The drip irrigation industry does not currently maintain standards concerning the various sizes of irrigation tubing. When you hear tubing referred to as 1/2", 3/4", 5/8" etc., these designations are just generalities. For example, many manufacturers produce 1/2” tubing (also referred to as "poly tubing" because it is made from polyethylene), but the outside diameters (OD) and inside diameters (ID) can be quite different. In fact, 1/2" poly tubing can refer to at least 3 different tubing sizes.
This lack of standard sizes often results in incompatibility with tubing and fittings between manufacturers. If you purchase all of your tubing and fittings from the same manufacturer, you can be certain that they will all be compatible. However, If you have an existing drip system and you want to add to it, you'll need to ensure that you get the proper sized tubing and fittings, or get the correct reducing couplings to mate the systems. In contrast, smaller diameter 1/4" tubing also comes in different sizes, but even different sizes of 1/4" tubing will usually work together.
To match your fittings to your tubing, you'll first need to obtain the OD and ID of the tubing. The OD and ID is usually listed in thousandths of an inch. For tubing with an OD of .700 and ID of .600, that translates to an outside diameter of 700/1000 of an inch and an inside diameter of 600/1000 of an inch. The thickness of the tubing can be calculated as the difference between the OD and ID, or 100/1000 of an inch in this example. Some suppliers also offer universal fittings to match a range of OD or ID dimensions to make it somewhat easier to mate unmatched tubing.
“Drip Line” differs from standard poly tubing as it has emitters embedded within the tubing at specific distances apart, such as every 12”. “Drip Tape” is generally the same as drip line, but it differs in the wall thickness and lays flat on the ground when not in use. Drip tape thickness is commonly listed by the manufacturers in metric units, and is usually available in thicknesses of 8, 10 or 15 mils (millimeters). Drip tape must be laid out in straight lines, while drip line can be curved somewhat. For sharper turns, elbow fittings should be used with drip line or standard poly tubing to avoid kinks in the line which can decrease or even cut off the water supply. Kinks in the tubing are common when it is coiled, bound and shipped. Kinks rarely result in leaks, and the tubing will usually revert to a naturally rounded shape once it is laid out, allowed to warm up, and the system is pressurized with normal use.
Vinyl irrigation tubing, most often found in micro tubing only, is made from flexible polyvinyl chloride (PVC) and performs the exact same function as polyethylene or “poly” tubing. The inside and outside diameters of the vinyl and poly micro tubing vary only slightly. The primary difference between the two are that vinyl tubing tends to be more flexible than poly tubing. This can be either good or bad depending on your needs. Vinyl tubing can make tighter turns without kinking, but vinyl tubing tends to have a slightly shorter lifespan than its poly counterpart. Poly tubing is made from a more stable polymer than vinyl tubing, and as such, is less likely to break down and enter the water stream. In the end, you should use whatever tubing material you feel comfortable with.
Different tubing sizes have different capacities in regards to maximum single run lengths and maximum flow rates. For ¼” tubing, the maximum single run length is 30 feet, and the maximum flow rate is 30 gallons per hour (gph). This relationship is sometimes referred to as the “30/30 rule”. For ½” tubing, the maximum single run length is 200 feet, and the maximum flow rate is 200 gph (the “200/200 rule”). For 3/4” tubing, the maximum single run length is 480 feet, and the maximum flow rate is 480 gph (the “480/480 rule”).
For example, if the distance between your water supply and the end of your planting area is over 30 feet, you would not be able to use the 1/4” tubing for that run, since you will have exceeded its maximum single run length limitation. As another example, if you are using 32 gph micro sprinklers with 1/2” mainline tubing, you can only use a maximum of 6 of these sprinklers to stay within the tubing capacity (32 x 6 = 192 gph required). Exceeding the tubing capacity in either run length or flow rate may result in inconsistent water flow at your emitters within your drip irrigation system.
Drip Irrigation poly tubing, drip line and drip tape can be buried underground and/or covered by mulch. Keep in mind though that burrowing rodents like gophers can chew through the tubing. If you decide to bury your tubing, make sure to monitor your system regularly so any problems can be identified and corrected in a timely manner.
Whether the difference is size, material or type, the differences in drip irrigation tubing can sometimes be confusing. These differences, however, also allow more flexibility in obtaining just the right components for all your varied watering needs. Happy gardening!