In this write up, I am going to explain about residual contents in service hoses, containers and techniques to avoid refrigerant loss.
Refrigerant emissions results in skewing of system charge, environmental harm as well as the cost of the lost refrigerant. Same precautions are needed even if the gauges are hooked up just to take measurements. This write up mainly applies to manual service tools rather than idiot-o-matic full automatic machines.
Have a cap available for the center port on manifold.
Air in manifold and hoses should be vacuumed, or pushed out with refrigerant vapor. If you're only hooking it up for measurement, connect both fittings and open both quick valves (if applicable). SLOWLY open each valve on manifold a little bit letting the gas escape, ONE AT A TIME, just enough to clear the hoses of air. While this seems like an intentionally release, this step is important to minimize refrigerant loss. If you skip this step, you will be introducing air into the system at the disconnect process.
Here, we occasionally come across questions over how much refrigerant remains in hoses and cans.
Much of it depends on service technique and to some extent, tools you have available. The automatic addition of whatever weight by machine is not an indication of what is lost.
In automotive air conditioning service, almost all service hoses used are the 1/4" inner diameter variety with length usually being 5-6 ft for each hose. There are three hoses. High side, low side, and center port. 5' hose has an internal volume of 48cm^3 and 6' hose is 58cm^3.
Most of us know that things move from high pressure to low pressure. A unique thing about condensible gases like refrigerant is that given the same pressure, liquefaction moves over to the colder side. The primary concern is the high side hose. The manifold gauge is at about ambient temperature, so the refrigerant will liquefy and migrate into the manifold gauge.
So add a slight amount of internal volume for manifold gauge and there's give or take 60cm^3 of internal volume.
Density of R12 is 1.31 and R134a is 1.21 at 25C. So, this translates to 79g for R12 and 73g for R134a. These amounts to about 2.5oz.
so, unless care is exercised, about 2.5oz of refrigerant will be removed from the vehicle.
To prevent refrigerant loss, this is what you need to do:
1.) Shut off car (safety)
2.) block the center port on manifold with a cap.
3.) CLOSE the valve on car end of high side. Disconnect from car.
4.) Star the car and A/C.
5.) Open both high and low valve on the manifold. This will flush the refrigerant trapped in high side hose back into the system and since the low side is the coldest part of the entire loop, liquid refrigerant will not become trapped in service hoses.
6.) Turn off the car, quickly shut off the car side valve and remove the low side connection. You will lose approximately 120cm^3 of gaseous refrigerant at 50psi or so from the system which amounts to 0.1 to 0.2oz by weight, which is negligible.
When adding refrigerant, shut off the refrigerant tank first if possible. If not, reopen the manifold's low side valve after shutting off the tank to move any refrigerant in hoses to the car.
If you don't have quick shut off valves, you can briefly open both valves at once with the middle port capped off immediately after shutting off the car to equalize through the manifold so that vapor from high side will push over any liquid in hose to low side. Close both side and remove quickly. Not recommended. Only do this if you don't have shut off valves on car-side hook-up.
You know you've done it right when you get a slight puff, but absolutely no liquid refrigerant escaping as you disconnect.
Residual refrigerant in large cylinders when there is no more liquid remaining and 50 psi present as vapor:
These figures are based on my own calculation (Leggie on autoacforum) based on available industry data.
R12 12oz can ~0.35 oz
R12 15 lb size: 6oz
R12 30 lb size 10.5 oz
R134a 12oz can ~0.3 oz
R134a 15 lb size 6 oz
R134a 30 lb size 9.8 oz
Your stock of 12oz cans of R12s can be effectively consolidated into your service station by running "recover" on each can as if they're a car each containing 12oz of R12. This will give you almost 100% yield on those small cans. Stop recovering from small cans promptly at about 15" of vacuum(EPA standards) to minimize ingestion of air through seal leak and can collapse. If you have 2 can adapters, hook up two of them to a manifold, one to high side, one to low side. If you have a 4 way manifold, you can do 3. Do the 3rd on center port. Recover through vacuum pump port. This will let you consolidate 2 or 3 cans of R12 at once into your service rig.
Read the disconnect process. The high side hose is opened to low side to return the trapped liquid refrigerant to the system. If there's air in the hoses, it will ingest all of the air along with it.
The internal volume in the little bridge within the manifold is negligible. If you do this step thousands of times on the same car, it could become an issue, but the amount of air that gets entrapped here is negligible. This is certain far less significant than removing the high side hose full of liquid refrigerant and losing 2 1/2 oz of R134a each time the gauge set is hooked up.
Just as air dissolves in water, a negligible amounts dissolves in refrigerant too.
Why remove the yellow hose from the can or tank? Why not just close the can tap valve or tank valve?
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