Introduction to 3M Audio Open Reel Tape List

Copyright © 2000, 2010, 2011 by Delos A. Eilers. Reproduction in any form prohibited without the written permission of Delos A. Eilers.

    Minnesota Mining and Manufacturing Co (3M) developed and manufactured magnetic recording tape from 1947 to 1997. During the winding down of the magnetic tape business at 3M, it struck me that audio archivists and historians would probably appreciate a listing of the products 3M had made over the years, and their basic characteristics. There have been a number of serious aging problems with several magnetic tapes of 3M and other brands, which had led to concern about all magnetic tape records. However, most products sold have not had aging problems when properly stored.

    This chart was devised to help the archivist determine which 3M Audio Open Reel product they have. Then, knowing the tape types they possess, they can monitor their libraries by product types. When a given product type of a given vintage is identified with a problem, those recordings in the library of the same binder type and vintage can be more closely checked for aging problems.

    The parameters chosen for this database were tape type/number, estimated year of introduction, binder type, oxide coating color, base material, base caliper (thickness), oxide caliper, total caliper, back treated (yes or no), remanence, coercivity, retentivity, and any special feature worth noting. These parameters were chosen because they can either be seen or measured to help determine which 3M product a tape might be when it is not labeled or identified. Electro-magnetic (recording performance) data was not shown since it is not absolute, but dependent upon a recording machine operating conditions and a reference tape. Frankly, a common set of test conditions and reference tapes was not used over the nearly 50 years of tape design, thus technical data sheet electro-magnetic data from one era can not be compared to another. Even the basic magnetic properties are not exactly comparable from the old technical data sheets to new as refinements and improved accuracy in measurement changed the data shown over the years. This list is our best guess of the physical and basic magnetic data that we could assemble from technical data sheets kept over the years. 

    Perhaps the most frequent question asked has been "I now have some 3M Type XXX tape. What recording level should I use?"  The standard recording level reference in the US (Ampex, MRL, etc) is the "Ampex Operating Level, 185 nWb/m at 700 Hz, which was set for 3M 111 tape. For other tapes, it is approximately equal to 20 log ( Remanance/ [Remanence of 3M 111 tape] ). We have now (2011) added a such a column to these tables -- Rel Rem Lvl re 3M111/[dB]. For example, if you have 3M 226 tape, you should record at +5.8 (in practice, +6) dB, by setting up with a 355 nWb/m ("+6 dB) Calibration tape.              

    We plan to add scans of the many 3M tape Tech Data Sheets to this web site.

    A word about binder type shown. The binder chemistries used to make magnetic tapes went through many changes over the years. We also used different identification systems over the years to describe them. Sometimes they were reflections of the binder chemistry make-up. Sometimes they were the initials of the developer(s). Sometimes they were just a code name. Rather than use this coded jargon, I chose to merely group them into their basic types and use a letter designation starting with A going through the alphabet till I ran out of group types. The binder groups were never referred to as A, B, C, etc. within 3M.

    Lastly the earliest tape data is very "sketchy" as there is little historical documentation in existence today. Talking recently with one of the chemists from that era, he admitted that the changes made to a given tape type in those days often were significant. As we learned more about making tapes and found improvements, they were incorporated in the products. We didn't really appreciate what a standardized product type/name/number meant. In some of our early Sound Talk bulletins we identify products as the same tape type but also show different internal numbers (3RBA, 4RBA, 5RBA, 6RBA). These were all product "improvements"/changes made to the same basic tape type. In some cases these changes were quite significant on performance. By the early 50's this practice was stopped, so that significant performance changes were accompanied by a new product number identification.

    We hope this database is of interest and is useful to you. While I believe it to be accurate, there may be some minor errors. Some of these can be "found" looking at different versions of technical data sheets for the same product. These "errors" are really slight changes in measurement accuracy or in product "fine tuning" that occurred over the years. For this database I had to pick one number to represent the product characteristic.

   At 3M, all measurements were done in what are now called "customary units".  In this version, we have added tables in the System of International Units, SI Units, to enable easy comparison with modern tapes. The conversion is thus:

        "Caliper, mils", in this sense, means "thickness". A "mil" is one-thousandth of an inch; for thickness in micrometers [um],  multiply the number of mils by 25.4 .

        "Remanance, flux lines per quarter inch" is the saturation remanance flux from a quarter-inch wide tape. A "line" is an alternate name for a maxwell, equal to 10 nanowebers. For saturation fluxivity in nanowebers per meter [nWb/m], multiply the number of flux lines per quarter inch by 1600.
          For coercivity in kiloamperes per meter [kA/m], multiply the number of oersteds by 0.08 .
          "Retentivity, gauss" is the saturation remanance flux density; for retentivity in millitesla [mT], multiply the number of gauss by 0.10 .

    The data files  "3M Audio Open Reel Tapes" are available for you to download in two formats:

        First, as  PDF files in customary units and in SI units, that you can read on your browser, print out, etc.

        Second, as Microsoft Excel (xls) files in customary units  and in SI units that you can import into many spreadsheets ("MS Excel", a part of the MS Office package; or "Quatro Pro", a part of the Corel "WordPerfect" package; etc). From these spreadsheets, you can also use "save as" to save the data in other formats such as Dbase, or tab- or comma-delimited text files. By putting the data into a database program, you can do various useful things such as re-sorting the data (which is now in Product Number sequence) by introduction date, or by binder type, etc. Finally, you can import the xls file directly into some word processors (e.g., WordPerfect).

Delos (Del) A. Eilers

Senior Technical Service Specialist
3M Company


 Rev de & jm: 2010-05-18 and 2011-07-24a,

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