All of these UAT-like schemes and my pleated paper clunk all work due to the same reason... the fuel's surface tension across a medium (the fine mesh bag in this and UAT's and the fine mesh of the pleated paper on my clunks) keeps bubbles from passing through, as long as enough surface (does not take much) is immersed in fuel. They all work great and if someone can sell one for $25, more power to 'em!!!