H.I.D. is the acronym for Hose I.D. (inner diameter).

T.O.D. means Tube O.D. (outer diameter).

You identify and measure an air line tube size by it’s outer diameter. The O.D. dimension is critical as it has to fit inside a specific type of fitting made for tubing.

You measure the hose size by it’s inner diameter as the hose will fit onto the barb or male projection of an entirely different style of fitting than the tube.

The T.O.D. versus H.I.D. phenomenon is pretty easy to demonstrate. If you take a 3/8” RVC (rubber vinyl compound) or as it’s also commonly known - PVC (poly-vinyl chloride) type of 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.

Be particularly careful when speaking with folks about air line hose. First, make sure they are talking about hose and not tubing, then you have to clarify where the dimension they gave you came from.

Often, the technician will give you a hose dimension as 5/8” or 3/4”, but they will be talking about the hoses O.D. If you size the necessary fittings based on that information, they won’t work, and you’ll be making another trip to the tool crib or the industrial distributor for correct ones.

If the hose, you need to know the I.D. to ensure that you get fittings that are sized correctly.

The vast majority of the air lines supplying compressed air through valves to cylinders or other actuators on equipment are polyethylene or polyurethane tube.

Polyurethane is softer, is more malleable, able to be bent in tighter circles, but may have some leakage or “blow off” issues when used with some types of instant fittings.

The ‘natural’ polyethylene tube is rated to handle industrial air pressure in the 120+ PSI range in normal ambient temperature environments, (this depends on the manufacturer, and you need to check to be sure) and it normally works very well with instant fittings, is readily available, and is inexpensive.

Colored forms of P.E. tube get more expensive and are sometimes harder to find.

Yet plumbing with colored tubing makes problem diagnosis easier on a larger machine. You might consider making the cylinder extend air lines one colour, the retract another. In this manner, it's easy to trace a specific air line from the valve to the actuator.

Or, make the signal air tube different in color from the power air to differentiate them from the power air lines.









Other tube materials are available besides polyethylene and polyurethane such as Nylon, copper, stainless steel etc.

If you are planning on using instant fittings, remember that the fitting has "O" rings inside it, and the "O" ring seals on the outside of the tube inserted into the fitting.

This is why an accurate O.D. is critical for tube. Once I blamed a batch of fittings for being out of spec, and it turned out that the poly tube was undersized by a couple of thousandths, enough, as it turned out, to allow a continuous leak and "blow off" of the tube from time to time.

Inside the fitting there will be a "ring" with "teeth" inside the tube orifice on the fitting. When the tube is inserted, these teeth penetrate the surface of the tube slightly to hold the tube into the fitting. When air is introduced to the line, it expands the poly tube slightly, and the teeth grip even harder.

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 different style of fitting as the tube O.D. will lessen as the tube stretches, and it might pop out of the fitting.

Standard P.E. tube (with an accurate O.D.) is appropriate for most if not all instant type fittings.

If it’s a P.E. tube that you are using for an air line, the fitting style can be;

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. Once again, selecting the right tube O.D. for the fitting is critical, as “instant” style fittings seal on the O.D. of the tube.

The same can be said for the compression style of fitting, which incorporates a device to seal the air line into the fitting based on the O.D. of the tube.

The exception is the barbed fitting for tube. This is quite a small fitting, and the air lines using this style are usually destined to be “signal” or “control” air rather than “power” air. Ergo, the flow requirements are low. If the fitting you’ve selected is a barbed fitting for tube, you must ensure that the barb size will fit the I.D. of the tube selected.

Different manufacturers of tube may have different wall thickness, even though the O. D. should be standard in dimension. You will either have to obtain the tube wall thickness information from the manufacturer, or a “hole size” chart to see if it will accommodate the barbed style tube fitting.

If it’s a hose that you are connecting to your fitting, usually it will be a barbed style, often in conjunction with a “quick connect” device, also known as a “coupler – connector”.

If your fitting has an NPT thread on one size, and a “barb” on the other, usually the manufacturer will identify the barb as being a specific fraction, and that is the size that will correspond to the I.D. of the hose.

Hose is often used as air supply on large pieces of equipment with high flow applications for power air.

Often you’ll see PVC hose used as a sort of extension cord for air. Either end of a length of hose will have a quick-connect of sorts that will ‘mate’ with similar connectors on air mains or on equipment, or to another hose. This allows an operator to ‘plug’ the air hose into an air source, and ‘plug’ the other end into a tool, quick-connect to a work bench, etc.

Usually one end of the air "extension" cord will have a "checked Coupler” so that when air is introduced to the line, it can't escape from the Coupler, until it fitting is connected to a piece of equipment or another air extension cord. This depresses a pin inside the coupler and allows compressed air to flow.







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