A tank blanketing valve in which a biased piston is positioned to control the flow of source gas into the tank. The piston is biased by a spring and a pilot gas stream from the source gas (the biasing pressure of force) to close the tank gas inlet. The pilot gas stream is controlled by a pressure sensing component of the valve, whereby the pilot gas stream is directed away from the biased piston when the pressure in the tank is less than a predetermined pressure. This reduces the biasing pressure which is holding the piston in the gas inlet line to the tank. The pressure of the source gas on the piston in the source gas inlet line then exceeds the biasing pressure and causes the piston to slide open and admit source gas into the tank. When the pressure in the tank reaches the predetermined level the pressure sensing component responds and allows the pilot gas stream to be redirected to the piston and thereby to combine with the biasing pressure of the spring which then exceeds the pressure of the source gas in the inlet, forcing the piston to close off the inlet to any further flow of source gas into the tank.