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What is a fitting? Fittings are small parts used to connect, adjust, or adapt other parts in a pipe/tubing system. In the analytical world, it will usually be screws and ferrules that are referred to as fittings.
Which fitting do I have in my system? There are 5 things that define a fitting:
Different threads: 1/4-28: the first number indicates the diameter of the thread and the second number tells how many threads per inch.
10-32: this is similar, but when below 1/4", gauge numbers are used, so gauge 10 equals 3/16", and there are 32 threads per inch.
M6x1: the first number again is the diameter of the thread, but in millimeters. The second number is the millimeter pitch between threads.
The holding part – the ferrule: Ferrules come in all shapes and sizes, but the similarity lies in the tapered end designed to grip around the outer wall of the tubing – external compression. Low-pressure ports often have a flat-faced configuration. Ferrules used in these types of ports have the tapered end facing the screw and away from the port.
Historically, low-pressure ports often used flanged tubing where the flange provided the seal. Because flanged tubing has certain disadvantages, ferrules as described above are now often used. These ferrules allow the tubing to be secured in the port without flanges, so they are called flange-less ferrules.
High-pressure ports, on the other hand, end in a conical seat just below the thread. For high pressure, the tapered end of the ferrule faces the port. PEEK and steel fittings are commonly used in high-pressure systems, while fittings made of softer materials like ETFE and PP are used in low-pressure systems.
FREQUENTLY ASKED QUESTIONS:
Is it possible to switch from metal to plastic fittings? Yes, metal fittings are often only used because many believe they have to, since the equipment was sold with them. This is actually why metal fittings remain popular today. Plastic fittings are cheaper and more chemically resistant. Always check that the pressure rating matches your system.
It is possible to use a plastic fitting on a metal tube and metal port. The opposite is not possible, as metal fittings can damage both the tubing and the port in the valve.
How is dead volume avoided when using capillary tubing in a standard 1/16” port? Using sleeves avoids much of the dead volume in the port/connection, as sleeves fill the cavity/lip in the port. Sleeves come in many different inner dimensions, but the outer dimension will always be 1/16”. The definition of capillary tubing is that its outer diameter is smaller than 1/16”.
Instead of sleeves, it is also possible to use PEEKsil™, which also always has an outer dimension of 1/16”. PEEKsil is fused silica tubing encased in PEEK polymer, which makes it more robust when tightened with metal or polymer fittings. Note, however, that PEEKsil must be used in cut lengths, as permanent damage can occur if cut with standard tubing cutters.
Additionally, special ferrules for capillary tubing are available, where the bottom part is 1/16”. Note that port length can vary between equipment, so it cannot be guaranteed that the ferrule will reach the bottom of the port.
TIPS FOR LEAKAGE:
Is the fitting installed correctly? When using a fingertight fitting, the tubing must be inserted fully into the port before tightening the screw and ferrule. Remember to turn off the pump when installing fittings.
The fitting may not be tightened enough. Metal fittings must be tightened with tools, even if they have been installed before.
The screw and ferrule may not match. Using fingertight fittings prevents the ferrule from locking onto the tubing and allows the ferrule to be replaced. Fingertight fittings can be reused multiple times.
Check the condition of your screw and ferrule. Often, the screw and especially the ferrule become deformed after repeated use and no longer create the seal they are designed for.
Check the port for visible damage. Sometimes a leak has nothing to do with the fittings but with the port they are mounted on. Ports with metal fittings have a higher risk of damage.
Is the chemical compatibility correct? Using fittings incompatible with the mobile phase will cause leaks. Always check the chemical compatibility list when selecting fittings.
Download fittings 101 guide