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(NOTE): The fittings that interface with the injector are NOT the fittings we used. These fittings were COTS fittings from McMaster that were initially had YorLok nuts on them (link), but were unscrewed. THESE FITTINGS WILL SHEAR. They were 10-32 threads, we scaled up to a custom fitting with a 1/4-32 male thread and we haven't experienced shearing when compressing the copper crush.
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The tube is a micro piece of SS hard line (0.05" OD) that you can get from McMaster, and is brazed to the inside of the custom fitting. This joint actually wasn't that hard to braze – you can put flux and filler metal into the larger hole on the inside of the died 1/8 NPT side. Then, you can slide the tube inside and heat up the fitting from the outside with a propane torch. IMPORTANT: In the CAD photo, the needle tube is only barely sticking out above the surface of the fitting on the inside; this is supposed to be mOsT oPtiMaL but in reality it needs to stick way above that to allow the flux to sit below and not clog the inside of the needle. Once the stainless steel fitting gets to a rose red (basically eyeballing that the temperature is ~650-870 C or 1200-1600 F, as this is the necessary heat range to apply the silver-based brazing flux (should turn clear at this temperature) and add in the filler metal)), the joint has probably heated up enough on the inside to melt the flux and seal it. Example below!
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Once the igniter solenoids close, there will be a trapped volume of ASI gas between the ASI chamber and the main chamber, which should "insulate" the ASI well enough. The main chamber pressure will push on this insulated pocket, which will push on the PT in the ASI and give a pressure reading.
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