As a driver, your primary objective at the pump is to get your tank filled so that you can get your car back on the road. The goal of the service station owner and the company that supplies the gas, however, is to know just how much gas you've pumped so they can properly charge you for it. That's where the flow meter comes in.
As the gasoline travels upward into the dispenser, it passes through a flow control valve that regulates the gasoline's flow speed. It does this via a plastic diaphragm that gets squeezed more and more tightly into the pipe as the flow of gas increases, always leaving just enough room for the proper amount of gasoline to get through. If you've set a predetermined amount of gas to be pumped, the flow of gas will slow down as you approach the limit.
This pipe also contains the flow meter, which is a cast iron or aluminum chamber containing a series of gears or a simple rotor that ticks off units of gas as they pass through. Information about the gas flow is passed on to a computer located in the dispenser, which displays the metered amount of gas in tenths of a gallon. As the temperature of the gas changes -- on particularly hot and cold days, for instance -- the density of the gas may change, causing an error in the amount of fluid measured by the flow meter. The computer compensates this error by taking the gas temperature into account as it records the flow and adjusts the price accordingly.
Wear and tear on the meter may degrade its accuracy over time, which is why periodic inspections are necessary. Typically, inspectors will use a container of a certain volume, pump gas into it and compare the amount in the container with the amount metered on the dispenser. If the amounts don't match, the flow meter will need to be recalibrated and possibly refurbished or replaced. Although regulations for pump calibration come from the National Institute for Standards and Technology (NIST), the actual inspections are performed locally, usually by a state's Department of Weights and Measures.
Now that the gas is flowing and the amount of flow has been measured, there's only one step left: getting the gas into the customer's car. But that's a trickier process that you might think. For instance, what if the customer doesn't know when to stop pumping? Will he or she get soaked in a potentially lethal eruption of runaway fuel? Let's find out on the next page.