I use a gas fuse and it seems to read as full for a lenghty period. When it starts to show half you need to fill up soon as it quickly drops to empty as it measures pressure not the volume of liquid.
Mine does the same...........
Pretty much tells you when you're just about empty.
Doesn't give a realistic idea of what's left in the tank.
As far as a safety device goes................that's a whole other can of worms probably best left to the electrical section
I guess for me - they work to a degree, but aren't really accurate.
I do know someone who bought a set of scales from target for around $10 and installed them under his gas bottle. When he wants to weigh it he just releases the quick release holder and it lets him know exactly what's left. Bit over the top for me....