A manometer quantifies the difference in air or liquid pressure by comparing it to an outside source such as a sample of Earth's atmosphere. It works when liquid is inserted in the tube like mercury and the other end of tube is filled with gas then sealed. The liquid gets balanced in the lower section of the U depending on the strength of the gas. Then measurements are taken to see how far the liquid in the sealed end has been pushed either underneath the point of the liquid in the open end or above it. In simple terms A manometer works by measuring the pressure of liquid or gas per inch in a U shaped tube. This lets you know when you have ran out, or have a leak. Each appliance or grill you have requires a certain amount of pressure to keep running, like a propane grill.