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As a general rule, stainless steel fittings are interchangeable initially, prior to the ferrule of the fitting being swaged into place. However, also as a general rule, once a stainless steel fitting has been swaged into place on a piece of tubing, it is best to use only that fitting with the mating part into which it was initially swaged.
The reason stainless steel fittings are not interchangeable after they have been swaged into place is due to what we will term “Dimension X.” This “dimension” refers to the length of tubing extending past the ferrule of a fitting that allows the tubing to have a flush connection at the bottom of the union or adapter into which it is being tightened. Depending on the manufacturer, this dimension can be longer or shorter than others. Keeping this in mind, let’s look at a couple of scenarios that might help explain this a little more in depth.
First, assume you have swaged a stainless steel nut and ferrule into a manufacturer’s union where Dimension X is long. Now, you remove the fitting from this union and try to attach it to another manufacturer’s union, where the requirements for Dimension X are short. Now your tubing extends past the ferrule too far, and no matter how much you tighten the fitting, you will never be able to create a seal between the outer surface of the ferrule and the inside surface of the union
In the opposite scenario, you swage a fitting into a manufacturer’s union where Dimension X is short. You then try to move this to a union where the required Dimension X is long. You are able to get a tight seal with the fitting, but you have introduced a dead volume chamber between the end of the tubing and the bottom of the union. Depending on the location of the union, this dead volume space can lead to remixing of your sample components and extra turbulence, among other problems.
Once again, as a general rule with stainless steel fittings, always use the same mated fitting initially used to swage one of these fittings in place.
If you are in a situation where you need interchangability, we recommends Fingertight Fittings. With Fingertight Fittings the ferrule does not permanently swage.
Please see our Fittings Primer for more information.