If there is an engine driven, or electrically driven, supply/lift pump on your engine (which in my experience there almost always is) I would tap the pressure between this pump and the primary/engine mounted fuel filter. Direct this working pressure to a normally closed hydraulic type selector valve. In the event of failure to your "machine" supply interruption would cause a pressure loss, thereby allowing the spring pressure to open the valve to the alternate fuel source (assuming that the supply at this point in your system is of negative pressure). If the alternate source is pressurized then the engine "tap" would be utilized as a pilot pressure to control your valve.In my experience the quantity of fuel contained in the filter or filters is sufficient to maintain operation of the engine until the alternate source is established. You may find that you will need to increase the primary filter size to insure that you do not experience fuel starvation.

As for flow measurement, I would seriously consider using a Floscan diesel flow meter. I have installed many of these and found them to be highly accurate and reliable. http://www.floscan.com/html/blue/generators.php?gclid=CPi18IDS1ZECFRkGagodMniaZw.

This set up should not be very difficult to accomplish.

Good Luck,

Dragonfly

P.S. I saw a post some time ago that lined out the method for inserting graphics, but I did not save it and now can not remember the procedure. I have a very simple schematic of how I would do this that was quickly accomplished using AutoCAD. If my idea is unclear contact me and I will forward the drawing to you.