that can be measured using this system is _________.