The capabilities of an MFL tool strongly depend on the strength of the magnet system. In general, the design strength of a magnet system depends on the wall thickness and material properties of the pipe to be inspected.

Various permanent magnet strengths are available. A magnet's strength is categorized by its maximum energy product, which is a measure of the magnet's force of attraction. Many older inspection systems used ceramic permanent magnets, which have an energy product of 1 to 4 megagauss-oersted ^[McCaig77]. A horseshoe magnet with a strength of 1 to 4 megagauss-oersted can lift about 10 to 40 pounds. Other systems used Alnico (aluminum-nickel-cobalt) magnets, which have energy products from 5 to 12 megagauss-oersted. Rare earth magnets, such as neodymium-iron-boron magnets and samarium-cobalt, became available in 1984. These magnets have energy products from 18 to 35 megagauss-oersted. Rare earth magnets can lift 180 to 350 pounds. Thus, the newer magnets have dramatically increased the magnet power available ^[Bal91]. They are also mechanically stronger than the other magnet types, which tend to be brittle ^[Atherton89], and they are significantly more expensive.

The temperature sensitivity of the magnets used on MFL tools should not affect tool operation. The maximum practical operating temperature for samarium-cobalt magnets is 400 F, and the range is 300 F for most neodymium-iron-boron magnets; for some grades of neodymium-iron-boron magnets, the maximum temperature can be as low as 140 degree F. Since many data storage systems require temperatures less than 140 F and typical in-line inspections are performed below 100 to 110 F, even the newer rare earth magnets should not significantly limit most pipeline applications.

The use of electromagnets rather than permanent magnets is limited. As discussed earlier, an electromagnet's strength is proportional to the number of coil turns and the current passing though the coils. Electromagnets are used when the power required to run the batteries can be met by the batteries carried by the tool. For in-line inspection tools, power consumption is a critical factor. So, electromagnets are not often used. A potential benefit of an electromagnet system is that the applied field levels could be changed during the inspection; this type of control has not been used in the past.