The Waterman Inner Pressure Bladder is EXACTLY what the name implies, a small 1.5 gallon bladder designed to be installed in the main fuel cell and receive fuel from the 2 combined scavenge pumps.  The primary fuel pump then draws its' fuel from the inner pressure bladder.  Since the 2 scavenge pumps produce 40% more volume of fuel than the primary pump can draw, the inner pressure bladder was designed to expand until it reaches 10 PSI.  At that point a bypass valve opens to maintain 10 lbs. of positive pressure to the primary pump.  A -3 fitting located in the end plate of the bladder sends a signal to a pressure sensor.  When the pressure drops below 10 PSI, that will be your notification that there is exactly 1.5 gallons of fuel remaining in the car.  Having the luxury of knowing exactly when to pit for fuel is obviously an enormous advantage.  The Waterman Inner Pressure Bladder is available in either a Vertical or Horizontal model.