What is the purpose of the vent on your cement scale anyway? Remember when you were a kid and you would stick a straw into your drink and put your finger over the top and pull it out and the liquid would stay in the straw until you let your finger off? The same principle is at work with the vent in your cement scale. The reason the liquid didn’t leave the straw is because you were blocking the path for air to replace the liquid. Once you removed your finger, you allowed air to rush in to fill the void left by the liquid leaving the straw.