Anyone who has ever disconnected a hose from a Schrader valve knows about the spray you get as a result. If you are taking a hose loose from the liquid line there can be quite a lot of very cold refrigerant spraying out. Many people don’t realize that the spray is usually NOT coming from the Schrader valve, but from the hose! The spray is from refrigerant that is trapped in the hose coming back out when you loosen the hose. By my calculations a standard 5 foot 1/4” hose holds about 3.5 ounces of R22 liquid. Releasing it is bad for the atmosphere, bad for the system, and bad for you if it gets on your skin. If the system was perfectly charged before you connected your gauges, it is no longer after you take them off. If you were to connect a 5 foot hose to the liquid line of a dorm refrigerator and fill up the hose, you would essentially suck most of the charge out into your hose and gauges. Of course you would first have to install a piercing valve or two – creating potential leaks to boot.
My point is that you don’t always need to connect your gauges to every system you see – especially small critically charged systems. Even on larger systems, if you are connecting gauges just to check the system operation, consider getting a couple of “short gauges.” These are essentially a gauge mounted on just enough tube to connect it to the system. They hold a minimal amount of refrigerant, so the amount released is much less. This saves the atmosphere, is better for the system, and is way better for your fingers. Here is a link to a picture of a set of stubby gauges used in an article on ice machine service found in Contracting Business Magazine.