Re: Mercury, dif. pump oil, etc

To: neon-l@netcom.com
Subject: Re: Mercury, dif. pump oil, etc
From: dead@netcom.com (John Anderson)
Date: Mon, 5 Feb 1996 15:55:18 -0600
Reply-To: neon-l@netcom.com
Sender: owner-neon-l@netcom.com

Mark,

First on the difusion pump oil - what you notice is normal.  But it may or
may not be a result of contaminated oil.  Silicone oil does degrade over
time, especially on repeated exposure (when hot) to air.  What you're
seeing may very well be outgassing either within your diffusion pump itself
or anywhere in your manifold.  The first step in tracking it down is to
isolate as many things as possible.  If your manifold it pyrex, then make a
plug for the fitting that transitions to lead glass.  Also isolate any
other area you can with caps, plugs, etc.   A _tiny_ , invisible smudge of
moisture can outgass for days, even weeks.  Often it takes the least amount
(and money) of time to just clean everything and replace the silicone
fluid.  I try to change mechanical pump oil every 6 months or so, but I
often let it slide much longer.

Is the DigiVac a themocouple or pirani gauge?  I have "heard" that TC's are
very squirrly in the 0-3 micron range, and not a good indication of when
you need to hunt a leak or contamination.

As to your questions about bombing - you basically want to heat the glass
first, _then_ the electrodes.  It takes gas in the tube to heat the glass,
so you usually begin bombing at as high a pressure as you can (up to 10
torr or so).  When the glass is hot (280-300C) reduce the pressure to 1-2
torr, increase the bomber current to 10x the milliamp rating of the 'trode,
and heat till they glow.  Every bombing proceedure incorporates these
elements.

Most u-gauges are calibrated in torr, which is synonymous with "millimeters
of mercury".  Millitor is synonymous with "micron.", and is also equal to
1/1000 of a millimeter of mercury.

    1 torr = 1mm Hg = 1000 microns

 -John