Measurement of the flow at low fluid flow rates can be a problem if attempted using conventional flow sensors which have a limited range of flow conditions for which they are applicable. Large errors in measurement can occur when the flow is outside this range. In particular, flow meters based on flow velocity such as turbine meters cease to operate as there is insufficient energy in the fluid to rotate the wheel. Differential pressure flow sensors can at times operate at low flows, but the smaller flow orifices required for low flows have been prone to obstruction if there were suspended particles in the fluid. Also, pressure drops across the orifices could be significant.
In response, a number of compact microflow meters have been engineered specifically for low flow rate, small pipe diameter applications.