From: rakoczynskij@gtephx.UUCP (Jurek Rakoczynski)
Subject: isobutane/propane (was Re: Charging A/C with Freon)
Organization: AG Communication Systems, Phoenix, Arizona
Distribution: na
Date: Mon, 27 Jul 1992 18:52:49 GMT

In article <1992Jul25.154235.29252@en.ecn.purdue.edu>, ghg@en.ecn.purdue.edu (George Goble) writes:
[ Various parts between the following text have been deleted ]

> Hydrocarbons:  Blend of 21% isobutane, 79% propane (by weight)
> 
> 	               If bought in bulk, hydrocarbons only cost
> 	about $.50 to recharge your A/C system.
> 
> 	Hydrocarbons are much less sensitive to moisture than is
> 	Freon.  Hydrocarbons do not break down into acids and cause
> 	corrosion like Freons do when moisture is present.  Lots if
> 	moisture in a hydrocarbon system will still freeze in the
> 	expansion device though.
> 
> 	                                     Don't use straight
> 	propane, as the pressures will be too high. Always add
> 	the isobutane first.
> 
> 	Go get a 6oz can of "isobutane" camping fuel.  Charge that
> 	into the evacuated system first.  Next follow up with 16 oz
> 	of propane (you will have to crock up a fitting to mate with
> 	a 16 oz torch tank).  That should be close to correct blend
> 	and amount of charge to run a typical car A/C.  It will
> 	be slightly lower in capacity then R-12.  This mix will be
> 	37.5% isobutane, 62.5% and will cause lower pressures than
> 	R-12, but should be close enough to work. Isobutane is lower
> 	pressure than propane.  Adding another oz or two of propane
> 	may help a bit.  When it is time to recharge again, vent
> 	the remaining charge, and start over with a new fresh charge
> 	as the isobutane and propane may leak at different rates.
> 	You can skip the vacuum if you still had pressure in the system.
> 	If it went clear flat, you need a vac to get the air out.

jgd@dixie.com (John De Armond) on 
Date: 17 Jul 92 06:25:20 GMT writes:

:           A mix of 79% propane and 21% isobutane has the same
: vapor pressure curve as R-12 and is compatable with refrigeration oil.
: Both gases are available from your friendly local welding supply 
: company.  Don't use fuel-grade propane, as it is often wet and 
: sometimes contaminated with ammonia.  Trivial to mix.  Just dump
: the required amount of isobutane in a propane cylinder and fill
: the remander with propane.

My questions:
My experience with a/c system in minimal; only the theory.  But
George's and John's comments have peaked a few questions about using
this mix.  Please rephrase my questions or expand your answers if
appropriate:

1. Moisture content!  Is using torch propane rather the fuel grade any
differant?  I guess so as you state but what would I have to ask for? 
Do I need to state it or do the specs define max moisture content? 
Isobutane I assume is typically only available as a camping
fuel?  I guess I could find a wholesaler for "bulk" purchase, but what
about the few pounds (< 15#) for the Saturday mechanic.

2. If I premix as John mentions, is there a problem with mixing so that
perhaps one liquid might drain fron the tank into the a/c first before
the other one?

3. If I use George's method of adding isobutane first, isn't there a
problem when adding propane that the pressure in the system will prevent
me from adding the entire contents of the propane bottle, and therefore
be short of propane.  Would it be better then to premix?  If I don't
premix, how would you use the pressure reading to determine if you need
more mix?

4. The ideal mix should maintain the same a/c efficiency as freon.  Can
you post some efficiency % change with other ratios; in case I screwup
the mix ratios!

Thanks for all your information!
-- 
Jurek Rakoczynski, AG Communication Systems, POB 52179, Phoenix, AZ. 85072
UUCP: ...!{ncar!noao!enuucp | att}!gtephx!rakoczynskij  Voice: +1 602 581 4867
Inet: gtephx!rakoczynskij@enuucp.eas.asu.edu              Fax: +1 602 582 7111
Inet: /DD.UN=J.RAKOCZYNSKI/O=GTE/PRMD=GTEMAIL/ADMD=TELEMAIL/C=US/@sprint.com