BTW, if anybody's looking for affordable refrigerant to keep those old AC systems running, there is a very low cost alternative that will give identical performance.
I had an old car that needed R12 charge. R12 is ridiculously expensive and hard to get. After looking around I found that a mixture of 21% isobutane and 79% propane can be used as an equivalent substitute for R12 (compatible with mineral oil, same cooling performance, same high/low pressure). Actually straight propane will give even better cooling, but head pressure shoots up and might be too high for some compressor, I burned out one clutch doing that, so had to dial pressure back down with Isobutane. If you fill with IsoPro by weight - fill to 40% and that will give equivalent volume. That's for R-12, so double check on R-22.
Here's my notes from when I did the fill-up:
Gas weights:
R12 120.91 g/mol
Propane 44.1 g/mol
Isobutane 58.12 g/mol
34.839 + 12.205 = 47.0442
0.389084442974113 should be the mix replacement ~0.39
To charge R12 system with isobutane and propane mix:
* Evacuate the system (draw vacuum for 20 minutes)
* Add liquid MSR IsoPro to high side with engine off (add liquid because in gas form propane will be added before isobutane, add to high side because it can all flow in to condenser without the danger of locking up the pump). For Civic - total: 8.91oz, 21% isobutane 79% propane => 1.87 oz isobutane, 7.04 oz propane => MSR IsoPro 2.34oz / propane 6.57oz. Isopro cans have a valve that stay opens only as long as it's being held manually open. Remainder of isopro in the hose can be extracted out by starting the engine and opening up low side/suction side valve. Use very thin hose to ensure that the amount of isopro in the hose is minimal.
* Add propane gas to low side with engine running
--- 07/18/2012 User opinion:
* How could I make an HC based replacement if I wanted to?
You want to upgrade to an HC based refrigerant for better performance. But you don't want to send a lot of money to OZ technology, or you can't find them. This section is for you. It describes how you can make your own refrigerant from commonly available hydrocarbons. The procedures described in this section were taken from rec.auto.tech archives, dating from June 1991 to the present. Most of the text can be attributed to either George Goble or John DeArmond.
************************************************************************ DisclaimerSome of the procedures described in this section may not be legal. Refilling some types of pressurized containers is illegal, as is replacing R-12 directly with a non EPA approved substitute.
Several states have banned flammable refrigerants outright. The current list is:Arkansas, Conn., Idaho, Indiana, Kansas, Louisiana, Maryland, North Dakota, Oklahoma, Texas, Utah, Virginia, Washington , DC.
Check with your state to see what the exact restrictions are, if any.
Hydrocarbons weight much less than CFCs, consequently much less mass is required to achieve the same pressures. Always charge using gauges to prevent overfilling. Check the pressure and temperature of your blend if you pre-mix and store. The pressure should be the same as R-12 at the same temperatures. Adding propane will increase the pressure at a given temperature, adding iso-butane will reduce it.
************************************************************************
The most commonly asked about hydrocarbon mix is 21% isobutane, 79% propane, by weight. Note that is isobutane, not butane or n-butane. This mix has the same temperatures and pressures as R-12, but performs better in your A/C system, partially due to being a blend. Hydrocarbons weigh less than CFCs, so you will need about half the weight of hydrocarbons to achieve the same performance. The paragraphs below describe how to charge this mix directly into your A/C system from the can, and how to pre-mix it in a large container for future use. Be sure to charge with gauges so you can stop at the appropriate pressure. Do not add the same weight as you would if it were R-12.
Side tappers are available from JC Whitney and other places. A side tapper is a device that taps a can by piercing the side of the can, rather than the top. It will work on the small cans of refrigerants as well as cans containing other substances, whereas the standard A/C tapper will only work on refrigerant cans. In a ruling from the summer of 1995, the EPA ruled that it is illegal to use side tappers on cans of approved refrigerants. Use them on R-12, or on your custom blend, but it's illegal to use them on R-134a or any other approved R-12 substitute. You're supposed to use the fitting on top of the can.
HOW TO GET IT-------------Isobutane is available as the fuel used in Gaz brand camping stoves. Read the label to make sure you are getting isobutane and not n-butane. isobutane is generally used in stoves designed for low temperature use. Gaz sells both plus a propane/butane mix so read the label. Propane is conveniently available in propane torch cylinders. The odorant does notharm its use as a refrigerant. Don't use gas grill propane. This stuffis generally fairly wet.
HOW TO PRE-MIX IT-----------------You'll need two old torch bodies fitted with refrigeration flare fittings, an empty propane torch cylinder, a side tapper, and some standard refrigeration service hose.
An empty propane torch cylinder makes a very good mixing container, particularly the large fat ones. An old torch valve fitted with a refrigeration flare fitting lets you use it with A/C equipment. The procedure is simple and requires only a scales of some sort. A postage scale will work fine. Evacuate the propane cylinder if you have a vacuum pump. Then using a side tapper for blow-off cans (the type that punches a hole in the side of the can), introduce the correct weight of isobutane from the Gaz cylinder. Then top the cylinder off with the required weight of propane from another propane torch cylinder. Warmingthe donor cylinder will drive the process. A second torch body fitted with a refrigeration flare will let you hook the two cylinders together with a refrigeration service hose. Be sure not to overfill the recipient cylinder. Check this by slightly lifting the safety valve with the cylinder sitting upright. If liquid comes out (white mist, real cold), bleed the cylinder until the liquid is below the bottom of the safety valve. Needless to say, do all this outside.
Before using the mix, double check the accuracy of the blend by comparing the vapor pressure in the cylinder to its temperature. This is easily done using refrigeration gauges. The vapor pressure should agree with that of R-12 +- 10 psi or so. When using your mix, charge with the bottle upside down, ie liquid into the system. Otherwise the propane will charge first due to it's higher pressure. Charging as a liquid ensures that both the propane and iso-butane charge according to their proportions. Be careful to charge slowly, however, because if the liquid gets back to the compressor it can destroy it.
When you modify a torch body, you'll need to find and drill out all restrictions that limit the propane flow to the torch. Typically there is a restriction and/or a check valve in the barb that taps the cylinder and another one downstream of the control valve.
BTW, propane torch cylinders make convenient replacements for blow-off cans. They are much more gas-tight than the typical disposable freon cylinder so transfering valuable R-12, GHG-12 or whatnot makes real good sense. And they are more reliable. Instant sickness is dropping a 30 lb can of freon onto something that punctures it. :-( Be sure to label the cylinder contents.
It is illegal to refill and transport disposable containers. BBQ grill containers are legal to refill, but are much less convenient. Be sure whatever container you use is clean initially.
HOW TO CHARGE DIRECTLY INTO THE A/C SYSTEM------------------------------------------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 27% isobutane, 73% and will cause lower pressures than R-12, but should be close enough to work. The best mixture is 21% isobutane, 79% propane, so adding less isobutane and more propane will help. When it is time to recharge again, vent the remaining charge, and start over with a new fresh charge as the isobutane and propane will leak at different rates.
.... Just joking. Please PM me and provide me the alternatives, Im open at this point to any and all brand recommendations. 
