Hmm..  20 cc maybe.   I will check with a vac engineering firm tomorrow in
Dayton.
On Nov 2, 2015 11:40 PM, "Sean Breheny" <shb7@cornell.edu> wrote:

> My gut feeling is that decent gas-service ball valves should be able to
> handle this. In my case, the pressure in a 20 liter tank rose by about 1
> torr per day. That's a leak rate of 20/760 =3D 26mL (at STP) per day or a=
bout
> 1mL per hour. 1mL represents a 20 micron pressure rise in a 38 Liter tank=
..
> OK, maybe my gut feeling was wrong, unless you are working with a volume =
of
> tens of liters :)
>
> On Mon, Nov 2, 2015 at 3:22 AM, David VanHorn <microbrix@gmail.com> wrote=
:
>
> > I need to pump down to 20 micron or so and then backfill to 0.5 to 2
> torr.
> > Hold time of tens of minutes.  Current system leak rate keeps me to 500
> > microns more or less and the breeze is killing my backfill.
> > On Nov 2, 2015 1:08 AM, "Sean Breheny" <shb7@cornell.edu> wrote:
> >
> > > David,
> > >
> > > What is an acceptable leak rate? I am just getting into the high vacu=
um
> > > world but previously I have had a chamber at a low vacuum (30 torr),
> > using
> > > ball valves from McMaster, remain under 60 torr for a month, if I
> recall
> > > correctly.
> > >
> > > It seems that high vacuum stuff I've seen often uses butterfly valves=
..
> > >
> > > Sean
> > >
> > >
> > > On Sun, Nov 1, 2015 at 1:07 PM, David VanHorn <microbrix@gmail.com>
> > wrote:
> > >
> > > > I have a vacuum project in the works, and I've just isolated one
> source
> > > of
> > > > leaks.
> > > > The system needs three shutoff valves.  I used ball valves, and I'm
> > > finding
> > > > that they are leaking. Probably designed for positive pressure I'm
> > > > guessing.
> > > >
> > > > Is there someone on the list with vac experience down around 20
> microns
> > > who
> > > > could suggest relatively inexpensive valves that I could use?   The
> > > system
> > > > is now built with 1/8 NPT but I could switch if I need to.
> > > >
> > > > These valves just need to be on or off, and they are used for
> > controlling
> > > > the flow of air and other gases. No liquids, nothing harsh.
> > > >
> > > > Thanks.
> > > > --
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