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One pump, three grades: How does it know?

So you’re at the gas station, filling up your car. You have a choice of regular, mid-grade, or premium fuel. Three buttons — but there’s only one fuel nozzle. How does the pump dispense three grades of fuel through a single hose?

“There are two fuel tanks in the ground,” says Glenn Phillips, service technician for Albert’s Gas Station Maintenance in Cambridge, Ont. “It blends at the top of the pump.”

You’ll find that at most stations, the octane ratings — the “grades” of gasoline — are 87, 89, and 91. Buried under the ground are huge tanks, as large as 45,000 litres each, which hold 87- and 91-octane fuel. The gas pump has two meters. If you select 87-octane, the pump turns on that meter and dispenses from the lower-octane tank.

Conversely, if you want 91-octane, that meter turns on and dispenses from that tank.

Should you want 89-octane mid-grade, the pump blends the two 50/50.

“On top of the meter, there’s a ‘pulser,’ and it counts the volume of fuel you’re getting,” Phillips says.

“If you buy 20 litres of mid-grade, you’re getting 10 litres of regular and 10 of super. If it sees that you’re getting more regular than super, it’ll open and close valves to compensate and adjust. It’s constantly monitoring to the blend ratio.”

The fuel nozzle has either a small hole in it, or a piece of metal on it, which reacts when fuel backs up against it in your vehicle’s filler neck, indicating that the tank is full. The pump then automatically shuts off. You should always operate the nozzle yourself and never use the gas cap or other object to hold it open, Phillips says.

“If there’s a spill, you can actually be held liable for the cleanup,” he says. “Think twice before you use something to hold it open.”

The pump compensates for temperature, “so every litre that comes out is calibrated as if the temperature was 15 C,” Phillips says.

“In the summer, you actually get a little less gas, and in the winter you get a little more, but the difference is pennies, and it only makes a difference if you’re buying millions of litres a year.”

The pumps are calibrated once a year. And as for the price of gas? “Please don’t take it out on the attendants,” Phillips says. “It’s really not their fault.”

Did you know?

The pump hose is always full and contains about half a litre of fuel, so the first 500 millilitres will be whatever grade the last customer purchased.

Fuel pumps contain filters to screen out any dirt. Even so, avoid buying gas right after the fuel tanker has filled the underground tanks, since it stirs up any sediment on the bottom.

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