3mm Tube Fittings: Sizing Tips for Ultra Compact Chromatography and Analyzers

Even small tubing decisions can count in the case of ultra compact chromatography systems and small lab analyzers. Where space is limited and flow must remain constant a 3mm Tube fittings is a popular choice. Poor connection or wrong size may cause leakage, pressure drop, and non-uniform reading, thus interfering with the test results. Most technicians end up with problems not due to the instrument, but due to poor fit of the tubing and fittings. Knowing the 3mm fittings will work within the system will prevent these minor, yet expensive, issues and keep your system running smoothly.

Ferrule grip on small diameter tubing

The ferrule grip is quite large with 3mm tubing so that the connection can be held in place. The tube is very small and therefore, a very tight Ultra High Purity Fittings can cause the wall to be crushed and a loose grip will cause leaks. The ferrule is formed to cut into the surface of the tubing to secure it sufficiently firm on the surface without obstructing the flow within it. When you fit a fitting the ferrule moves around the tube and is sat between the nut and the fitting body. It then compresses as you tighten it and grips onto the tube. The contact area on small diameter tubing such as 3mm is small so the grip must be even and controlled. When one side bites more than the other, the tube can tilt a little and this can influence the sealing. Another problem that is usually seen in laboratory installations is the re-use of tubing whose ferrule marks have already been made. After a ferrule has bit into a 3mm tube the surface is no longer smooth. When reinstalled in a new fitting, there may be uneven sealing. A common method used by technicians is to cut off a new end of tubing and reconnect. The choice of materials is also important. A soft tubing can easily collapse when pressurized, whereas a very hard tubing can also fail to hold on to the ferrule. Stainless steel ferrules are commonly combined with stainless steel or PEEK tubing in chromatography systems to provide more control. A real-life example is when a technician was working on a small HPLC system and encountered unstable pressure reading. The problem was identified after inspection of the system and it was found that the problem was due to ferrules that had been over tightened on the lines of 3mm. Tubing was good at the surface, but variation of flow occurred due to restriction inside. The readings stabilized after the replacement of the ends of the tubing and re-setting the ferrules to the appropriate torque. The best tip is to tighten the nut to the point of it becoming snug, and then turn it another turn depending on the Tubing and fittings design. This maintains the ferrule grip unstressed without stressing the tubing.

Temperature limits for PTFE and PEEK ferrules

Both PTFE and PEEK ferrules find use in chromatography and in analyzer tubing systems, however, with heat, they act quite differently. Being aware of their temperature limits will assist in avoiding leakage, loose fittings, and damage in the long run. Ferrules made of PTFE are pliable. They close easily at room temperature and can be installed easily, and thus are common in many simple laboratory systems. But they can not take heat. After reaching temperatures over about 80 C to 100 C, PTFE begins to lose its form. It has the ability to creep when pressured over a time, particularly when the system is operated under a hot condition over a long duration. In practice this manifests itself in a fitting which was initially tight but starts to leak after a certain number of heating cycles. In a lab water analysis system, PTFE ferrules were applied to a hot line and a few weeks later the operator observed that there were small leakages at the points of connection. The ferrules were somewhat softened, and no longer offered the same hold. PEEK ferrules are a great deal stronger in the presence of heat. They find application in HPLC and small-scale systems of analysis where temperature is a factor. The majority of PEEK ferrules have the ability to operate continuously up to about 120 to 250 C, based on the grade and system pressure. They remain straight and retain their form even when heated up in line of operation. This assists in keeping the seal constant, and minimizes the frequency of re-tightening. Nevertheless, PEEK cannot be used in all cases. It has the potential to creep slowly at very high pressure and heat. This makes proper torque during installation not irrelevant. Ferrell life can be reduced by over-tightening, which does not enhance the sealing. An easy rule to follow during daily lab work is to check the system temperature initially. PTFE might be sufficient in case the installation is operated at room temperature or in mild conditions. PEEK is generally the safest bet in case it involves heating or in case the analyzer has extended operation. Corresponding ferrule type to temperature conditions aids in maintaining stability of 3mm tubing systems, particularly in small instruments where a small variation can result in a rapid change in results.

Installation in tight spaces

The tightness of tube fittings can be fussy to install (3mm tube fittings are recommended), particularly in tight spaces used in chromatography systems as well as in small analyzers which use every millimeter. It is easier said than done, not only in order to get the fitting to fit, but also to do it without scrunching the tube or breaking the ferrule when your hands are barely big enough to go into the cabinet. One of the common problems with these arrangements is the lack of alignment prior to tightening. In the case of slightly off-angle tube, the ferrule can be bitten unevenly. It is very difficult to notice in narrow areas since one can see little. The easiest method to deal with this is to first pre-align the tubing outside the instrument. Before inserting the tube in the cramped area, make sure it is straight in the fitting body. Another issue is finger access. Most technicians will attempt to tighten fittings to the maximum extent immediately with the aid of tools, which may result in over-tightening. Where there is space, it is useful to begin by hand, where possible, and then complete the work with a short turn of the wrench. Tightening with short strokes is more controlled and stress on the 3mm tubing is minimized. The way the tubing is routed is important too. Excessive bending of the tube before insertion into the fitting may cause the tube to push back on the ferrule and compromise the seal. Maintaining a slight curve into the connection point will aid the ferrule to settle flat even in a tight space. We experienced persistent leaks during one project involving a compact UV detector. The tubing was crammed into tight spaces behind the panel causing it to enter at a skewed angle. The ferrules were not damaged but could not seat evenly because of the angle created when forcing the tubing into place. After adjusting the tubing run out and clipping it into place, leaks were no longer occurring and we did not need to replace any fittings. Lighting is another thing that can help. Having a small flashlight or headlamp allows you to see how the ferrule is seated before fully tightening it down. A lot of mistakes made when installing in tight quarters is due to estimation instead of actually seeing where everything lines up. Take your time. 3mm systems will magnify small errors. It's better to go slowly than to have to come back and fix a problem later.