Ghatika Yantra - Water clock to measure time

During ancient times, some people used sundial to track time while others used water clock. Indians called this water clock 'Ghatika'. To calculate time, empty container with a hole was placed inside a larger container filled with water. When empty container with hole was filled, it used to mark the passing of certain time. This sequence used to repeat.

I tried this mechanism with Aluminum bowl. Small hole was drilled and bowl is placed in water. It takes more than a minute to fill it.

Similar mechanism can be built with the help of plastic bottle. Coins are used to make it heavy. Small hole to the cap is the entry point for water.

This is a classic case where buoyant force is constantly changing. It is always equal to net weight of the plastic container - including water in it at that time. When container sinks, net weight of the container is more than the buoyant force!

Try this activity by changing the weight as well as the size of the hole.

Can you build one which will indicate when your one period in school gets over ?

Thank You.