I did look it up. Sorry, I don't have the flowmeter with me, but specs on this sort of equipment are very difficult to find. Just discovering the fact that it's RF pickup was about all I could find.
I believe their interface circuit costs about $350, and I'm very certain it's just as simple as you've stated. When I get back to the meter this weekend, I'll write down the numbers. If I recall correctly, it's made by FTI.
There's high margin and lots of trade secrets with vendor lock-in with this industrial equipment. I think this meter cost about $1500 when new.
---------- Post added at 11:44 ---------- Previous post was at 11:27 ----------
Okay, I can do better than my last post. Let's assume it's for one of these.
**broken link removed**
and let's pick the RF4. Are they modulating in the MHz range? Can a simple RC circuit with a diode -> amplifier get me in the ballpark?
---------- Post added at 12:03 ---------- Previous post was at 11:44 ----------
My, what a big difference a small change in Google search makes. I just found this gem:
**broken link removed**
It appears that they are exciting at 50KHz. However, it's not completely clear if they are working with an amplitude modulated response, or a frequency modulated response. I would prefer the on/off as you mentioned.