When considering about compressed air lines, know that the least expensive and most user friendly compressed air line is polyethylene tube.
Polyethylene tubing is available in a host of diameters, is relatively heat resistant, has a burst strength suitable for typical compressed air systems and is relatively low cost.
Polyethylene tube is connected to the compressed air valve or air cylinder by a fitting.
There are a host of fitting styles available, from the “old fashioned” nut & compressed-ferrule type, to the more modern “instant fitting” like the ones shown in this next image which are from the Camozzi fittings selection.
A typical instant fitting will have a thread on one side – to thread the fitting into the air valve or air cylinder, and an opening on the other, into which the polyethylene tube is inserted.
Inside the tube side of the fitting will be an “O” ring and a retaining ring, through which the tube slides. The teeth on the retaining ring “bite” into the surface of the tube to prevent it from sliding back out of the fitting, and the “O” ring prevents compressed air blow by.
There will be a ring on the outside of the fitting which, when depressed, distorts the retaining ring teeth allowing the tube to be removed. This is rarely easy, particularly if the tube has been in the fitting a long time.
Since the fitting seals on the outside surface of the poly tube, the O.D. of the tube must be within a certain tolerance. I have had very few instances where purchased tube was undersized to the point where it could not be used in an instant fitting, but I have had it happen.
Polyethylene tube is measured on the O.D. Remember the acronym “TOD”. TOD stands for – Tube O. D. to help recall that when you are buying fitting for poly tube, you want one sized to fit the tube’s outside dimension.
Don’t forget the thread size. Make sure it’s the right NPT or metric thread to fit the port in your air valve or cylinder.