Throttle pressure drop devices are working according to Bernoulli's principle and measure the average flow rate and the mass flow. They are operated subcritically.
According to the Bernoulli equation the mass flow follows the differential pressure and the density in a square root relationship at subcritical flow by an orifice, nozzle or venturi tube. A range of flow of 1:4 consequently requires a differential pressure range of 1:16. Due to the square root relationship between flow and DP it has to be made sure that in case of a low flow of, e.g., 10% of full scale, a differential pressure of approx. 1% of full scale is not exceeded as maximum measurement.