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 abo=
ut
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 vacuum
> > 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 recal=
l
> > 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 sour=
ce
> > 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 micro=
ns
> > 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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