by 84-1074663779 » Sun Dec 12, 2004 5:22 pm
The density of water and the earths gravity do not change much from place to place or over time. A homemade water manometer costs just about zero, and it will be very accurate and reliable, even after a few little accidents.
A purchased instrument will have a specified accuracy, that may degrade over time. This is especially true if the instrument is bumped, or dropped, or over pressured. How can you be sure an instrument is reading correctly ? Can you trust it completely ?
One day you will suck a rag into your bench and blow all the water out of your manometer tubes. That can be a nuisance. A sensitive pressure measuring instrument may be permanently damaged beyond economic repair, or at the least never be quite as accurate again.