Hi Bob;
This is a differential pressure device with the dial scaled
(square root) to read in flow instead of pressure. It will create a
small pressure (flow) loss. These can be accurate, but it is important
to ensure a straight run for 10 dia. upstream & 5 downstream.

Todd


See here:  http://www.nciweb.net/liquids.htm

My guage is for a 1 1/2 inch pipe and the orifice is around 3/4 inch or
so.  That may introduce some restriction, but I'm hoping it isn't enough
to change the readings too much.  This could be used in flight if I
ignore the requirement to not change the pipe diameter for a length of
10 diameters.  The guage includes a transmitter so that I could get
readings remotely.

Bob White