11 12 Read fuel quantity here Drip hole Locking pin Fuel tank  access panel 0 5 10 15 20 25 30 35 40 Nozzle pressure gauge calibrated in pounds per hour of fuel flow Fuel injection system flow divider From fuel injector regulator Fuel nozzle 14-41 Figure 14-76.  The pressure drop across the fuel injector nozzles is used to represent fuel flow in light reciprocating-engine aircraft.  Figure 14-75.  A fuel drip stick is lowered from the fuel tank bottom  until fuel drips out the hole at the bottom. By reading the calibrated  scale and adding readings from all tank drip sticks, a chart can  be consulted to arrive at the total fuel quantity on the aircraft by  weight or by volume.  closely approximates fuel flow and provides useful flow  information for mixture control and flight planning.  There is a major limitation to the use of fuel pressure as a  flow indicator. Should an injector become clogged, fuel flow  is reduced. However, the pressure gauge indicates a higher  fuel pressure (and greater fuel flow) due to the restriction.  Operators must be aware of this potential condition and check  the flowmeter against EGT to determine the nature of the  elevated indication.  [Figure 14-76] Large reciprocating engine fuel systems may use a vane- type fuel flow meter that measures the volume of the fuel  consumed by the engine. The fuel flow unit is typically  located between the engine-driven fuel pump and the  carburetor. The entire volume of fuel delivered to the  engine is made to pass through the flowmeter. Inside, the  fuel pushes against the vane, which counters the force of  the fuel flow with a calibrated spring. The vane shaft rotates  varying degrees matching the fuel flow rate through the unit.  An autosyn transmitter deflects the pointer on the cockpit  fuel flow gauge the same amount as the vane deflects. The  dial face of the indicator is calibrated in gallons per hour or  pounds per hour based on an average weight of fuel.  engines, systems have been devised to measure fuel volume.  The actual mass of fuel flowing to the engine is based on an  assumption of the average weight of the fuel per unit volume. The simplest fuel flow sensing device is used in conjunction  with fuel injection systems installed on horizontally opposed  reciprocating engines. A pressure gauge is used but it is  calibrated in gallons per hour or pounds per hour. The amount  of fuel that is flowing through the fuel injectors has a direct  relationship to the pressure drop across the fuel injector  orifices. Therefore, monitoring fuel pressure at the injector(s)  Since fuel fed to the engine must pass through the flowmeter  unit, a relief valve is incorporated to bypass the fuel around  the vane should it malfunction and restrict normal fuel  flow. The vane chamber is eccentric. As more fuel pushes  against the vane, it rotates further around in the chamber.  The volume of the chamber gradually increases to permit  the greater flow of fuel without restriction or pressure  buildup.  [Figure 14-77]