;D I like your findings, Mark.
There's another instrument around with "inches of Mercury" I think, sounds toxic  :P
This source is better than any of my texts http://en.wikipedia.org/wiki/Inch_of_water

But anyway, this little gauge shows readings without having something to read.
Turn all air systems on or off, always the same there, no fluctuation.
If you cut off electrics, it goes to zero but that's probably the case on all electrical driven instruments.

I think I'll put that on the list soon.