Do you mean a flow
indicator, rather than a flow
metre?...
I believe RS make one in 1/2 & 3/8inch sizes. I'll look for a link...HERE!!:
http://www.rswww.com/cgi-bin/bv/sear...quickSubmit=Go
Here's a link to a forum were the dudes making an autoshutdown from one of these electrical flow sensors:
http://arstechnica.infopop.net/OpenT...5&m=3040918135