Differential Pressure flow meters measure the velocity of fluids by reading the pressure loss across a pipe constriction. These meters can contain laminar plates, an orifice, nozzle, or Venturi tube to create an artificial constriction. Highly sensitive pressure sensors measure the pressure before and after the constriction. According to Bernoulli's principle, the pressure drop across the constriction is proportional to the square of the flow rate. The higher the pressure drop, the higher the flow rate.