"What happens, if your flowmeter or microcontroller will fail?"

Yes this isue has been addresed. When the 4-20ma loop fails, I generate an alarm for the user to read; and for me to integrate, I am reading a "OK" bit from the client.

"Think of a brown-out situation, where microcontroller will stop running."

No, I had not thought of this. How can MY design and my MCU fail ;-)
Anyway, it is a valid point and since the MASSVAL will have to be valid till overflow ( 999,999 kg) I had thought of a battery backup. But I now have to address this brownout issue.
And finally :The MCU does have a good reset controller.

Thanks for the ideas.

Raghu