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  • Bootleg pressure chamber feedthrough design

    Discussion in 'The main mechanical design forum' started by TomM, Jul 16, 2013.

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    Good idea?

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    1. TomM

      TomM New Member

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      I'm going to do some experiments with a pressure chamber requiring me to get electrical signals out from a strain gage. That means I need at least 4 wires. Directly in the chamber wall, there's a ball valve connected using a 3/4" NPT male end. I'm thinking of replacing that section with a T-joint - one end goes into the chamber wall, one end into the ball valve, and the third end will be capped. Previous tests haven't shown many problems with the NPT connections throughout the system, despite the pressure being 800 psi.

      My idea is this: I want to drill small holes in the cap, then use copper nails as the pin feed-throughs. The head of the nail is on the inside of the cap, so it's pushed outwards by the pressure. The shaft is wrapped in electrical tape, to prevent conduction to the cap itself. Then, the wires would be soldered on either end of the nail. Lastly, of course, an o-ring right under the head of the nail provides the seal.

      I've been staring at this idea for awhile now, and feel like I must be missing some obvious problems, because it seems too easy. Does anybody have some feedback on potential complications? And also, any tips on O-ring selection?
       
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    3. Dana

      Dana Well-Known Member

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      Better would be a plastic or ceramic insert in the metal NPT plug, with holes for the pins, rather than trying to insulate with electrical tape. But why reinvent the wheel? Google "NPT electrical feedthrough".
       
    4. beenaround

      beenaround New Member

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      Abbess Instruments offers a nice variety of chamber kits and components for do-it-yourself-ers, including pass-thru ports.

      Search for "abbess VFT-01" and you'll see one good example.

      Hope this helps.

      -Mike
       
    5. TomM

      TomM New Member

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      I'm trying to avoid commercial feedthrough components, as they all seem pretty expensive ($200 and up).

      Dana - I wasn't too worried about insulating with electrical tape, since the highest voltage is the 10 V excitation for the strain gage. Also, to avoid drilling more holes in the chamber wall, I need to use a T-connection. I'd prefer not to have to drill more holes in the thick steel and worry about adding more stress concentrators
       
    6. TomM

      TomM New Member

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      I'm trying to avoid commercial feedthrough components, as they all seem pretty expensive ($200 and up).

      Dana - I wasn't too worried about insulating with electrical tape, since the highest voltage is the 10 V excitation for the strain gage. Also, to avoid drilling more holes in the chamber wall, I thought of the T-connection. I'd prefer not to have to drill more holes in the thick steel and worry about adding more stress concentrators.
       
    7. ChrisW

      ChrisW Well-Known Member

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      I do feel a bit uneasy, "explosive decompression" is a dirty phrase? I'm hoping the pressure vessel is small, or alternatively can be hydraulically tested prior to pressurizing.
      I think the "O" rings will be extruded from under the nail heads. If you want to pursue this method I suggest using PTFE sleeving as your insulator, then flood the fitting with epoxy to make the seal and allow to fully cure. Test at 1.5x operating pressure for a prolonged period before using in a situation involving stored energy.
       

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