The 12oz. Gats Jar fuel testers are used to collect, pre flight inspection, fuel samples. It then separates any co-mingled fuel and water, by allowing only the fuel to be poured back into the tank through our special built-in filter which traps the water behind. This allows the fuel samples to be returned to the aircraft instead of being thrown on the ground. After the water is trapped in the container, and the fuel is back in the tank, the captured water can then be thrown on the ground without polluting the soil, and the ground water with leaded fuel.Studies indicate that over 3 million gallons of AVGAS gets poured on the ground in this country every year just by sampling fuel during pre flight inspections. This product is the only "environmentally responsible" fuel testeron the market. It also filters out particulate fuel contamination, and makes octane recognition simple and certain.