My sole tip is to be sure that there isn't enough suction to suck up a big plug of water__you have a considerably larger reservoir than the average tube-type manometer! A hole in the sensing tube, something that can be partially blocked with your finger, would be one type of failsafe (I know you can take that little tidbit of info and run with it; no shortage of
imagineering on your part
).
I once did just that, it was even tinted with some
Dwyer green dye; fortunately, the U-tube that I used doesn't have much capacity, but one good blip of the throttles, and it was gone!