Enclosures hermetically sealed

Arcing contacts in Division 2 areas must be installed in explosion-proof enclosures, be immersed in oil, be hermetically sealed, or be non-incendive. High-temperature devices must be installed in explosion-proof enclosures. Fuses must be enclosed in explosion-proof enclosures unless the fuses are preceded by an explosion-proof, hermetically sealed, or oil-immersed switch and the fuses are used for overcurrent protection of instrument circuits not subject to overloading in normal use. 

Seals are required at entries by conduit or cable to explosion-proof enclosures containing arcing or high-temperature devices in Division 1 and Division 2 locations. It is not required to seal IM in. or smaller conduits into explosion-proof enclosures in Division 1 areas housing switches, circuit breakers, fuses, relays, etc., if their current-interrupting contacts are hermetically sealed or under oil (having a 2-in. minimum immersion for power contacts and 1-in. for control contacts). 

When dealing with small, hermetically sealed parts where the enclosure is leaky, krypton 85, a gaseous, radioactive isotope, can first be forced into the device by applying pressure from the outside. Once an exactly measured holding period has elapsed the pressure will be relieved, the component flushed and the activity of the gas charge will be measured. In the same way it is also possible to use helium as the test gas (see Section 5.7.4, bombing test). 

Recourse must now be made to the use of a certain hermetically sealed enclosure in order to impede an indefinite diffusion. Flat Men say that they are dealing with un gaz, and they call pression or tension the results of a certain bombardement against the enveloping wall which is carried out by millions of molecules shot out forth in all directions from Plane P. This operation is the only phenomenon captured by the senses of a Flat Man... . They give this operation a specific name, HEAT, and they only experience it through an artificial unity, DEGREE OF TEMPERATURE. . .. Voild, the transformation of the operation into heat, which is the reverse of the preceding operations. 

Acceptable protection techniques for electrical and electronic valve accessories used in specific class and division locations include explosion-proof enclosures intrinsically safe circuits nonincendive circuits, equipment, and components dust-ignition-proof enclosures dusttight enclosures purged and pressurized enclosures oil immersion for current-interrupting contacts and hermetically sealed equipment. Details of these techniques can be found in the National Electrical Code Handbook, available from the National Fire Protection Association. 

The converter enclosure is hermetically sealed so that the atmosphere between the electrodes can be controlled. In the conventional thermionic converter, the interelectrode space is filled with cesium vapor from a liquid reservoir at a pressure of 1 torr. The cesium performs two functions. First, it adsorbs on the electrodes to provide the desired electron emission properties. Second, it provides positive ions to neutralize the electron space charge so that practical current densities can be obtained from the converter. 

The major advantages of this technology include the ability to provide high-density interconnections with relatively short path lengths, which lead to higher-speed computers excellent thermal dissipation the ability to operate at elevated temperatures, excellent structural strength provision for a hermetically sealed transistor enclosure, excellent dimensional stability, and an overall low thermal coefficient of expansion. 

Ventilation of battery enclosures, which must not be hermetically sealed. 

Volatile compounds reach the protective vapor concentration rapidly, but in the case of enclosures that are not airtight, the consumption of inhibitor is excessive and the effective protective period is short. Low vapor pressure inhibitors are not rapidly exhausted and can ensure more durable protection. However, more time is required to achieve a protective vapor concentration. Furthermore, there is a possibility of corrosion occurring during the initial period of saturation, and if the space is not hermetically sealed, an effective inhibitor concentration may never be obtained. Therefore, the chemical compound used as a volatile inhibitor must not have too high or too low a vapor pressure, but some optimum vapor pressure. ... 

Due to their small feature sizes, microelectronic circuits need protection from environmental hazards such as mechanical damage and adverse chemical influences from moisture and contaminants. Several approaches are currently in use, for example, hermetic encapsulation of the device in sealed metal or ceramic enclosures, application of soft silicone gels as a cover over integrated circuitry, and encapsulation by transfer molding, which is the topic of this report. Both silicone resins and epoxy resins are used for this purpose. As the quality and performance of the epoxy encapsulants improved, the need for the generally more expensive silicone resins diminished. The present work is exclusively devoted to epoxy transfer molding compounds. 

A double tamper-evident cap sealer has been introduced by Paklink. It has a ring style enclosure incorporating an inner foil seal which, once heat sealed, provides a hermetic barrier suitable for products... 

Vitreous materials comprise a third type of attachment material. Glasses may be used to seal lids to packages for hermetic enclosures. Silver-glass pastes have been developed for die attachment. They have high thermal and electrical conductivities and are used extensively for single die attachment in single packages. 

---