Ferrules may be utilized to form leak-free fluidic couplings between two components. Ferrules may be employed in applications entailing small-scale fluid flows, such as analytical instruments and microfluidic devices, and thus may be sized to join a small-bore conduit such as a capillary tube or fitting with another component, or to create a sealed connection between two conduits with the use of a union or tee connection. The coupling may be established by a solid-to-solid seal that is secured by mating two surfaces together, one of which is an outer surface of the ferrule. The coupling may be formed under mechanical compression achieved by applying torque to a compression nut or equivalent component such that the nut bears against the ferrule. Depending on design, torque may be applied manually (i.e., hand-tightening or finger-tightening) or with the aid of a wrench or other tool.
In a typical example of a conventional fluidic coupling utilizing a ferrule, a conduit is inserted through the bore of the ferrule and the conduit and ferrule are inserted into the interior of a union or other structure with which the ferrule is to form a sealed interface. The conduit also passes through a compression nut. The nut is threaded onto the union and rotated (screwed). Rotation axially translates the nut directly into the contact with the ferrule. Consequently, the ferrule is axially translated into contact with an inside surface of the union under a compressive force imparted by rotation of the nut, thereby creating a sealed interface between the ferrule and the inside surface against which the ferrule bears. The ferrule may also be shaped so that the compressive load also causes the ferrule to bear against the portion of the conduit residing in the ferrule's bore.
This type and other types of conventional fluidic couplings have disadvantages. A conventional fluidic coupling provides no mechanism for a user to feel and limit the torque or force developed during rotation of the nut, which may result in a wide variation of compressive loads applied by users from one coupling site to another. Moreover, depending on the system or environment in which the fluidic coupling operates, the fluidic coupling may be subjected to thermal cycling. The thermal cycling may be significant, ranging for example from −80° C. (cryogenic liquid N2) to 400° C. Thermal cycling may cause thermal expansion and contraction of the solid components, and material relaxation due to annealing. Hence, thermal cycling may adversely affect the reliability of sealing interfaces. In particular, thermal cycling may reduce the sealing pressure at the solid-to-solid interfaces and thus cause undesirable fluid leakage, requiring the nut to be retightened to reestablish the sealing pressure or in some cases requiring one or more components of the coupling assembly to be replaced. The sealing pressure is proportional to the compressive load maintained on the solid-to-solid interface. Such an interface is very sensitive to the level of contact pressure being applied because there is very little elasticity in the system. Thus, a slight reduction in pressure due to thermal cycling or other causes may result in fluid leakage.
In general, there is an ongoing need for improving fluidic couplings. There is also a need for providing a fluidic coupling that enables compression to be applied consistently, without needing to measure torque or compressive load. There is also a need for providing a fluidic coupling that maintains reliable sealing interfaces over many cycles of operation before requiring maintenance. There is also a need for providing a fluidic coupling that minimizes sensitivity to thermal cycling.