A wide range of applications take advantage of capillary for precision control of liquid and gas flow rates.  This general application area is referred to as Mass Flow Control, and relies on several unique properties of capillary.

• The smooth internal surface of capillary results in stable laminar flow profiles and good adherence to Poiseuille’s Law (Eq. 3-1), allowing accurate prediction of flow rates.

• Capillary flow rates can be adjusted by changing either the ID or overall length.  Polymicro offers a broad range of standard products with different ID’s so that users can select one that best fits their flow rate requirements.

• Capillary can be easily cleaved, providing users great latitude in manipulating flow rates through length adjustments.

• The tight tolerances of Polymicro’s capillary provide the needed dimensional consistency, both within each lot and from lot to lot.

• Capillary, if handled with care, has excellent strength and durability. This, coupled with high temperature capabilities and chemical resistance properties, allows for usage in harsh and challenging environments.

• Combinations of capillary are often utilized in flow splitting devices, with LC/MS interfaces being a classic example.

Capillary is a robust solution in any application that demands stable, reproducible control of fluid or gas flow rates. Insertion of a segment of appropriate length and ID generates a fixed restriction, given that the other key parameters of viscosity and pressure drop are controlled. Tips on estimating the expected flow rate are discussed in this chapter (see Estimating the Flow Rate in Capillary).