Rotameter is most commonly used device for flow measurement. Great representation of this device is the result of its simplicity, wide range of applications and large measurement range. The measuring tube has a variable cross-section, closest at the entrance of the tube, and widest at the exit of the tube. Fluid (in this case, chlorine gas) flows through the ring between the measuring tube and ball, and depending on the gas flow, ball is lifted to a certain height. They are made in capacities up to 20 kg/h.