Newsgroups: sci.space.shuttle
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: He Signature test
Date: Mon, 5 May 1997 14:42:00 GMT

In article <01bc5418$6e83a200$8410f6cd@bzentis-ppp>,
Brian Zentis <bzentis@ncinter.net> wrote:
>What is the He signature test that is done as part of the orbiter
>preparation?  I read in the status reports that it is to take place Tuesday
>for STS-84.  Is it a leak check and how do they go about doing it?

As I understand it, yes, basically it's a leak check.  You pressurize the
plumbing with helium, partly because it's inert (i.e., safe) and partly
because its very small molecule will leak vigorously through even very
small holes.  Then you wave mass spectrometers (the only practical way of
detecting small amounts of helium in air) around until you're sure that
none is leaking, or more precisely that leak rates are within tight limits.
--
Committees do harm merely by existing.             |       Henry Spencer
                           -- Freeman Dyson        |   henry@zoo.toronto.edu