7 Tips for Pressure Window Design

Recently, I was interacting with a customer regarding a pressure window RFQ and after my first set of questions regarding the design they asked, “how does this affect the quote?” My response was, “what is it you are trying to do, because you have high-end polishing specs on an expensive material?” To which they responded, “oh, it doesn't have to be that glass and the polishing specs can be relaxed.” Now, there's a certain degree of stress, (pun intended) with designing pressure windows; trust me, no one wants to see a window fail. So, this conversation led to our marketing guy to push me to write down tips for pressure window design. We also have an excellent blogpost for pressure window formulas and material properties available here.  
 

1. Material Selection: Choose a material that can withstand the specific pressure range and environmental conditions where the pressure window will be exposed. The material should have appropriate mechanical strength, corrosion resistance, and thermal stability. Common materials used for pressure windows include fused silica, fused quartz, and N-BK7/BSL7. While sapphire is another option for its strength, it will have a significantly higher cost than the other materials listed. I will also add Zerodur^® and ClearCeram^®-Z if low-expansion materials are needed. 
   
   
2. Pressure Rating: Determine the maximum pressure the window will encounter during its operation. Ensure that the selected material and window design can handle this pressure without deformation, fracture, or failure. There is some math involved, but if you are reading this article, I am guessing you can handle the formula. The standard measurement is usually PSI, but in some cases, you will use megapascals.  
   
   
3. Optics and Transmission: If the pressure window is intended for optical applications, such as sensors or imaging systems, consider the optical properties of the material. Namely, ensure that the selected material has the desired transparency and minimal optical distortion in the desired wavelength range which is where customer conversations expand. For laser applications, the surface quality should be tight to reduce any beam scatter or defects in the glass. For viewports and monitoring, 20/10 scratch dig specs are overkill and add unnecessary cost to the part from additional polishing time and finishing. Since surface quality is crucial for optical applications as it affects light transmission and image quality, ensure that the window surfaces are polished to the required specifications and free from defects that could impair optical performance of the pressure window. 
   
   
4. Window Shape and Thickness: The shape and thickness of the pressure window can affect its strength, pressure resistance, and optical performance. Optimize the window's geometry based on the specific requirements of your application. Thicker windows generally offer higher strength, but they may also introduce more optical distortion.  
   
   
5. Sealing Method: The pressure window needs to be sealed properly to prevent leakage or contamination. Consider the sealing method based on the pressure range and the compatibility with the selected glass. Common sealing techniques include O-rings, gaskets, welding, or adhesive bonding. 
   
   
6. Temperature Considerations: Evaluate the temperature range the pressure window will encounter. Different materials have different coefficients of thermal expansion which can cause mechanical stress and potential failure. Consider thermal management strategies or select materials with a low coefficient of thermal expansion if temperature variations are significant. 
   
   
7. Coating: If a coating is required, consider the environmental conditions where the pressure window will be exposed including humidity, chemicals, or abrasive particles. Evaluate protective coatings or additional barriers to mitigate the impact of these factors on the window's performance and longevity. Hydrophobic anti-reflection coatings and, in some cases, DLC should be considered to protect the optics performance and lifespan.  

By considering these factors during the design process, you can create a pressure window that meets the specific requirements of your application in terms of pressure resistance, optical performance, durability, and reliability. Our team is always available to advise on fabrication methods and material selection. For any questions, please reach out to me or our sales team.