A hermetic seal is the quality of being airtight. In common usage, the term often implies being impervious to air or gas. When used technically, it is stated in conjunction with a specific test method and conditions of usage.
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The word hermetic comes from the name of the Greek god Hermes, via the vocabulary of alchemy. The alchemists invented a process for making a glass tube airtight, which was used in distillation. The process used a secret seal whose invention was attributed to the legendary patron of alchemy, Hermes Trismegistos.
Several types of packaging need to maintain a seal against flow of gasses: foods, pharmaceuticals, several chemicals and some consumer goods. The term can be used to describe the Food preservation practices such as Vacuum packing and Canning. Barrier packaging includes containers such as glass, aluminum cans, metal foils, and high barrier plastics.
Building envelopes designed with sustainable architecture use airtight technologies for energy conservation projects. Under the low energy building, passive house, low-energy house, self-sufficient homes, zero energy building, and superinsulation standards, structures are required to be extremely air-tight compared to conventional construction. Air barriers, careful sealing of every construction joint in the building envelope, and sealing of all service penetrations through it are all used to achieve this. Airtightness minimizes the amount of warm (or cool) air that can pass through the structure, enabling the mechanical ventilation system to recover the heat before discharging the air externally. In green building, windows combine triple-pane insulated glazing with the airtight void between panes filled with argon or krypton gas to reduce thermal conductivity and increase R-value (insulation) efficiency. In landscape and exterior construction projects airtight seals are used for general service and landscape lighting electrical connections and splices; and in other specific applications, needing to meet "airtight" standards to be "waterproof" and/or "vapor-proof" for human safety and proper functioning.
Applications for hermetic sealing include semiconductor electronics, thermostats, optical devices, MEMS, and switches. It is used for electrical or electronic parts that are designed and intended to secure against the entry of water vapor and foreign bodies in order to maintain the proper functioning and reliability of their contents.
Hermetic sealing for airtight conditions is used in archiving significant historical items. In 1951, The U.S. Constitution, U.S. Declaration of Independence and U.S. Bill of Rights were hermetically sealed with helium gas in glass cases housed in the U.S. National Archives in Washington, DC. In 2003, they were moved to new glass cases hermetically sealed with argon.[1]
Standard test methods are available for measuring the moisture vapor transmission rate, oxygen transmission rate, etc. of packaging materials. Completed packages, however, involve heat seals, joints, and closures which often reduce the effective barrier of the package. For example, the glass of a glass bottle may have an effective total barrier but the screw cap closure and the closure liner might not.