Hermetically sealed connectors you can always rely on

Paul Boughton
Hermetic sealing is crucial in preventing gas leakage in numerous operating environments and applications, especially those exposed to vacuums and high pressures. Christophe Boillat reports.

Severe conditions call for robust equipment you can depend upon at all times. In vacuum, pressurised or underwater conditions, there is a high risk of gas leakage into or out of equipment via gaps, or diffusion through material. Leakage may lead to equipment failure or even bodily harm. High-quality airtight connectors and sealing barriers are thus prerequisites for reliable equipment performance and long-lasting protection.

Fischer Connectors' extensive range of hermetically sealed connectors - like Fischer Core Series and Fischer UltiMate hermetic panel mounted connectors - are specially designed for vacuum applications and pressurised vessels, as well as for long-term immersion and exposure to strong jets. They all undergo a 100 per cent leak test to verify gas tightness, and their sealing is rated IP69K. Moreover, they have consistently demonstrated total reliability in even the most demanding environments.[Page Break]

Hermeticity in vacuum applications

A vacuum forms when gas molecules are removed from a given volume. Because air molecules act as barriers to other molecules or electrons, a vacuum has to be created in a chamber before specific physical reactions can take place. A vacuum chamber is needed in, for example, equipment for analysing gas (mass spectrometers) or specimens (electron microscopes). Atmospheric' pressure is equivalent to one bar, or 1000 millibars (1000mbar). Applications vary in their vacuum level requirements (10mbar for 'low vacuum', 1E-5mbar for 'high vacuum' or 1E-7mbar for 'ultra-high vacuum').

A minimal volume of gas will always pass through a connector on a vacuum chamber per unit of time.

Hermeticity is vital to stop gas molecules entering the vacuum chamber, increasing the residual pressure and preventing the equipment from functioning properly. A 'leak' is quantified by measuring a 'certain volume of gas per second' traversing the connector, expressed in mbar.l/s. In the vacuum industry, 1E-4mbar.l/s is considered a 'big leak', tolerated only in very rare low vacuum applications, while 5E-7mbar.l/s is considered a very small leak, tolerable in most vacuum applications.[Page Break]

Pressure gradients

A low vacuum of 10mbar induces a 'pressure differential' with atmospheric pressure (1000mbar) of 990mbar. An ultra-high vacuum of 1E-7mbar (0.0000000001 bar) induces a differential of 0.9999999999 bar. More than one bar differential pressure is unattainable because a vacuum chamber cannot contain 'fewer than zero air molecules', which explains the 'standard gradient' of one bar. A given leak for a given connector differs if the connector is exposed to overpressure. The leak is larger if the vacuum chamber is replaced by a '10bar pressurised chamber'.

Since leaks are most commonly measured using 'helium tracers', they are often called 'helium leaks'. However, since vacuum applications do not work with helium, leaks are also indicated for air molecules.

Air leaks '2.6 times less' than helium (standard calculation based on the square root quotient of the molecular mass of N2 and He). A connector leak of 4E-7mbar.l/s measured under a helium atmosphere is actually 1.5E-7mbar.l/s in the real vacuum application (air).[Page Break]

Leakage or diffusion?

Leakage and diffusion are two major mechanisms whereby gas can permeate a hermetic barrier. 'Leak' phenomena are due to 'gaps' through which gas passes, while diffusion occurs when gas passes through the material. Defects or cracks in the barrier generally result in gross leakage and device failure. Yet even intact barriers allow small quantities of gas to diffuse through them, a natural phenomenon observed in most plastics and rubber.

Connectors or other devices mounted on hermetic systems therefore exhibit residual leakage. Connectors for high-vacuum applications usually have O-rings made of Viton, which produces a tiny residual leak. Optimising the design of connectors or other devices mounted on hermetic systems, and selecting material carefully, ensures that the residual leakage rate remains extremely low and suitable for most ultra-vacuum applications.[Page Break]

Measuring leaks

The standard IEC 60068-2-17Qk method 3 for leak measurement applies to a 'short-term leak'. Connector hermeticity is usually tested by spraying helium onto the connector and measuring the quantity of helium traversing it. This test detects 'leaking' connectors without measuring the real long-term leak value. If the threshold is established at 1E-8mbar.l/s (as at Fischer Connectors), the test can effectively identify connectors unsuitable for vacuum applications. Lower thresholds (1E-9mbar.l/s or 1E-10mbar.l/s) are irrelevant because the diffusion leak through O-rings inevitably reaches higher values after several minutes. Although the diffusion effect depends on numerous factors, a value of 1.5E-7mbar.l/s (air) for standard-size connectors can be given as a 'medium- and long-term leak value' (equivalent to a leak of 4E-7mbar.l/s for helium).

Connector sealing techniques

Fischer Connectors' hermetically sealed products have several independent sealing barriers - including panel, block and interface seals - designed to control the connector's performance precisely according to requirements. Panel seals, joining the connector housing to the panel or equipment housing, have large cross-sections contributing to hermeticity. All Fischer Connectors' standard hermetically sealed connectors feature a Viton panel seal. Block seals ensure long-term hermeticity. Combining sealing gasket and advanced polymer compound minimises permeation and optimizes reliability over a wide temperature range. Interface seals protect the interface between two connectors, preventing water or harmful particles from entering the connection where male and female contacts mate.

Sealing is a highly complex science and designing a totally reliable seal necessitates expertly combining mechanical design, materials science, surface science and fluid behaviour. Selecting the right connector and seals depends on thoroughly analysing the operating environment and determining specific application requirements.[Page Break]

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Christophe Boillat is Product Quality Manager at Fischer Connectors SA, Saint-Prex, Switzerland. www.fischerconnectors.com

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