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Understanding Degaussing

In spite of all the material written about degaussing, most of it is in a format not easily understood by an average person. Let's try to answer some of the questions that cause about 90% of the confusion. But first, a glossary of terms and a couple "Rules of Thumb". 

Glossary of Terms


Origin: Karl Friedrich Gauss, 1777-1855, was a German mathematician, astronomer and physicist.

Definition: Gauss is a unit of measurement of magnetic flux density produced by a magnetic force (coils).

Translation: Gauss is the amount of energy that is produced by a magnetic force (coils). Gauss is a measurement of coil strength.


Origin: Hans Christian Oersted, 1777-1851, was a Danish physicist.

Definition: A unit of magnetic intensity equal to the intensity of a magnetic field in a vacuum.

Translation: Oersted is a measurement applied to magnetic media. Most of "today's" tapes have an energy level rating of 1800 oersted or higher.


Definition: The amount of applied magnetic field (of opposite polarity) required to reduce magnetic induction to zero.

Translation: The ease (or difficulty) by which magnetic media can be demagnetized. A tape with an oersted rating of 1800 might also be called a high coersivity tape.

NOTE: Oersted and coercivity are often used interchangeably, which only adds to the confusion. Newer technology tapes generally have a higher oersted rating than older tapes (DLT or LTO for example).

Rules of Thumb

Rule of Thumb #1

The higher the oersted rating, the more energy needed to degauss it.  Or...

Rule of Thumb #2

In order to degauss magnetic media, a magnetic force (gauss) of 2 to 3 times greater than the media to be degaussed must be created. Or...

In theory, an 1800 oersted would require a magnetic field strength in the neighborhood of 3600-6400 gauss in order to properly degauss it. 

Degaussing Why?

Why would anyone want to degauss magnetic media in the first place? Interesting question! No one degausses media for the fun of it. Media is degaussed because there is either some laws on the books that requires that media be degaussed or you simply want to reuse it.

For starters, we degauss media:

  1. Declassification for security purposes or to conform to privacy laws
  2. For reuse of media, it eliminates possible recording errors from residual magnetic signals
  3. For disposal purposes

I suppose that there are people out there that fake it. You know, say that the tape is degaussed without actually doing it.  But, if they are ever challenged, they better have a very provable reason why they didn't degauss the media.

There are dozens of Federal and State laws relating to data privacy - Fair Credit Reporting Act, Freedom of Information Act, Title 28, Buckley Amendment (Educational Records), etc.

By now, you are getting the message. Today's records relative to insurance, consumer credit, bank records, etc., all must meet various standards of data privacy. How about national defense? Now, you've got it!

In short, if magnetic records get tossed out without being degaussed first and someone recovers the data - POW!!! Translation - You just got socked with a very expensive lawsuit or you just landed in jail.

So, degaussing make sense. But, you say, "I don't know anything about degaussing." Don't be alarmed - you've got a lot of company!

First of all, degaussing is merely a fast, efficient way of removing all audio, video, and data signals from magnetic media.

Incidentally, IBM has published information on degaussing. Chapter 1, page 3 of the Care and Handling of the IBM Magnetic Cartridge states "AC erasing all the magnetic records from tape for security reasons will not damage the tape." However, there are a couple of exceptions that are mentioned at the end of this webpage.

Let's suppose that you have a cartridge or reel of magnetic tape that is already encoded and you want to dispose of it. OK, we'll degauss it.

A long time ago, someone developed this thing called a degausser. The early versions were crude by today's standards and looked something like a hot plate. Confidentiality in those days wasn't as critical as it is today. Times have changed and we are concerned that sensitive information might fall into the wrong hands.

So, how's it done?

Basically, all you do is introduce the recorded magnetic media into a very strong magnetic field and uniformly withdraw it from this same magnetic field. As it is leaving the magnetic field, the magnetization of the recorded data becomes weaker and weaker until there is no residual magnetic signals remaining on the media. In other words, you put it in (magnetize it) and you take it out (demagnetize it).

"Oh yeah, I'll bet there is a lot more to it than that." You can bet there is! There have been books written about it but since you don't intend to make it a lifetime study, we'll keep it simple.  Don't try to get too technical.

Some media is harder to degauss than others. For example, a standard VHS cartrige tape at 650 oersteds is easier to degauss than a cartridge at 1800 oersteds. In fact, most cartridges being used today have a high coecivity rating - most around 1800 oersteds.

All this means is that you should choose a degausser depending on the coecivity rating of the media to be degaussed. A degausser designed to degauss media with a maximum coercivity rating of 650 (VHS) oersteds will not properly degauss media at the 1800 oersted rating.

Coercivity is a measure, in oersteds, of the magnetizability or erasability of a tape, cartridge or hard drive.

The higher the coercivity number the more energy needed to degauss it. The higher the coercivity the harder it is to erase.

So, all you really have to know is the bit about withdrawing the media from a strong magnetic field in order to degauss it. Given the type of media and the quantity, you will be able to choose the proper degausser.

When choosing a degausser, cost should not be the deciding factor. Consider the sensitivity of the data to be degaussed; the frequency of degaussing; the laws relative to the destruction of records and then choose the proper degausser for the job to be done. Please call our sales staff at BOW Industries for any assistance you may require.

For your information, several of BOW Industries cleaners (DLT, SDLT, etc) have an optional erase feature. You may be very surprised when you compare rices.

IMPORTANT: DO NOT degauss 3590 and LTO cartridge tapes or hard drives. If you do, you will destroy the servo tracks (or chip in the case of a hard drive). This should only be done for destructive purposes.