When maintaining, repairing, and/or installing drip/low-volume irrigation systems, there is a good chance that the technician will be dealing with polyethylene supply tubing of some type and size. Determining the size (or “outside diameter”) of the tubing is essential to know which size of compression fitting to use.
Unlike PVC pipe, polyethylene tubing is “nominally sized.” That means that there are actually several different sizes of so-called ½” (and ¾”) poly tubing, and each size is compatible with different sized, color-coded compression fittings.
Most major manufacturers produce 3 sizes of so–called ½" poly tubing: .520 (ID) X .620 (OD), which accepts compression fittings with green insert, .600 X .700, which accepts compression fittings with black insert, and .620 X .710, which accepts compression fittings with blue insert.
With the exception of “blue stripe” tubing, (which is .710 OD), so-called ½" poly tubing is not color coded, so if the OD is in doubt, check the outside of the tubing for written information regarding the OD. Most manufacturers print the OD and ID and other specifications every five feet or so on the tubing.
If determining the outside diameter proves to be too problematic, consider utilizing spin lock or universal nut lock fittings, which (unlike compression fittings) can be easily re-used if necessary. These fittings are available in a variety of configurations, including tee, elbow, and coupler, and are compatible with all three sizes of half-inch poly tubing. They are more costly than compression fittings, but can be more convenient in certain installations.
Insert fittings are a third type of fitting available for use with ½" and ¾" poly tubing. These fittings have a male barbed design that is forced into the tubing. Unlike compression fittings, with insert fittings, the chance of leakage or separation increases with higher pressure. Since tubing inside diameters vary, these fittings are best used when dynamic pressures are 15 psi and lower.
EDITOR’S NOTE: Stuart Spaulding, CLIA, is a technical service manager for DIG Corporation.