Difference: MWardWorkDiary2017Log1 (13 vs. 14)

Revision 1422 Nov 2017 - MarkWard

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-- MarkWard - 27 Sep 2017
-- Public.MarkWard - 27 Sep 2017
  Previous Logbook


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16 / 11 / 2017 4.76x10^-4 mbar 7.59x10^-4 mbar l / s
20 / 11 / 2017 3.27x10^-4 mbar 4.00x10^-4 mbar l / s
21 / 11 / 2017 3.37x10^-4 mbar 5.00x10^-4 mbar l / s
  • Some numbers and discussion
    • Turbo was added 15 / 11 / 2017 , type is a HiCube 67 l/s turbo molecular, reaching an ultimate pressure of 3.27x10^-4 mbar
    • An effective pumping speed of 33.7 l/s predicts a massive gas load of 0.011 l/s... Id not trust this... too many assumptions made about the pumping configuration
    • Opening the vacuum seal pumpout port causes a large rate of rise... approximately 3.2mbar l/s!!! <--- AAAGH!
    • Above suggests there is no effective seal between the Silicon (pressure seal) and the Inner vacuum seal. Sam sort of ummed and arrrd his way through telling us that during one of the phone meetings, I expected a partial seal, not an enormous leak like that... thats more than a virtual leak
    • Other option for our observed rate of rise is... Permiation
    • Silicon is the worse material to make a high vacuum seal out of due to Permiation constants being quite high.
    • Qperm = Kperm * Pressure (external) * Area (exposed to pressure)/ Thickness
    • Silicons N2 permiation is roughly 2.7x10^-10 m2/s
    • Put in the spec for our seal and you find a number roughly 6x10^-4 mbar l / s
    • To put it bluntly... it is likely the majority of our rate of rise is permiation through that seal. frown frown frown
    • Clear we need to make that inner seal work, or concider a procedure for backfilling which involves minimum time at vacuum.
    • Viton and Buna are factors of between 800-4000x better, which would put us into 10^-7 mbar l/s region, if that volume is pumped out, the issue reduces to zero
  • Workshop have been test fitting our components, I saw a few final products this moring big grin we should have multiple in our possession Soon


  • Complete DMTPC and HPTPC internal comparison drawings, for design consistancy
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