Pipe is most always in the form of round and it's thickness is called schedule, schedule 80 is thicker wall than schedule 40. Those are the two common pipe schedules or wall thickness. Pipe is hard to understand for an advanced mechanic because the measurements if read on a digital caliper do not correspond to the exact size of the dimension numbers. For example a pipe is measured by inside diameter and a schedule to determine it's outside diameter or circumference. As seen in the pipe chart link below a 3/4" pipe rating is around that number somewhat. It is simply a number closest to 3/4" and not exact.


Pipe Chart Link

You can see by the link size chart for pipe that the 3/4" pipe is  around .820" I.D. and 3/4" is actually .750" and that's a mile in tolerance to metal parts. Also if you notice the schedule 40 thickness on each size pipe is different so that makes it confusing also to a mechanic. Mechanics need to work within a few thousandths of an inch, .003" thickness of a human hair. A piston in an engine only has .0015" or 1/2 diameter of human hair otherwise at around .003" it will cause a rattle noise over the .0015" clearance piston.


Pipes are not common to equipment, tubing and rubber hoses are.






Tubing comes in round, square and rectangle, it has a specific size that is measured on the outside surface or diameter. A 3/4" round tube measures .750" around O.D. A 2" square tube measures 2" on all outside sides. A 3/4" x 1" rectangle tube measures 1" on two sides and 3/4" on the other 2 sides. All tubing is ordered based on the outside dimensions. The thickness of the tube is called gauge but that also can be measured with a digital caliper. In the case of a round tube 3/4" is the O.D. and if the wall thickness is 1/16th inch or .0625" decimal equivalent the thickness is doubled because of 2 sides and comes to 1/8". So the tube would come out to .750" minus the 1/8" so the I.D. (inside dimension) of the tube would be really close to 5/8" or .625" decimal. 



An easy way to determine if you have pipe or tube is by the very close measurements using a digital caliper. Pipe is round and tubing also comes round so sometimes out in the field or workshop an advanced mechanic needs to determine exactly what type of metal conduit he or she has.


NOTE: Conduit is a generic name engineers and mechanics use to broadly detail a means to conduct gas, air or liquid. Like a wire is to electricity so is the conduit to fluid and air. A means for containment.


   Pipe (round only)    -   Measurements    - O.D. Measurement



Use the pipe size chart link above to establish the size of your pipe and determine if you are working with pipe or tubing. Use the teeth (top short) of the caliper for the I.D. and the jaws (lower longer) for the O.D. For the schedule # or wall thickness the jaws can be used or the I.D. measurement can be subtracted from the O.D measurement and divided by 2 to find the wall thickness.

          Tubing                         Hose             Hose measurement 


Tubing is ordered by it's O.D. and you as the customer make the choice on the I.D. by specifying the gauge or thickness you need. For example an advanced mechanic will use tubing material to make very accurate bushings to fit specific bolts tightly. If I have a bolt that is 3/8" O.D. and need a strong tight fitting bushing I would order 1/2" tubing with a wall thickness of 1/16th inch or .0625" decimal. This would give me an I.D. of .375" or 3/8" which my bolt will fit snug.



Tubing is also used for hydraulic lines and building things out of metal which the pipes are generally used for low pressure gas or water lines and for other fluids also. Very important is to determine in fact if you are working with round pipe or round tubing.



Rubber Hose



Rubber hose is ordered by the inside dimension or I.D. and depending on the quality and pressure needed to contain inside that determines the outside dimension.



So pipe is ordered by inside dimension schedule 40 or 80 and does not have exact measurements outside or inside regarding the 1/16th method of measurement. Pipe is used for low pressured fluid or gas not exceeding several hundred pounds of pressure and it usually has a seam where it is welded together and protrudes on the inside diameter of the pipe.



Tubing is ordered by outside dimension and has an exact size with slight amount of tolerance. The inside dimension can be calculated based on the thickness of the wall. Normally used in high pressure fluid or gas applications, bushings, building metal objects and is also used to cool down hydraulic systems by convection.

Pipe / Tube / Hose - conduits to flow fluid, air or gas.


The mechanical function of a pipe tube or hose is simply to transport fluids and gasses without loss of either. They are conduits for those elements to travel within and be used in a circuit which allows either fluid or gas to travel from a reservoir to a work load and then back to reservoir. The circuit is part of an entire system that translates horsepower from the primary mover to a pump of some type either fluid or gas and through conduits that supply valves the fluid or gas and distribute them according to whichever valve is allowed to pass the fluid or gas (open). The purpose is to produce work or load.



Without the use of pipe tube or hose we could not use the hydraulic fluids or pneumatic gas or air. It is vitally important to understand the specific uses of a pipe, a tube or a hose, they all three have different applications.



To understand a little more about how pressure is developed in a circuit of conduit the Hydraulic page of this site can be referenced for a more in detailed description. Each pipe tube or hose has a rated running pressure maximum, and a burst pressure rating. For example a small plastic hose could take possibly 20 pounds per square inch (PSI) of pressure, a pipe possibly 800 PSI, a tube possibly 10,000 PSI and a heavy steel braided wall of a hydraulic hose 15,000 and so on. Pressure determines the ability to work with heavy loads so the choices are simple to determine what to use in a conduit needed situation.


A rubber hose is easy to replace and bends around corners without using special bending tools, most steel tubing get's replaced here because of rusting. Steel tubes are used in hydraulic systems to dissipate heat from the oil as well they take up less room and can be bent to fit perfectly in a situation. It's a choice made by the operator of the equipment. It is very difficult to bend pipe so those are usually straight sections and can only contain lower pressures than the tubing usually because of the fittings on each end of the pipe. They are threaded with a taper or straight called pipe thread which limits the choices or pipe when high pressure is used, when pipe is made there is a seam in the inside bore so pipe is good for water lines and lower flow applications.



It is important to understand that pressure ratings are per square inch where each square inch the fluid or air is exposed to it exerts the same exact force onto. For example a simple metal barrel that holds oil is filled with fluid, on the top of the fluid is a gap of air between the top of the fluid and the inside surface of the lid or top which is sealed with the cap. Essentially no pressure exists in the barrel at this point until air is pumped into the barrel at a volume to allow one pound of pressure to be contained and acting on the inside surfaces of the barrel. The top is exactly 20" diameter so the surface area would be 20 / 2 = the radius @10" - pi 3.1416 x R squared (10 x 10 = 100) would be equal to 314.16 pounds of pressure in force reacting on the inside of the barrel lid.



Fluid and air work exactly the same, it's how much exposed surface is acted on by the fluid or air, it is equal in all directions inside the conduit. What this amounts to is the more pressure needed to perform the work the thicker wall needed for strength in a pipe tube or hose.



A small tube with a wall thickness can handle more pressure than a larger diameter tube with the same wall thickness. Larger ID means more exposed surface area and more pressure exerted on the walls of the pipes, tubes and hoses. The same applies to all the other components within the system.