Nowadays, when you ask people how to check the charge on a heat pump during low outdoor temps, they will say that you need to “weigh in and weigh out” the charge. While this may be an effective method, it isn’t always practical.
Now… If you are making a refrigerant circuit repair, weighing out and weighing in makes perfect sense, especially since microchannel condensers and scroll compressors make pumping down less viable anyway. But there are many cases where you just need to check the charge to make sure the system is working properly, and in these cases, weighing in and out is just plain silly.