Xuan,

There are two areas you have not addressed.  One, is the flexible line (bellows
I guess) super clean and dry?  Second, is the flexible line old.  They crack
after a while and create small leaks that are hard to find.  KF flanges can hold
down to the 1E-7.  My system goes 4E-8 with the high vac open and has many KF
flanges.

Brent

Guangchi Archie Xuan wrote:

> Hi, I have a leaking problem that annoyed me very much the past two days.  We
have a Varian pumping station: Turbo pump back by mechanical pump, nothing
special.  The system is using a flexible stainless steel line with K flange
connector to pump chambers we need to be pumped.
>
> Now we put an end-cap to this flexible stainless steel line to seal it.  Then
after pumping this line for 20 minutes and the Ion Gauge (which is located
directly to the line, above the gate valve between the line and the turbo pump)
reads 1e-6 torr, we close the gate valve.  However the pressure (read from the
Ion Gauge) rose quickly (less than 10 seconds) from 1E-6 to close to 7E-4 and
up.  We need to shut the Ion Gauge off at anything above 7E-4.
>
> To trouble shoot it, we put the ion gauge directly above the gate valve and
pump the ion gauge. So now the system is like this:
>     Ion gauge ==gate valve===Turbo pump == mechanical pump.
>
> 10 minutes later we turn the ion gauge on and it reads like 1E-6.  However,
the moment we close that gate valve, the pressure reading for the ion gauge
shoots up again.
>
> I took out ALL the o-rings and grease them carefully and put them back
together, however they still do the same.
>
>
> So my question:   Why can't these K-flange connectors keep the pressure at
1E-5 Torr range?  Is the pressure shoots up indicating a leak or is it just some
out-gasing?  Or is it because the K-flange connectors are not supposed to hold
any vacuum beneath 1E-3 torr?