Nancy,

There are several strategies that you can take to try to stabilize the
relative humidity. The first thing that is very important is to try to
get a handle on why the humidity is fluctuating so much. As a
conservator I would start with any hygorthermograph charts to see if
any patterns were present and also I would want to contrast that data
against the records of the exterior air temp and RH. This would give
me a good view into how well the building and its system are buffered
(or not buffered) with the exterior environment. I would also talk to
the engineer or staff member responsible for the general HVAC to get
information on the set points of the unit and how it is operated. In
most cases you can ramp the set points on an HVAC system seasonally to
slowly move the building's RH to be more in tune with the exterior
environment. What trully hurts objects and artifacts are the sudden RH
swings and fluctuations along with extended super high RH that can
boost biological activity as well as corrosion in metals, etc. You can
also try adjusting the temperature of the HVAC unit to compensate
somewhat to changes in RH.

So much also depends on the display room (are there exterior walls?
windows? exterior doors? insulation?). If the objects are largely
within cases then those cases can be treated as microclimates and can
be independantly buffered from the room environment using trays of
media such as conditioned silica gel.

You did not say where you are located. There is no absolute in ideal
temperature and RH set points because so much depends on the
environment that the objects have resided in. If you are in an arid or
semi-arid climate (Phoenix for example) humidifying to 50% RH would
not only be very expensive but would also damage organic objects that
have accclimated to much lower RH conditions. And the reverse is true,
if you are in a very humid and warm environment (Miami for example)
you would not want to kick the RH way down low if your collections are
used to higher levels. In lower RH regions you see embrittlement and
adhesive failures, in higher RH environments you see much more
corrosion in metals and fungal growth in collections. The best setting
is one in which you can reliably buffer and stabilze the environment -
conservators love to see that flat, straight line on the
hygothermograph charts.

If you do go to portable dehumidifiers (you should have some on hand
anyway for flood events, disasters, etc.) you can use distilled water
to avoid any potential issues with the minerals and chlorine in the
water, and you should never have the air blast from the unit pointed
directly at objects. You want to condition the entire room and not
locally dehumidify just one area. It is also just sound practice that
if you have any water collected into a pan in a device that circulates
air that you need to remove that water religiously so that no
microbial growth occurs. This is true of portable dehumidifiers as
well as with HVAC units. And with the portable units you still have to
do alot of monitoring to ensure that you are achieving a more stable
environment.

Since this can be a complex issue I would suggest that you look into
consulting with a knowledgable conservator in your area who can survey
the data, the collections, and the building and who can really focus
in on the best solution for you.

Cheers!
Dave

David Harvey
Conservator
Los Angeles, California

On 7/17/07, Jenner, Nancy <[log in to unmask]> wrote:
> I checked the listserv archive, and it doesn't look like this has been
> discussed for a few years, so....I'm curious about what the current
> thinking is on small portable humidifier and dehumidifier units.
>
> We have a display area that does not have its own climate control, and
> the humidity tends to rise and fall dramatically, depending on what is
> happening with the building's main air conditioning units.
>
> Although my preference would be for a console unit hooked into the
> building's drainage and water supply, so that we don't have to
> constantly refill and empty pans, I don't think it's going to be
> possible.
>
> I don't want to just buy a cheap home unit, however, because I keep
> coming across cautionary tales about bacteria growing in the units,
> about humidifiers depositing lime on collections, etc.
>
> Are portable units acceptable when there are no other alternatives?  If
> so, any recommendations for humifiers and dehumifiers (separate units or
> combinations) that are good in a museum setting, do the job, do no harm,
> and are relatively quiet?
>
> Thanks for your insight,
>
> Nancy Jenner
>
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