TDC1000: Receive pulse is not generating.

Hi,

    I am working on water flow meter using TDC1000 and TDC7200.  I am able to get a transmitter burst for 1MHz.

 a) The start pulse is generating automatically even though the Transducer is not mounted on a pipe.

 b) receiver pulse (echo pulse) is not generating.

please let me know what to be done to receive an echo pulse?