Dust collector vacuum guage

A simple, low cost way to measure the pressure of your dust collector system,   I probably have $2 investment in it and it is used almost every time I turn on the dust collector.   It will indicate a forgotten gate left open or a plugged line somewhere.
Simply a tube with some water in it (I used some food coloring for a visual aid).
The photos show 1) system turned off   2) one gate open  3) all gates closed.
Technically it is called a manometer and has something to do with the sea level...........   whatever.  I've been using it for about 20 years and have forgotten the original source,  certainly not me.