[hatzie]

I've never had good results trying to extrapolate pounds of R12 to R134, HFC152, or R290/R600 blends.

I saved this method from some posts Jungle Eddy made about using hydrocarbon and other alternative refrigerants on a Fidonet Aircon message forum that I frequented on a dial in BBS system. Around 2001 or so. I thought it was older than that.
This method always gives me decent results no matter what refrigerant is used.

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Posted by JungleEddy on September 05, 2001 at 08:17:44: via: or 66.25.151.188

More important stuff.

How to charge an A/C system.

Note: This method will work for any automobile A/C system regardless of refrigerant type. The pressures I list here will only be correct for HC’s.

I have been trying for the past couple of weeks to come up with a “system” that would allow just about anyone to properly charge their A/C system. While this method I have developed is not fool-proof, it yields the closest thing to a perfect charge I have found. I have tested it on three “non A/C” mechanics in the past few days with great success.
First: If you have not read my previous post on vacuum, flushing, oil supply, leaks and fans etc etc, please do so now. You still need a properly assembled system to get proper cooling!

This ENTIRE process makes a few IMPORTANT assumptions!!
1. The ambient air is OVER about 84 degs.
2. You have a set of A/C gauges
3. You do not take shortcuts.
4. You are able to read and comprehend enginsh.

This process will work down to about 80 degs ambient air, but will be MUCH MUCH easier to over charge below 85 degs ambient air!! Relative high side pressures will be lower at 80 degs ambient and pressure drop after wetting the condenser will be less dramatic.

Pull a hard vacuum for a minimum of 15 minutes, 60 minutes is perfect. Static charge the system to about 65 psi while the engine is NOT running.

If you have a low pressure cut out switch on your system you will need to adjust it down to 18-19 psi sometime during this process. Personally, I find it easiest to do this about midway (now) through the charging process. Remove the connector from the switch and turn the adjustor screw about 1 full turn counterclockwise from its factory position. (counterclockwise is less psi, clockwise is more psi, 99% of the time) Start the engine and turn on the A/C, recirc, max fan, and engine at idle. Max fan is important, since it will help keep the evaporator from freezing as you charge. Charge the system slowly (if you have to!) until the compressor stays on fairly continuously at IDLE. The adjustment I illustrated above should give you a cut out psi of 12-18 psi. Watch your low side gauge and SLOWLY increase engine rpm. The low side pressure will drop slowly but substantially because you are still undercharged. Watch it drop from 20+ psi down through the teens and carefully note where the compressor cuts out. Return the engine to idle and pull the connector to the cut out switch and adjust it accordingly. Try this several times until, as the engine is slowly revved the compressor cuts out at about 18-19 psi on the low side.

The Charge:
Bring the engine rpms up to a continuous 1200-1800 rpm. Begin to add refrigerant (if you have to) SLOWLY until you notice that the air at the vents is noticeably cooler than the ambient air, say at about 65-75 degs or so.
At this point grab your garden hose and hose down the condenser; soak it once only. Your pressures will drop dramatically. The high side should drop below 150 and the low side should drop low enough to cycle the compressor, or if the switch is temporarily “jumped” the low side should be well below 20 psi. I prefer at this stage to jump the connector to the pressure switch to keep the compressor running continuously. It makes the process go much faster, but you risk freezing the evaporator. Now, watch the system pressures rise as the water evaporates and the heat in the system and stabilizes and equalizes. This can take a long time if you have a mechanical radiator fan. The point where the pressures remain relatively constant is called equalization.

Here is the key:
(After wetting the condenser) As you are watching the high side pressure rise from 160psi through (potentially) about 205psi the LOW side should REMAIN from 21-24 psi. If the high side never sees 145+psi you are still low on charge as long as the ambient is 85 degs or above. If as the high side needle swings through 160psi the low side is still below 20psi, you will add more refrigerant AFTER wetting the condenser again and dropping the pressures. Keep doing this until the low side remains at 21-24 psi while the high side swings through 160psi and finally settles at equalization (no more rise) Keep the engine rpms constant and wait for equalization (or close) each time before wetting and charging. If you are going carefully and slowly you could see a high side over 225 at equalization BEFORE you reach a full charge. The high side will DROP as you come closer to a full charge. Be aware that if you over-charge, the high side will climb again and never come down.

Note: If the ambient air is above 95 degs, stop watching the low side after the high side climbs past 205 or so. Especially if you have weak fans.

Note: Take your time and wait for equalization and water evaporation off the condenser before adding refrigerant. An overcharge can occur with no more than an extra 1.5 ounces of refrigerant!!

You can double-check your work at any time (and I suggest doing so) by waiting for full equalization and stabilization of pressures. Then, carefully MIST water into the condenser SLOWLY SLOWLY dropping the system pressures and watching for the same readings on the gauges as listed above.

When finished you should have a sweaty return line all the way back to the compressor. When the return line begins to feel chilled over the first portion of its length, you are approaching full charge. Do not forget to reconnect the low pressure cut out switch!

Do not consider your vent temps accurate until the vehicle has been driven for about 10 minutes at moderate highway speeds!!!! Idle low side pressure should not exceed 40 psi (34-38 actual) if everything is working well; good fans etc….

The idea here is that the compressor should NOT cycle when ambient air temps are above 81-84 degs.

Interesting note for tech heads: If you are charging SLOWLY. You will find at first that the high side will be at a higher pressure, after equalization, on a low charge than at a correct charge! Remember, raised heat = expansion and/or pressure.
Why: (basically)
Within a certain range, the pressure in the condenser (high side) is MUCH more affected/determined by the temperature of the refrigerant than its volume.
ALL THINGS BEING EQUAL, the condenser has the ability to lower the temperature of the refrigerant passing through it “X” amount and no more. The compressor, in compressing the refrigerant, heats or raises the temperature of the refrigerant “Y” amount. Lets say the temperature of the refrigerant entering the compressor via the return line is “Z”. So the final temperature “T” of the refrigerant that gets to the orifice/exp valve is: Z+Y-X=T
If we could lower the value of “T” the entire system would work more thermally efficiently and at lower pressures on the high side. So, ALL THINGS BEING EQUAL, you could 1. get a larger condenser and raise your “X” value or 2. lower the value “Z”.
A full charge on an A/C system will not only have enough refrigerant in it to keep the evaporator “chilled”, but JUST ENOUGH that the line leaving the evaporator and returning to the compressor will also have substantially cooled refrigerant in it THUS LOWERING YOUR “Z” value!! Hello!!
Note: Over charging will allow actual condensed refrigerant (liquid) to make it all the way back to the compressor. As we all know, you cannot compress a liquid…boom/screech.
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