Some of my friends and relatives are on wells that have sulfur in them. Using this water in your house eats away at copper and electronics, so it is common to get water hauled in and store it in a tank underground. However how to tell how much water is left in the tank requires some kind of gauge or dropping down some kind of measuring stick, or you risk running out of water.
Retrofitting a tank that did not have a gauge when the tank was buried can be a challenge. These tanks generally have a fill pipe with a cap and a vent pipe that has an elbow pointing down. I got something labeled a rain tank gauge from Amazon and with some additional plumbing pieces added it the vent gauge. This is a very simple mechanical gauge that has a string with a float that hangs below it. As the float goes up and down the string winds the hand on the gauge. The empty and full marks are just pointer hands that you set when you figure out the levels for your particular tank.
In this particular application, the gauge is next to the sidewalk to the door the is used every day so it is easy to monitor.