When I was young, I worked at a gas station. Procedure is to regularly measure the level of fuel in the tanks with a very long stick (a 14 foot long ruler basically) and cross reference that with the tank's measurements to get the volume of the tank- it should go down the same number of gallons as you sold.
You also coat the end of the stick with a paste that detects water (i.e. turns red). The water settles to the bottom. You can have a bit of water down there and not harm anything, because the pump pickup is a few inches above the bottom of the tank.
BUT.... remember that when a load of gas is being delivered, it churns the bottom of the tank, mixing water with gas. Rule of thumb should be to NEVER purchase fuel from a station that is getting a delivery. Never never never.
You also coat the end of the stick with a paste that detects water (i.e. turns red). The water settles to the bottom. You can have a bit of water down there and not harm anything, because the pump pickup is a few inches above the bottom of the tank.
BUT.... remember that when a load of gas is being delivered, it churns the bottom of the tank, mixing water with gas. Rule of thumb should be to NEVER purchase fuel from a station that is getting a delivery. Never never never.
