Issue link: http://read.dmtmag.com/i/131533
70 • JUNE 2013 • WORLD FENCE NEWS Myths and facts about saw-cut loops continued from page 68 sealant is better encapsulating the wire in the wider 1/4" groove, and the twisted lead-in is preventing the wires from moving in reference to each other, preventing false detections. To really drive the twisted lead-in myth to bed, both the U.S. Department of Transportation and the Texas Transportation Institute commissioned Texas A&M University to conduct a test that found crosstalk between leadin wires – whether twisted or untwisted – was not a problem. This report can be located at http://library.ctr.utexas.edu – Report No. FHWA/TX-94/1392-2 – Induction Loop Detector Systems Crosstalk. After doing this research, we developed a saw-cut loop that does not have a twisted lead-in; instead, the wires lay on top of each other secured together by a tough polyethylene outer jacket that prevents movement and allows installers to use the same blade size for both the loop and lead-in. Do installers still use 1/8" grooves and say they never have issues? Of course, and when I come across such an installer I have made a challenge to them – Let me take a jackhammer to their loop site and do a ground vibration test with the detector set to max sensitivity. Unfortunately, no one has taken me up on this test! The main reason I think installers do not realize they are having issues related to blade width is that this problem can be masked by setting the sensitivity of the detector to a lower setting – which makes the problem appear to go away. The only issue with setting the detector to a lower setting is that high bed vehicles and motorcycles might have a harder time being detected. We often find that installers who use 1/8" blades also often make the short leg of their loop at least 6'. The better detection height offered by a 6' short leg verses a 4' short leg can help mask the issue of air pockets as well as cover for the detector being set to a lower sensitivity. In fact, several installation schools teach that the short leg of a loop should never be less than 6'. The best solution for air pockets is to use a 3/16" or larger blade if you are wrapping your own loop, or to use a preformed loop that is wrapped in a continuous flow outer jacket that prevents movement of the wire. Even when installers use the proper sized blade, air pockets can still form when wrapping a loop by hand, but they are smaller and less likely to cause false detections than wires run through a narrower 1/8" groove. I mention a continual flow outer jacket because some installers and preformed loops manufacturers wrap wires with tape to try to hold the wire together. The tape can easily become unraveled during the installation process, which makes getting the loops into the groove more difficult and can also lead to intermittent loop problems caused by air pockets. Do you have to cut an exact size groove for a preformed loop? Moving on to other myths about preformed saw-cut loops, the biggest myth is the saw-cut pattern has to be the exact size in order for the loop to fit. This is true where the preformed loop has a "T" junction where the loop meets the lead-in. But if the preformed loop has a "Y" yoke then the loop has the ability to be flexible in size. You can't make a preformed loop any larger, but if it has a "Y" yoke you can make it smaller by taking up loop excess in the lead-in run. For every inch of excess loop you pull into the lead-in run you shrink the loop by two inches. This type of design makes it easy to fit a preformed loop into a saw-cut groove that is not exact size. A preformed loop with a "Y" yoke will fit every time as long groove is slightly smaller than the loop. Type of wire Many years ago using TFFN or THHN for wrapping loops was the standard. Now we know that TFFN / THHN is not a good choice for inductance continued on page 72