Paying 30-40 € for a heavy ball doesn't make sense. when you search for medicine ball instructables most people use sand for filling a basketball . The density of sand is 1.6 kg/liter and for saturated salty water 1.095 kg/liter. since I don't want a 12 kg ball, the medicine balls on the market are homogeneous and elastic and I don't want to cover it in duct tape , my idea was to use salty water.
The total cost for this project was under 4€
what we need for this instruments is a basketball, syringe with needle, salt and a recipient with warm water
Add salt to the water until it does not dissolve anyone.
place the needle in the valve, fill out the syringe, place it in the needle and introduce the salty water in the ball.
I left the needle in the ball because I thought that literally removing the needle 200 times might harm the valve.
You might need to let the ball loose some pressure before you start fill it and close to the end, you might have to keep the needle covered while you fill the syringe, to maintain pressure in the ball.
It ended up becoming a 5.7 kg medicine ball.