From: ghg@en.ecn.purdue.edu (George Goble)
Newsgroups: rec.autos.tech
Subject: Re: isobutane/propane (was Re: Charging A/C with Freon)
Date: 28 Jul 92 20:38:23 GMT
Distribution: na
Organization: Purdue University Engineering Computer Network

In article <1992Jul27.185249.18998@gtephx.UUCP> rakoczynskij@gtephx.UUCP (Jurek Rakoczynski) writes:
>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.
>> 

[stuff deleted]

>: 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.

The consumer grade propane/isobutane is all the same I think.
I used hamburger-gas and it worked fine for me.. but you never know
what you will get.  I know it has *SOME* water in it, since work
with self-sealing refrigerants later in the same car, and sight glasses
showed it was wet.  AGA speciality gasses (via welding shops) has 15lb or
so of isobutane, but it was $150 or so.  Phillips-66 will sell 115lbs or
270lbs (?) containers.  The 115 lbs is around $300 or so + $100 deposit
on the cylinder.  This is "pure" (99.0%) grade, no moisture, small amounts
of butane, ethane, etc, which are harmless. No odorant.  You need to sign
a release that you will not use it for motor vehicle fuel. Their number
is 800-858-4327.  If it was me, I would probably get hamburger grade
propane & camping fuel isobutane, mix it first in a propane cylinder
(20 lb grill tank).  Then take a 2nd tank, with a vac on it, and use
a heater blanket and liquid transfer it back and forth to the 2nd tank.
Heat the tank you want it to leave.  Put in a large refrigeration
liquid line drier (About $10) in the line and a moisture indicating
site glass (about $8).. when the glass goes "green"  (or blue if it
is the pink-blue type), you have dried it out enough. One could also
set one tank outside and one inside to get a temp difference to make
it transfer.. invert the tanks for liquid transfer.  If using a new
propane  cyl, it will probably have grit in the bottom.. waste a tank
and invert it and blow it all out in some field to get the dirt out.

>
>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?

You must use liquid transfer to keep the liquids together.. if vapor
transfer is used, the mix will change

>
>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?

Static (non running pressure) should be around 70 PSIG (+- 5 PSIG) at
70F.   For 80F, pressure should be 83 PSIG +- 8 PSIG.  Just dump in some
isobutane into a propane tank with a Vac on it, then slowly add propane
until the above pressures happen. take your time, and let the tank
warm up. Shake every now and then.
>
>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!

more propane will run up the pressures, but cool more. Pure propane will
give head pressures of 700-800 PSIG, and may even burst the evaporator
if the car is parked on the hot sun (270 PSIG @ 120F). same for R-22.
Anything above 400 PSIG on the head will break something on a car A/C.

Watch the head pressure when charging, stop if it goes above 200 or so
on a cool (70F) day.. and 350 PSIG on a hot (85-90F) day.  You either got
too much charge or too much propane in.
>-- 
>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


WOuld there by any market in selling a "premix" camping fuel, which
would just happen to be the correct pressures for R-12?

--ghg