The description is very vague.
- What is the ball size?
- What is the diamater of the glass tube?
- What kind of steel does the ball consist of?
According to the picture it isn't a tube, but a bottle.
The standing liquid in a bottle is never of equal temperature, not only considering height, but also concidering the air flow around the bottle which adds to errors.
So a steel ball dropped into it won't descend straight down, but also tumble sideways which will result in erroneous speed measurement.
Using soft steel (not stainless steel!) the ball can be magnetized (and maintain magetized for some months) and then use salvaged Hall sensors of a worn out DVD-burner motor drive. Depending on the magnetic field strength of the ball and the distance the sensors are arranged at the tube the position of the ball can be pinpointed.
That is a practical, cheap and accurate way.
Of course, if your budget allows you might use a high speed video camera (100,000 frames per second) which is normally used at companies developing bullet proof glass to study the destruction caused at impact and the shock wave influence.
Boncuk