There are scores of casing collar locator (CCL) designs and variations, but all modern designs use coils and
permanent magnets. Most CCLs use a single coil, while a few use two or more coils. The coils are very simple devices consisting of nothing more than thousands of wraps or turns of fine
magnet wire wound on a steel spool or mandrel, sometimes referred to as the coil form. The inner wire end is usually grounded to the spool by one method or another, but a few versions terminate both coil ends in lead wires (coil isolated from ground). Some sort of insulating material is ordinarily installed on the inside faces of the spool flanges, and less commonly on the inner spool diameter if the inside winding termination is brought out as a second lead. An insulated wire (usually Teflon) is attached to the outer end of the magnet wire and the assembly is wrapped in
Teflon tape usually, but Polyester (Mylar), Polyimide (Kapton), or Glass Cloth tapes are also used. A few manufacturers potted their coils in epoxy, or used "wet winding" where a bonding agent fuses the windings together; both are nightmares when rewinding is necessary.
AnaLog Services, Inc. rewinds all types of CCL coils using only the best materials with proper tensioning to insure long coil life. We also offer a proprietary resin impregnated coil for the most demanding shooting CCL applications, but this extraordinary measure is seldom necessary. See
Coil Winders, Dereelers, and Tensioners for a look at some of the equipment we use to wind CCL coils.
Coils that are "perfect wound" (technically called orthocyclic winding) are aesthetically pleasing, but random wound coils work just as well. Perfect or orthocyclic winding yields a higher copper density (maximum number of turns per unit volume) than any other winding method, approaching a theoretical maximum of 90.6% for a large number of wraps. By comparison, square lay (where successive layers rest directly on the turns of the layer below) achieves only 78.6% fill efficiency, but random winding (also called scramble or scatter winding) can achieve approximately 84% fill efficiency. Clearly the advantage of orthocyclic winding over random winding is relatively trivial for most CCL coils. A true orthocyclic wound CCL coil is seldom seen since perfect winding becomes increasingly difficult with smaller wire gauges. Proper wire tension must be maintained when CCL coils are wound if there is to be any chance of coming close to a perfect wound coil, and to insure long coil life, especially for shooting CCL coils. See
CCL Magnet Wire Trivia for more information on magnet wire and wire tensioning.
Coil health can best be evaluated with resistance measurements. Measure the resistance from the spool or mandrel (ground) to the insulated wire connected to the coil, or between the insulated leads if the coil is the two lead type. The coil must be stationary, and no metal should be moved near the coil especially if the magnets are in place; otherwise, wild variations in observed resistance may occur. Shooting CCL coil resistance is typically lower than logging CCL coil resistance. Shooting CCL coil resistance ranges from maybe 1,000 to 2,000 ohms (1-2kW) with a few manufacturers offering higher resistance coils, while logging CCL coil resistance ranges from maybe 2,000 to 5,000 ohms (2-5kW). For example, the CCL coil resistance in a 1-11/16 inch GO COSMOS tool is normally around 3.5kW.
GO/MLS (and perhaps other manufacturers in a few cases) inadvertently created a shorted winding in the CCL coils used in COSMOS tools, and some of their other CCL products. The green Teflon lead wire was stripped far enough back to make a full loop around the windings on top of a wrap of high temperature tape in a slight depression created during the winding process, and was soldered to itself. While this solution is mechanically strong and gives a stable point for connection of the fragile magnet wire end, it does create an undesirable shorted winding. While it is unlikely that this practice significantly reduces coil sensitivity, it does have some effect and should be avoided. If these CCLs must be serviced, the shorted winding should be removed.