The most common casing collar locator (CCL) design features a
coil sandwiched between two permanent magnets. These magnets are almost always cylindrical with like poles facing each other (north to north or south to south). A fairly common alternate design has a single magnet between two coils. A relatively small hole through the center of the magnets allows wiring to pass through the CCL assembly, and sometimes is used to facilitate mechanical attachment of the coil(s) and magnet(s) with a hollow threaded rod, or a solid insulated threaded rod in the case of many shooting CCL designs. Some designs have a groove or grooves milled longitudinally on the outside surface of the magnets to facilitate wire routing (see
illustration below).Common Magnet Figures of Merit
Residual Induction (Br) in gauss: How strong is the magnetic field?Some Magnet Types
Coercive Force (Hc) in oersteds: How well will the magnet resist demagnetizing forces?
Maximum Energy Product (BHmax) in gauss-oersteds x 106: How good is the magnet overall?
Temperature Coefficient in gauss / °C: How much does the strength change with temperature?
Magnets are common items in our lives; most refrigerators would not be complete without a few stuck to the side or door. Iron, nickel, and cobalt are the only common metallic elements to exhibit ferromagnetism, the capability of remaining permanently magnetized. Prior to Alnico magnets, most commercial magnets were steel, with carbon steel the most common (various steel alloys make a much better permanent magnet than iron). Alnico magnets are the most commonly used for CCLs (see
illustration below).Magnet Charging
AlNiCo Magnets derive their magnetic properties, as well as their name, from the main constituent metals - Aluminum, Nickel, and Cobalt (iron is in there too, along with additives that can be varied to give a wide range of properties). Alnico magnets were developed in the early 1940s, and said technology has grown to dominate the commercial permanent magnet marketplace. Alnicos have the widest range of temperature stability of any standard magnetic material. They maintain 85-90% of their room temperature magnetic properties up to 1,000 degrees F, and become demagnitized at approximately 1,600 degrees F (Curie point). Alnico magnets are either cast or sintered; sintered Alnicos have marginally lower magnetic properties, but better mechanical properties. Both types are hard and brittle and can only be shaped by grinding, explaining why chipped, cracked or shattered magnets are a problem, especially in shooting CCLs. Alnico magnets are usually shipped from the manufacturer magnetically uncharged; they are charged or recharged with ease. Alnico magnets are made in several grades, with 2, 5, and 8 commonly available. The non-oriented or isotropic Alnico grades (1-4) are less expensive and magnetically weaker than the oriented or anisotropic alloys (5-9). Maximum energy product ranges from 1.3 to 10 (Alnico 5 is around 5 to 5.5). The vast majority of CCL magnets are cast Alnico 5, but Alnico 5 has a relatively low coercive force (fairly easily demagnetized if handled improperly). Alnico 8 has a higher coercivity, and has therefore been used by some CCL manufacturers, but it is considerably more expensive than Alnico 5, and may be somewhat more brittle. Alnico 5 magnets have been used in CCLs for decades with excellent results, many are in use after 30 years or more without having been recharged even once.
Ceramic Magnets (hard ferrite) were developed in the 1960s as a low cost alternative to metallic magnets (steel and Alnico), and have become the most widely specified magnetic materials. Low cost, light weight, a relatively high energy product, and good resistance to demagnetization account for their popularity. Ceramics are sinterd from barium or strontium ferrite (or another element from that group) and are very hard and brittle. Maximum energy product ranges from 1.0 to 3.5. Because they retain only about 45% of their room temperature magnetic properties at 350 degrees F, they have not been used in commercial CCLs. We have seen only one oddball prototype CCL with ceramic magnets. Plastic / rubber magnets are barium or strontium ferrite in a plastic matrix material with a maximum energy product range from 0.2 to 1.2.
Rare Earth Magnets are high energy devices (they can actually hurt you if you get in their way) that come in two main types - neodymium and samarium cobalt. The neodymium iron boron magnets are limited to operating temperatures below 180 degrees F, and have therefore not been used in CCLs. Maximum energy product ranges from 7.0 to 15.0.
Samarium cobalt magnets were first commercially available around 1970. They retain most of their energy up to 575 degrees F. Samarium cobalt is sintered and is extremely hard and brittle. Nevertheless, these expensive and delicate magnets have been used in CCLs in recent years with excellent results. If small size or centralized operation is an issue, samarium cobalt may be the answer. Maximum energy product is approximately 16.
Recharging CCL magnets is generally accomplished with a capacitor discharge type magnet charger (these devices can kill a mechanical wrist watch deader than a door nail). Years of observation, coupled with some actual research, reveals CCL magnets do not need to be recharged very often, despite the common practice in the past of recharging them at frequent intervals. There are 30 year old logging tools in daily operation that have never had their CCL magnets recharged! It may be more important to have stronger magnets in a shooting CCL than in a logging (amplified) CCL, but even that assumption is subject to debate. It has also been suggested that shooting CCL magnets may tend to weaken faster than logging CCL magnets. See
Magnet Chargers for a look at some of the equipment we use to charge and recharge CCL magnets. For detailed information, see Methods of Magnetizing Permanent Magnets (external link), a paper from Oersted Technology Corporation.