What and How should I use my Transducers for Ultrasound Flow Metering ?

Hey Fellas,

I can get my hands on HC-SR04, US-015 and US-100 sensors. I want to build a transit time flow meter so I have to use two sensors.

So, would it get the job done if I purchase just two HC-SR04s ?

or do I need to get a transducer designed specifically for this purpose like this one ?

And my other question is how should I mount these sensors on pipes ? [u]Do they have to face each other directly[/u], meaning I'll have to mount them on the pipe surface with a slight degree so that the waves will have to travel an exact straight line ? like this :

If yes, what kind of hardware do I have to purchase to be able to place sensors at the exact correct distance and angle ?

Or just mounting them [u]facing the pipe surface directly[/u] would be sufficient ? [ but at a 15° distance (from datasheet, Induction Angle: Less than 15°) The horizontal distance between sensor on top and the one on the bottom has to be less than d = diameter x cot 15° so that they'd be able to send/receive waves. ]

anyone?

last try ...