I have a 5000 ltr overhead tank atop a 20 meter tower. I wish to make a liquid level indicator, mechanical type, which would indicate to me the liquid level therein the tank, as the tank is made of opaque material. The indication to be at the foot of the water tower or closer outside my kitchen window for viewing convenience from within my house.
Probably the simplest indicator would be to have a float that raises or lowers a rod of some kind. In this setup a float is attached to a vertical rod that goes through the ceiling of the tank. The rod then angles horizontally to the right to the side of the tank, then down vertically towards the ground. You would then build an indicator with lines painted for every unit of height of water in the tank. As the water level raises the float goes up, lifting the rod assembly, and as water us used the whole assembly drops.
The other mechanical way I can think of would be to use water pressure to indicate the amount of water in the tank. Assuming this is gravity fed you could install a pressure gauge on the main water feed and then calibrate the pressure levels with the water levels.
There are other mechanical ways to do this, however if it were me I'd go with an electronic solution of some kind rather than a mechanical one. You could install a low power (even solar powered) electronic system with some LEDs to show your water levels. You could even install a webcam in the tank itself and measure water levels based on lines painted on the inside.
Install a small pulley near the top edge of the tank. String a cord through the pulley and attach it to a float that is in the tank. On the other side of the cord connect to a small weighted object that serves a highly visible pointer that slides up and down in front of a vertical scale that you mount to the leg of your water tower. You could add a eye bolt near the top of the scale to guide the cord and keep it in position when there is some wind activity. The pointer at the lowest position indicates a full tank.
The float and counterweight are in the tank attached to pulleys. the indicator is attached to the counterweight as well and helps to balance the weights. From the diagram you can complete the project. Some water must be added to the float so as to balance the weight of the counterweight. This is trial and error until you get it right.