There are others with knowledge here for sure and I appreciate all of the information that gets exchanged on this site. I learn a lot from others here.
HVAC systems are designed to hold their refrigerant charge and never need topping off and some systems can run without issue for decades. I have an R22 based residential system that has never needed an ounce of refrigerant after almost 20 years. That said, some systems are more prone to leaking than others. Boats and cars have multiple flared fittings or threaded connections with o rings combined with lots of banging and vibration so even a well installed system can have leaks at the connecting points. Most residential systems are brazed all the way around so any leaks there are usually schrader valves or the evap coil (a growing issue in newer systems). Gauges are the best initial assessment but there are some rule of thumb measurements that give you a lot of information.
Get a baseline delta T by measuring the air temp going into the evaporator and the temp coming out of the vent closest to the evap (if there is more than one). Do this when the cabin air is near the set point so that the humidity is stable. More humidity puts more load on the evaporator heat transfer process and changes everything including gauge pressures. This dela T should be around 20F, mine is 22F. When this starts to drop more than about 2 degrees under similar conditions, something is underperforming. It could be fan efficiency, dirty evap coils, poor heat transfer in the heat exchanger but usually low charge. I use this all the time because it takes 15 seconds with an IR gun. I do put gauges on the system at least once per year but I check this at least once per month.
Also near set point, grab the suction line near the service valve. It should be at least cool and sweating in the summer. Carefully touch the liquid line coming out of the compressor can. It should be hot. The top of the can should be no more than around 250F and most Cruisair units will be considerably less, in the mid to high one hundereds. The process is gas is compressed by the compressor and it gives off heat as the pressure goes up, then the gas is cooled in the condenser coils and in doing so it becomes a liquid and is less hot. This liquid goes to the evap coil where a metering device allows it to expand back into a gas which absorbs heat from the evap fins to complete the phase change. This gas should be slightly cool as it gets back to the compressor but not too much so or the refrigerant may still have some liquid content which is bad. If you like to learn technical stuff look up the ideal gas law which is the law at work in refrigeration.
There are many problems that are not charge related that can be diagnosed with a volt meter, amp clamp meter, and a capacitance meter - all possible with relatively low cost meters these days. The faults can be OL sensors, pressure sensors, capacitors, start relays, cooling water pump relays. You don't need an EPA license to check any of these and some of them fail routinely. All you need is the wiring schematic and some basic knowledge to make sure that the energizing signal is present, the high voltage power is proper, the compressor is getting appropriate power and its running with proper amp draw. BTW another symptom of low charge is lower amp draw. A locked rotor on the other hand has a very high current draw for a few seconds and then goes to zero abruptly as the thermal overload switch opens, only to reset in a minute or two. That spec is actually in the compressor data sheet as well as the normal current draw spec.
Hope that helps.
George
