Case Study 02
O-Ring Testing

About

AEON are designing and manufacturing large (and high-transmissibility) optical windows to be used to test and calibrate the Ultra-violet, Visible and Near-infrared Sounder Instrument (UVNS), the key instrument aboard the Sentinel-5 satellite. Part of this project demands adherence to a tightly controlled vacuum leak rate specifications.

In order to address compliance, AEON have developed test hardware to examine and quantify the relationship between a specified seal material (e.g. FKM Viton), groove geometry and surface finish. These key parameters must work together to assure a high level of hermeticity ultra-high vacuum (UHV) applications. Using the hardware AEON was able to empirically demonstrate a representative performance of the final product early in the product development process, providing our customer with a high level of confidence that these stringent requirements will be met.

Project

The specific purpose of the test campaign was:

• To validate AEON’s calculations and Finite Element Analysis (FEA),
• To empirically define the leak rate and pressure decay (ISO 20484) when using different sealing materials, groove dimensions and surface finishes of the contact surfaces.

Ultimately, tests were completed to determine that the sealing arrangement specified by AEON’s designers would be able to withstand the pressure delta during the pump down. We took the decision to perform physical testing of the sealing arrangement to assure that the design met the customer’s requirement of: “a leak rate of <1×10^-9 mbar l/s at a pressure lower that 1×10^-6 mbar.”

To perform the campaign, AEON developed test fixtures, representative of the end product, that would seat polymeric seals of the same material and hardness as the ones specified for the end product. Different fixtures provided variation in groove dimensions, therefore altering the magnitude of compression exhibited on the seals. Whilst it may be intuitive that a high compression provide a more leak-tight seal, it’s important to realise that the forces involved can be very high, easily enough to fracture sensitive components, such as the optical window itself, so it is important to strike a balance between compression forces and the compressive strength of the optical window material.

Test fixtures were pressurised to a magnitude greater than the pressure difference seen by the optical windows during a vacuum pump-down, and then closely monitored over an extended period of time to determine any pressure decay. For improved accuracy, a pressure transducer was integrated with AEON’s very own Multichannel Verification System to periodically capture pressure readings within the pressurised vessel. From the data gathered, it was possible to calculate the leak rate for each configuration.

Results

Testing was completed with four different groove geometries, resulting in compression sets of 10%, 15%, 20% and 25%. Detailed engineering and test reports highlighted:

• Test parameters
• Test equipment used
• Descriptions of each test that was conducted
• A graphical representation, showcasing test data collected
• Additional comments that detail how calculations were completed
• A final statement drawing on experience to provide conclusions of any key findings

Test results showed that the as-designed groove depth provides most fit-for-purpose seal arrangement for the optical windows.

The test campaign has allowed us to validate our design at a very early stage, evidencing the adequacy of the design and providing assurance and confidence to the customer for what is a high-criticality performance parameter.

Can we help?

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