You can see the principle of the check valve in action by inserting a straw vertically into a glass of water so that the water flows upward into the straw. Once the straw is partially submerged, place the tip of your finger on top of the straw, forming an airtight seal, and then lift the straw back out of the water. The water will remain inside the straw, even after the straw has been completely removed from the glass, and will only flow back out when you release your finger. In effect, your finger has been serving as a check valve on the straw.
The Check Valve
The route that the gas takes from the tanks to the aboveground dispenser isn't terribly complicated, though it may take a few minor twists and turns. When pumping is complete and the pump motor is turned off, the gas inside the pipe doesn't simply fall back into the tank. Instead, it's held inside the pipe by a check valve. The check valve, which is located above the gas inside the pipe, creates an airtight seal above the fluid. Although the bottom of the pipe remains open, the vacuum pressure created by the check valve holds the gas in place. This is a process known as keeping the prime.
Using a check valve to hold the gas inside the pipe prevents unnecessary wear and tear on the suction pump and assures that a supply of gas will remain in the pipe so that the next customer won't have to wait for it to be drawn all the way up from the tank. It may not seem like a big deal, but the process can take 10 to 15 seconds. That isn't a very long wait by any means, but it can be an eternity when you're waiting for gas to be pumped.
The power that drives the pumps usually comes from the same electric grid that powers the lights and appliances in your home, though a few states require that service stations maintain a backup power supply in case of power failure.
Now that the gas is on the way to the car and it's time for the customer to start pumping, how does the dispenser know just how much gas the customer has pumped? Considering the volatility of gas prices these days, that may be the only thing the customer may care about. Find out the key to this mystery on the next page.