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LI-610 Portable Dew Point Generator Used for Charters of Freedom
Encasement Project
The Declaration of Independence, the Constitution,
and the Bill of Rights, collectively known as the Charters of Freedom,
are on permanent display at the National Archives in Washington,
D.C. In 1951 the Charters of Freedom were encased in helium-filled
glass and metal cases built by the National Institute of Standards
and Technology (NIST, formerly NBS). An examination in 1995 showed
signs of deterioration in the encasements’ glass; experts
at the National Archives and NASA determined that the humidity
in the cases was too high, causing alkali ions in the glass to
dissolve and leave a white residue. Scientists at NASA and NIST
recommended new high tech cases that incorporated pressure, temperature,
and humidity sensors to protect the delicate parchment on which
the Charters of Freedom were written.
As part of a major renovation of the National
Archives Building that began in February 2000, the Charters of
Freedom were removed from their original encasements, and reencased
in new airtight containers made of aluminum, titanium, and glass.
The new encasements were filled with argon gas to provide an inert
atmosphere.
The argon gas that fills the encasements was humidified
to a water partial pressure of 903 Pa (5.50 °C dewpoint) using
the LI-610 Portable Dew Point Generator. The LI-610 was also used
to calibrate humidity sensors installed in the encasements.
When the National Archives Rotunda reopened in
September 2003, all four pages of the U.S. Constitution were displayed
for the first time, along with the Declaration of Independence
and the Bill of Rights. |
