Glass tube flow meters are instruments used to measure the rate flow of fluids (such as gases and liquids). The rate of fluid flow is measured using glass tube flow meters through the use of a float (e.g., a small glass bead) lying within the tube. As the fluid enters the tube, it will push the float up the tube, with gravity counteracting the fluid's upward motion. Once the two forces equalise, you can measure the rate of flow directly on the scale in a few simple steps.
Connect the glass tube flow meter to a source of fluid flow (e.g., a garden hose).
Turn on the fluid flow source, and watch the float rise within the tube. Once the float has stabilised, record the value from the measurement scale. If you have a direct reading tube, the value you record is simply the fluid flow rate, in units specified by the measurement scale (e.g., litres per second).
If you have a reference scale tube, the measurement scale is in arbitrary units on a linear scale. In this case, calculate fluid flow rate using a reference scale flow correlation table, which is usually supplied by the manufacturer of the glass tube flow meter.
The value of the fluid flow will affect the type of glass tube flow meter you use. A small flow meter will be unable to measure a high fluid flow rate, as the float will be pushed past the measurement scale. For the float to stabilise within the glass tube flow meter, the fluid flow rate must be constant. If the flow rate is not constant, the bead will constantly move around.