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RE: [AV Media Matters] MIN RH ?

First of all I would like to acknowledge Peter Dahlbeck for his great
contributions to this Listserve.  I have been a fan of Munters-Cargocaire
since I bought one of their systems at Ampex in 1982.  Peter as been involved
in several archival air condioning installations and understands the issues.
I especially like the Hollywood Vaults system which has three independent A/C
units.  Any one of them can be shut down for repair or maintenance and not
upset the room air.

You don't need a Phd to understand a Psychrometric chart but an IQ of at
least 150 would help.  The Munters-Cargocaire handbook has illustrations that
show you how to read the chart.  "The Dehumidification Handbook" is an
excellant document.  I have had it since 1982.  I made a simplified version
of the Psychrometric chart and it is in the June 1983 edition of the SMPTE
Journal (Society of Motion Picture and Television Engineers).

I agree with Peter Brothers that I am not aware of any problems with storing
tape at low humidity.  I don't know of anyone who does store tape below 20 %
RH.  I have had a lot of experience OPERATING tapes in low humidity
environments.  I have operated tape at 5 % RH for several days with no
problems.  Also, the Ampex  tape recorder manufacturing facility has been
located in Colorado Springs since 1965 and the normal winter humidity is
around 5 % RH in the building.

Saying that, I am concerned that some tape manufacturers may have
plasticisers in the tape that would disperse at a low vapor pressure.  Until
someone runs long-term tests at low humidity, I would not store tape below
about 20 % RH.

All the test data I have seen shows that the ideal RH is around 25 % RH.
That is difficult for many facilities to maintain during the humid months of
the year but I consider that to be critical for long-term storage.

I have tape collections from 1943, 1948 and 1954 that play just fine and they
were stored in houses and garages most of their life.  But, they were
formulated differently then today's tapes.

Jim Wheeler

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