Compressed air tube is an inexpensive and quick to install compressed air supply line for almost any compressed air using application.
On earlier pages I’ve talked about the various types of fittings for plumbing your compressed air to your tools and cylinders and other air-using applications.
Now, let’s talk a bit about the air tube that you will use these compressed air fittings on.
What’s The Difference Between Air Tube & Other Air Lines?
What’s the difference between air tube and air hose?
One difference between compressed air tubing and air hose is how the size of each is measured.
Measuring Tube Size – remember TOD
The ‘T’ in TOD stands for TUBE.
The O.D. in the acronym TOD stands for outer or outside diameter.
So, you measure compressed air tube by its outer dimension, not the hole in the middle.
A 1/4″ poly tube (the compressed air industry workhorse air line) will be exactly 1/4″ on the O.D.
Want to check?
If you take a 3/8″ hose and then you take a piece of 3/8″ P.E. (polyethylene) tube, the 3/8″ P.E. tube will slide inside the 3/8″ hose perfectly.
The same for 1/4″ tube into 1/4″ hose, 1/2″ tube into 1/2″ hose, etc.
Air tube applications
The vast majority of air lines used in industry are polyethylene or polyurethane tube.
Polyethylene is the most common as it is least expensive, easily obtained, usually is purchased in 100 foot rolls, is easily cut, and quickly attached to an instant type fitting.
If you are contemplating doing a lot of compressed air plumbing, talk to your supplier. You can purchases compressed air tube in 1000′ plus rolls if you need that much.
Air Tube For Tools?
I would not normally hard poly air tube to supply an air tool. The fittings used for poly tube do not lend themselves to any kind of sideloading with a hard or rigid tube.
An air tool is in constant motion, and in moving an air tool to do work, you would be sideloading the tube-to-fitting connection all the time. It might not be too long before the connection failed or started to leak.
Hard poly tube does not take to bending in a too tight coil. The right poly tube for an air tool is the “coil hose” type which is designed to stretch as the tool is being used, and have fittings suitable for the side loading using an air tool commonly imparts to the connection.
Polyurethane is sometimes used to plumb air to valves and air cylinders too, as it is softer, more malleable and able to be bent in tighter circles without kinking.
It is also more expensive than polyethylene tube.
Polyurethane also may have some leakage or “blow off” issues when used with some types of instant fittings that aren’t designed for a soft-surfaced air line.
The ‘natural’ polyethylene air tube is rated to handle industrial air pressure in the 120+ PSI range in normal ambient temperature environments.
Colored Poly Tube
Colored forms of P.E. tube get more expensive and are sometimes harder to find. Why consider them, then?
Plumbing a machine with dozens of air valves and air cylinders with colored tubing makes problem diagnosis easier on a larger machine. For example, you might consider making the cylinder extend air lines one color, the retract another. Or, if you have pneumatic logic on a machine, you might make all of the control air lines one color, and the power air lines another.
In this manner, it’s easy to trace a specific air line from the source to its air component, and thus make it easier to determine if there is a problem with air flow to that unit.
As I mentioned earlier, you normally use poly tube with instant type fittings, though sometimes mini-barbed type fittings are used as well.
The mini-barb is just as it sounds. One side has a barb that is sized to fit inside the tube. The other side has a fitting to connect to the boss. When using a mini-barb on tubing, typically hose clamps are not used. The “sticktion” inside from barb to tube is normally sufficient to keep the barb inside the tubing.
The instant fitting, into which you insert a square-cut end of poly tube, has some type of “O” rings inside it. The “O” ring seals on the outside of the tube to render the tube-to-fitting connection air tight.
This is why an accurate O.D. is critical for tube. Once I blamed a vendor for a batch of fittings that I thought were out of specification. Further investigation revealed that the whole lot of poly tube was undersized by a couple of thousandths of an inch. That was enough to allow continuous leaking in the tube-fo-fitting connection, and even allowed “blow off” of the tube from the fitting periodically.
Inside the tube orifice of the instant fitting there will also be a ring with teeth, a collet. When the tube is inserted, these teeth penetrate the surface of the tube lightly to hold the tube in the fitting.
When compressed air is introduced to the poly line, it expands the poly tube slightly, and the teeth grip even harder preventing air line blow out.
The use of an instant fitting then requires a certain surface hardness of the tube to work properly.
Air line tubing that is quite soft or stretches easily may be better installed with a fitting specifically for them.
Since with softer tube the tube O.D. will actually lessen as the tube is stretched for the application, or in dynamic equipment, the stretching may reduce the O.D. of the tube sufficiently for blow off to happen.
Tube To Fitting
Since you measure the tube on the O.D. then the mating connecting hole on the fitting must be the same size as the tube.
A1/4″ tube will have an actual 1/4″ mating size hole in the fitting.
Over recent years, the “instant” style fitting has become the predominant style of fitting to connect air lines in the 10-32 through the 1/2″ tube size.