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Subject: Active and passive storage

Active and passive storage

From: Jerry Shiner <info>
Date: Monday, October 28, 2002
Nora Lockshin <misslockshin [at] yahoo__com> writes

>Can anyone clarify as to whether there is a consensus on the
>definitions "passive" storage vs. "active"? The terms originate in
>climate and microclimate control, but I see the term passive used
>much more often, but also in combination with using "active"
>materials such as Microchamber board, Silvercloth, zeolites, etc.

I prefer to use the terms "active control" and "passive control"
rather than active or passive storage. You are quite correct to say
they are terms often used when describing the creation and control
of micro climates in display or storage cases.

In my experience:

    Active control almost always involves the mechanical supply of
    air to maintain the desired microclimate in an enclosed space.

    Passive control usually involves only the addition of a some
    material to absorb or release the desired elements in a
    controlled manner inside a sealed chamber.

Of course, it is true that the reactions in silica gel, zeolites,
activated charcoal or other materials are "active". After all, at
some point, energy was applied to create the conditions needed for
absorption, desorption or adsorption. However, a passive system
generally does not need an outside source of energy to proceed.

Most of the MCTI Micro Climate Generators use fan driven air. They
also use physical reactions (usually done by cooling, heating, or
filtering the air), as opposed to chemical changes, to create the
desired moisture levels.

Another difference between active and passive, is that truly
"active" devices do not need to be "refreshed".  The capacity of a
passive system will eventually be exhausted, whereas an active
system will continue to operate as long as energy is supplied.

We also classify our very dry machine, which uses only silica gel to
maintain very low moisture levels as "active" not only because it is
fan forced but also because the silica gel is automatically

I can understand your puzzlement. From my point of view, you need
simply ask whether the air is mechanically circulated, and whether
the elements need periodic replacement. Does it have a plug or
batteries? Will you have to maintain a replacement schedule?


One important point I failed to note in my comments on the
definitions of active and passive climate control is the difference
in efficiencies.

Passive systems are limited by the speed of natural air movements
and by the transfer of moisture and gases to and from the moderating
materials. Active systems are probably some orders of magnitude more
efficient. [For example, the MCG8 is designed to control humidity in
a volume up to 750 cubic feet that is leaking at four air changes
per day. The MCG30 will do the same for up to 6000 cubic feet, up to
500 feet from the base unit.]

Even a small active micro climate control system can modify a very
large volume of air. This capacity can be used for a large, well
sealed volume of air OR for a smaller case volume that experiences
substantial case leakage.

The storage or display cases form an integral part of active micro
climate systems. While determination of case leakage, and
modification of the air supply are the basis of a both active and
passive systems, there is far more leeway in an active system.

Jerry Shiner
Keepsafe Systems
Supplies and Solutions for oxygen-free and micro climate storage

                  Conservation DistList Instance 16:30
                Distributed: Wednesday, October 30, 2002
                       Message Id: cdl-16-30-003
Received on Monday, 28 October, 2002

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