Self-contained systems will normally have listed on their data plate the amount of refrigerant weighed in at the factory when the system was built. To correctly charge these systems, simply add the same amount of refrigerant as the factory. However, when using this method, you do need to have the system in an evacuated state, so if you want to top off the system, this method will not work. If the system has some refrigerant in it, you will need to recover the refrigerant, evacuate it, and then you can weigh in the refrigerant. It is a more time-consuming method than simply topping off the refrigerant charge but is definitely more accurate.

When using this method, always look to see if any of the system components have been modified, as this could cause the amount of refrigerant needed to change and the value on the data plate to be inaccurate. Another potential issue with this method is how you handle the refrigerant contained within your hoses — mainly your high-side hose if it is connected to the liquid line. Since you are generally dealing with small amounts of the refrigerant, the amount of refrigerant within this hose could cause some inaccuracies, especially if you are using a long hose.

There are several ways you can factor in the amount of refrigerant in your hoses — again, mainly the high-side hose. One way is to determine the amount of refrigerant your hose holds, which will be based on the internal diameter of the hose and the type of refrigerant used. Some manufacturers will offer charts showing the amount of liquid held per foot for some of their hoses and based on some common refrigerants.

Another method is to use a high-side hose with a ball valve on the end connected to the system. When your scale shows you used the correct amount of refrigerant, shut off the ball valve and allow the refrigerant from the high side hose to be drawn into the low side.

Since these systems will typically use a Schrader valve for both the low- and high-side connections, another method is to use a valve core remover tool on both the high and low Schrader valves. This will also help speed up the evacuation process, since the cores will be removed from both of these valves. Before charging the system, close the ball valves on each core remover and purge your hose to the core remover. Then zero your scale and weigh in the charge. When you are done, be sure to once again shut the ball valves on the core remover and reinstall the cores before removing core remover tool.