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.
If you would like to compare a bit, here is information on air-hose.
One difference between compressed air tubing and air hose is how the size of each is measured.
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.
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.
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.
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.
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 colour, 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.
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.
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.
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.