Gerry, from Jim's Chicken Wagon blog here: http://formicapeak.com/~jimc/cwair.html
" The next step is to try it out. I fully charge the system, using a mix of Isobutane (R600a) and Propane (R290). This mix (70% Isobutane, 30% Propane) is usually available at sporting goods stores as high-altitude campstove fuel. An 8-oz (12-oz by weight) can will run $4–5, you only need one. You want Isobutane (R600a), not Butane (R600), because Butane has a too-high boiling point. It's possible for your compressor to ingest liquid if you use it instead of Isobutane, which can damage its reed valves. Lots of Butane/Propane mix is available out there, so beware. One of the pitfalls of converting an A/C system away from R12 is that there are a number of things that are dialed in for R12's exact characteristics. If you use a refrigerant with a different temperature/pressure curve, the system ends up operating inefficiently. One of the beauties of this hydrocarbon blend is that you can get a very close match to R12's curve. The system then operates well. A usable charge of this hydrocarbon blend is less than 50% of an R12 charge. In part this is because hydrocarbons are more efficient refrigerants than hydrochlorofluorocarbons. Also, most R12 systems are actually overcharged to accommodate slow leaks. If you're doing your own servicing, a lesser charge is acceptable, because it's pretty easy to re-do. And, having a lesser charge reduces the fire risk in case of a sudden large leak. What I find works is to feed in one can of the 70/30 mix, and then augment this with straight Propane until the temperatures and pressures are right. (With the system set for maximum cooling, and the engine at 2000 RPM.) What I do is to monitor the temperature of the pipe next to the high-side fitting using an infrared thermometer. As you slowly leak in Propane, you watch the R12 *temperature* scale on the high-side gauge and the actual temperature of the fitting. When they are close to each other ipso facto you are done. (This seems to be at around the 1/3–1/2 of a propane torch bottle on the cars I've done this on.) You also watch the absolute high-side pressure to ensure it's not too high, and you watch the low-side pressure to make sure it's still in the 20–30 PSI range. You have to make changes slowly enough to let temperatures and pressures reach equilibrium so that you get accurate readings." ------------- Max Charleston SC _______________________________________ http://www.okiebenz.com To search list archives http://www.okiebenz.com/archive/ To Unsubscribe or change delivery options go to: http://mail.okiebenz.com/mailman/listinfo/mercedes_okiebenz.com
