MSP430FR6047: DOF(time of flight ) is fluctuating too much

Part Number: MSP430FR6047

Tool/software:

Dear engineers,

I'm using the TI recommended setup for flow ,meter as follows:

When I use USS software to read out the DOF it is as follows, it seems there is too much fluctuation, but the flow rate of water is zero inside the pipe (the pipe is full of water without any movement), so my question is this normal? if not how can I fix this issue?

Thanks