Did someone say... "Beer"???
Damn, I can't help but respond to this one...
Back in the day... aaahhh, those foot-loose and fancy-free days... I used to have a pony-keg sitting in an old refrigerator. The tap was mounted in the door. The beer-keg and the gas bottle were contained in the fridge.
I did indeed place the keg on top a bathroom scale.
You gotta know... this was in the days before PLCs and various displays were available for a reasonable price.
Q: What's a reasonable price?
A: Anything that doesn't cut into Beer-Money!
The scale idea worked pretty good. That is, if, before beginning the day's watering, one were to open the door and check the scale - reading upside-down, of course - then you had a general idea as to whether or not you were close to empty.
Of course, the problem was in the amount of unexpected participation...
If a few extra folks showed up then... the beer level went down faster, and the faster it went down... the less inclined anyone was to open the door and read the scale!
As the usual routine went, and I dare say still goes... you might get up thinking that you would open the door and check the scale... but then, as you approached the door, you would forget your original intent because you see that HUGE tap on the door just begging to be pulled!
All in all, it worked very well on the starting end of the... vital routine. However, it quickly became more and more ineffective, especially if a few friends showed up. And I have to say, the latter case occurred more often than I expected.
These days, I would go with one of those shut-off valves that react to high-volume flow.
But then again, that still depends on the answer to... "What is a reasonable price?".