Re: 704 in manometer

To: neon-l@netcom.com
Subject: Re: 704 in manometer
From: dead (ja)
Date: Sun, 26 Nov 1995 07:48:06 -0800 (PST)
In-Reply-To: <199511260007.OAA24651@lehua.ilhawaii.net> from "Ted Pirsig" at Nov 25, 95 02:07:51 pm
Reply-To: neon-l@netcom.com
Sender: owner-neon-l@netcom.com

> > I like 704 as a
> >manometer fluid since its viscosity is much lower, and is much more
> >responsive and easier to degass. 
> 
> Wouldn't you need to recalibrate your scale to match the specific gravity of
> this? Or is it close enough to butyl pthylate (sp?) to not matter?

Absolutely.  In fact, it will have a different calibration than 705.  I'm
not sure of Butyl's exact specific gravity - but 704 is 1.07 and the 705
is 1.09.  I've calibrated scales for both silicones, and they are very 
close but enough to make up separate scales. 

Besides lower vapor pressure, silicone oil is easier on any o-rings it
happens to contact.  An o-ring supplier in Dallas looked at Butyl Pthalate
in a compatability chart for me awhile back.  It's surprising how many
elastomers it's "not compatable"  with.  The list _included_ Viton, common
in greasless stopcocks. FETFE is somewhat better. If a manomoeter made
with such cocks can slam fluid up to the seal, it will ruin it in short
order.  This was what brought me into the o-ring supplier in the first
place - a recurring leak at the u-gauge stopcock. 

The silicones don't have that degrading action (in fact are recommended
for conditioning Viton o-rings). Butyl is still lower in viscosity, and a
bit easier to see because of its color. 

I bit the bullet and bought an Edward's 0-50 mbar gauge for filling. 
Mostly so I could entrust others to pump without slamming fluid all over
the place.  In many ways I miss the simple, accurate, and self-calibrating
u-gauge!  May go back to it some day.

     -John Anderson