Radar and Ultrasonic Level

No. There are some questions that have no such thing as a one-size-fits-all "exact answer."

Not real exact, but the operation principle is pretty much the same. Radar is radar, and I assume you know how that works. Signal out - signal reflected by measured surface - signal returns - time differential versus wave speed gives a distance. Basic physics. The ultrasonics I have used (top mount/wave emitted down ward) are the same principle, but the wave length of the emitted signal is obviously grossly different.

With the top mounts I have used, the radar works poorly in small tanks, where the ultrasonics do well. I am told radar is much better on larger tanks (I deal with tanks in the 100 to 1000L size usually). Neither work well in the type of tank I usually have to measure, as they have dished bottoms which means the sensor "sees" the dish wall before the liquid is drained, unless you can mount the sensor exactly in the center of the top, which is ALWAYS where the customer wants the entry port, so you can't.

Radar is radar ? actually i am asking about deatailed working principle of both ?

ok leave it about level "

if the principle is pretty same i wanted know why the radar is not using for flow measurement ?

Anyone can give the exact answer of radar and ultrasonic level measurement principle ?

and which one is more reliable ?

Yes but it won't really help you.

The exact answer is "it depends on the application".

The more detailed answer is it depends on numerous factors that are application dependant (everything from material composition to accuracy to material container characteristics to size, etc, etc, etc). Radar won't work for some applications, likewise ultrasonic won't work for some applications.

General comparison information can be found on either the internet through a search or off the level sensor manufacturers websites (including details on advantages and disadvantages between applications).