Air exchange rate in exhibition cases

Chao-Chun Lu <lucc [at] npm__gov__tw> writes

>When purchasing exhibition case, should air exchange rate always be
>take into account or only considered special uses, such as
>nitrogen-filled cases. What level of the air exchange rate would be
>best?

The air exchange rate (AER) or leakage rate of an enclosure such as
a display case dictates the flow of air (a mixture of gases
including water vapour, pollutants and particulates) in and out of
an enclosure. The rate will greatly influence how that enclosure
will buffer external environmental conditions and limit the ingress
of dust and externally generated pollutants or potentially
concentrate any internally generated pollutants.

There are very few instances where a well sealed display case is not
a sensible approach. The air exchange rate you should aim for does
depend on the application and whether you want a 'passively'
controlled enclosure or are using a mechanical system. The classic
Garry Thomson maintenance free case buffered with silica gel uses a
value of 1 air change per day (ac/d) to calculate that 20kg of
silica gel per cubic metre is required to provide an internal
relative humidity (RH) stabilised around the annual mean internal
RH.

An air exchange rate better than this, say 0.25 or 0.1 ac/d (levels
quite commonly specified in the UK) means that less silica gel or
other buffer is required per unit volume or the hygrometric half
time of the enclosure is increased--especially important if you are
trying to maintain a case at an RH different from the annual mean
such as for the display of archaeological iron. Generally reducing
air exchange is generally more effective than using more buffer
which can be problematic to locate within the enclosure volume. In
my opinion the majority of the reports citing problems using silica
gel to buffer display cases are a result of not knowing the air
exchange rate and having very leaky cases--it is quite common for
cases to have rates very much greater than 1, making the use of a
buffers impractical.

The AER is also relevant when using mechanically controlled
enclosures to the extent that the air or moisture handling capacity
of the plant serving the enclosure volume must be appropriate for
the leakage rate of the case.

There are many published papers on the role of air exchange rate and
enclosures. A paper I co-authored has a list of relevant references
and is available online at

  <URL:http://www.cwaller.de/
    vitrinentest/calver2005_air_echange.pdf>

A later paper stressing the importance of air exchange can be found
at
   <URL:http://www.conservationphysics.org/
    mm/thickett_2/thickett_2.pdf>

    **** Moderator's comments: The above URLs have been wrapped for
    email. There should be no newlines.

Andy Calver
St Albans Museum Service
Verulamium Museum, UK
+44 1727 751 827


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                  Conservation DistList Instance 23:25
                Distributed: Wednesday, January 13, 2010
                       Message Id: cdl-23-25-009
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