Liquid is forced through a tube of constant cross-section and precisely known dimensions under conditions of laminar flow. Either the flow-rate or the pressure drop are fixed and the other measured.  Knowing the dimensions, the flow-rate can be converted into a value for the shear rate and the pressure drop into a value for the shear stress.  Varying the pressure or flow allows a flow curve to be determined. When a relatively small amount of fluid is available for rheometric characterization, a microfluidic rheometer with embedded pressure sensors can be used to measure pressure drop for a controlled flow rate.
Capillary rheometers are especially advantageous for characterization of therapeutic protein solutions since it determines the ability to be syringed. Additionally, there is an inverse relationship between the rheometry and solution stability, as well as thermodynamic interactions.