For those underground fire protection water supply tank installations using an ETT Site Vent assembly to indicate water level, it is important to remember that the site vent simply shows "full" or "not full." The float that you see in the site glass does not go to the bottom of the tank. The green and red markings are just a quick way to see if the tank is full or not during an inspection. These site vents work great, we have installed several of them and they are easy to use. Just remember, if you see red, the tank is not full and you will need to further evaluate the situation. Simply add some water to the tank until the vent overflows or you see the "green" indicator.