I want to measure the distance a glass hydrometer bulb drops over time in a solution of suspended solids. This distance is only about 4 inches. Since I can not modify the hydrometer bulb dramatically I thought I could add a drop of metallic paint on the top tip. Then I want to measure the distance from a fixed point to the top of the bulb/metallic paint. My resolution needs to be about 1 mm. I am looking to collect the data with some type of acquisition system.
What is the best type of sensor that could measure the vertical distance assuming the bulb might float form side to side about 1 inch max?
Thanks