Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Vacuum Terminology
Message-ID: <1990Feb5.162530.7095@utzoo.uucp>
Organization: U of Toronto Zoology
References: <108800006@uxa.cso.uiuc.edu> <9505@nigel.udel.EDU> <1990Jan30.045241.20029@utzoo.uucp> <13027@cbnewsd.ATT.COM>
Date: Mon, 5 Feb 90 16:25:30 GMT

In article <13027@cbnewsd.ATT.COM> knudsen@cbnewsd.ATT.COM (michael.j.knudsen) writes:
>> Not even close.  Glass apparatus peters out around 10^-10 mm because of
>> atmospheric helium seeping through the glass (!), metal goes on for
>> several more orders of magnitude with hydrogen seepage an increasing
>> nuisance...
>
>Many of the late '30s tubes were metal instead of glass, mostly
>to get better RF shielding -- according to Henry these should hold
>their vacuum even longer, tho they don't look nearly as nice :-).

Helium seepage through glass is so rapid, actually, that the interior of
most any glass apparatus is in equilibrium with atmospheric helium.  The
glass tubes have had that helium in them essentially from the start; it's
not something that would happen gradually over a period of years.  Just
as well that there isn't much of it in the air.  (There wouldn't be any
if not for radioactive decay -- that helium started out as alpha particles
from radioactive elements in the Earth's crust.)

>Could someone tell me what is the format definition of a Pascal?

A Pascal is a newton per square meter, i.e. the obvious SI pressure unit.
Fortuitously, one atmosphere is almost exactly 100 kiloPascals.
-- 
SVR4:  every feature you ever |     Henry Spencer at U of Toronto Zoology
wanted, and plenty you didn't.| uunet!attcan!utzoo!henry henry@zoo.toronto.edu