Anoxia

Smadar Gabrieli <smadar.gabrieli<-at->sydney<.>edu<.>au> writes

>I work for a small archaeological museum...
>
>We have recently been contacted by a company that supplies
>controlled-air systems to hospitals and libraries.  They suggested a
>system that will supply filtered air, rich in nitrogen (10% oxygen)
>at the required humidity to individual display cases.

It sounds to me as if you are being offered a nitrogen generation
system modified to control humidity.

A nitrogen generator removes oxygen and humidity from the air using
a compressor and filters, and while the output is usually set to
produce a trickle of air at very low oxygen levels (below 1%), the
system can be adjusted to produce a greater air flow with
consequently higher oxygen levels.

At first glance this offer sounds intriguing, as you are describing
a microclimate control system that will deliver a relatively robust
flow of pressurised and filtered air, with humidity adjusted to a
preset level.  These systems have been on the market for some years
now.  However, from what I can glean from your posting, I would
offer a few caveats on the environmental control aspect:

    I am not aware of any nitrogen generation systems that do not
    use a compressor.  Compressors generate much more noise and heat
    than do fans, blowers, or simple pumps, Compressors and pressure
    vessels must be carefully maintained, and regularly examined and
    certified.  They are consequently more expensive to operate and
    service.

    The air we breathe is already "rich in nitrogen", as it is about
    79% nitrogen and 21% oxygen.  Air enriched with nitrogen (read
    oxygen content reduced by 10%) offers no advantage for museum
    display or storage; oxygen levels must be kept very low to
    affect oxidation rates or to kill insects.  I see no advantage
    to the slightly reduced oxygen levels offered.

    I'm a believer in positive pressure microclimates.  Just a
    slight pressure should prevent ingress of exterior air and
    reduce levels of trapped pollutants.  Only a little pressure is
    needed to induce leakage (in the area of 2 pascal).  Typically,
    airflow pressures are in the range of 750 pascal or less, well
    within the capability of a small blower, fan or pump.  The high
    pressure generated by a compressor (>500,000 pascal) is not
    needed.

Some further comments on the anoxia component:

    My anoxia antennae twitch when a 10% oxygen level (hypoxia) is
    specified, as personal experience has shown that the maintenance
    of specific oxygen levels in showcases is challenging at the
    best of times.

    It is possible that extremely low levels of oxygen (anoxia)
    could have an effect on textile dyes.  Permanent fading has been
    noted on some watercolour pigments kept at for an extended
    period of time.  However, I am not aware of any fading of dyes,
    or studies done on textiles and anoxia or hypoxia (partially
    reduced oxygen levels) and welcome any comments or direction
    from readers.

I encourage you to explore more common active and passive
microclimate control systems, which are likely to be both more
reliable and less expensive to operate.

Jerry Shiner
Keepsafe Microclimate Systems


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                  Conservation DistList Instance 29:37
                 Distributed: Sunday, February 14, 2016
                       Message Id: cdl-29-37-002
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