DoD 5200.28-M requires that degaussing equipment be tested and approved by a laboratory of a DoD component or a commercial testing laboratory where the evaluation tests may be certified. Test methods and performance criteria are promulgated in DoD 5200.28-M. National Security Agency/Central Security Service (NSA/CSS) Specification L1 4-4-A, Magnetic Tape Degausser,  is an updated version of DoD 5200.28-M degausser testing requirements. The NSA/CSS has ensured that degausser testing criteria are current by publishing NSA/CSS Specification L1 4-4-A.
Data are stored in magnetic media by making very small areas called magnetic domains change their magnetic alignment to be in the direction of an applied magnetic field. This phenomena occurs in much the same way that a compass needle points in the direction of the earth's magnetic field. Degaussing, commonly called erasure, leaves the domains in random patterns with no preference to orientation, thereby rendering previous data unrecoverable. There are some domains whose magnetic alignment is not randomized after degaussing. The information that these domains represent is commonly called magnetic remanence. Proper degaussing will ensure that there is insufficient magnetic remanence to reconstruct the data.
Erasure via degaussing may be accomplished in two ways: in AC erasure, the media is degaussed by applying an alternating field that is reduced in amplitude over time from an initial high value (i.e., AC powered); in DC erasure, the media is saturated by applying a unidirectional field (i.e., DC powered or by employing a permanent magnet).
The DoD has adopted the National Security Agency security standard for degaussing equipment, which requires degaussers to reduce a special worst-case analog test signal by 90 decibels (db). More simply stated, degaussing must reduce the test signal to one billionth (1 part in 109) of its original strength. However, the signals recorded on magnetic media are easier to erase than the worst-case test signal. This signal is a test signal that magnetically saturates a tape and is set forth in references 1 and 13. After the test signal is recorded on the tape, the tape is degaussed and the residual signal is evaluated against the 90 db standard. This quantifies degausser effectiveness.
It is difficult to distinguish the different types of magnetic tape from appearance alone. For this reason, it is recommended that responsible personnel ensure that type labels (i.e., Type 1,11, or lll) are applied to the tape reels upon initial use. The label should remain on the reel until the tape is cut from the reel or the reel is destroyed.
In some cases, adding another label to the tape could introduce the possibility of operator error in shops where the reel is already crowded with labels. Some facilities require the security officer to use the manufacturer's label to determine tape coercivity. In any case, strict inventory controls should be in place to ensure that tapes can be identified by type so the correct purge procedure is used.
The list of magnetic degaussers that satisfy the requirements in NSA/CSS Specification L1 4-4-A is included in the NSA's Information Systems Security Products and Services Catalogue  as the DPL. The catalogue is updated quarterly and is available through the U.S. Government Printing Office.
Because of the possibility of equipment failure, degaussing equipment should be periodically tested to verify correct operation throughout the life cycle of the equipment. Preventive maintenance should be done on a regular schedule to preclude mechanical or electrical problems. Some manufacturers have maintenance contracts and recommended maintenance schedules to ensure the integrity of the degaussing procedure.
To provide a rough estimate of degausser effectiveness, an on-site test of generated magnetic field strength may be done by using a gaussmeter for some models of Type l degaussers. (Some Type l degaussers cannot be tested in this manner because the degaussing field is not accessible.) However, a more extensive test is required to maintain an adequate degree of assurance that the degausser is operating correctly.
Both Type l and ll degaussers may be periodically tested more extensively by testing against the 90 db test signal strength reduction requirement in NSA/CSS Specification L1 4-4-A using the following procedure: have the tape prerecorded with the specified test signal (in a testing laboratory), degauss the tape, then return the tape to the laboratory where it can be tested for the remanent signal level.  Check with local authorities or engineering personnel to determine if such a service is available to your organization. There are two companies listed in the DPL, Integra Technologies Corporation and Data Security, Incorporated, that can test an installed degausser's effectiveness.
Although this periodic test is not a DoD requirement, it is highly recommended. After a degausser is installed, it should be tested periodically (approximately every six months) for its first two years of operation. This data can be used to develop a histogram of the degausser's operation. Based on this information, an informed decision can be made about extending the interval between testing, e.g., every 9 months, yearly, every 18 months, etc.
Note that it is erroneous to assume that even a newly installed degausser, let alone a degausser several years old, is providing sufficient erasure. It is not prudent to rely upon one DoD evaluation of the degausser manufacturer's product line because of possible product failure.