What is "Super heat"? You will hear and see this term all the time in reference to refrigeration. Simply put it is the difference between the temperature of a vapor line in relation to the temperature scale on a pressure gauge for a particular refrigerant or how much liquid is feeding the evaporator in relation to how fast it is being boiled off. for example (R22) if the suction gauge reads 70 psig then the evaporating temperature is 41 degrees but if the tubing is 51 degrees then you have 10 degrees of Superheat. A typical range for residential air conditioning is 8-18 degrees with some error based on extreme conditions. Once you understand Superheat you can diagnose obvious problems. For example a system that is under charged or has a stuck (closed) metering device will have high super heat (over 20 degrees) at the compressor and a system that is grossly overcharged or has a dirty indoor coil will have very low Superheat about 3-7 degrees with low suction pressure and the suction line will be very cold. It is ok and quite normal for the Superheat to change dynamically while the system is running, you will have to interpret what you are seeing.
What is "Sub cooling"? Sub-cooling is similar to Superheat but happens in the condensing portion. Refrigerant when condensing will happen at a particular temperature which is very close to the temperature scale corresponding to head pressure for a given refrigerant. After the refrigerant is condensed it will try to assume ambient temperature but will never reach it. The difference between liquid line temperature and condensing saturation temperature is Sub-cooling and is a very good indication of "refrigerant level", but only when proper Superheat is indicated or you could have a misleading indication. Typically 20 degrees of Sub-cooling is desirable and the closer the liquid line temperature is to ambient the better (indicating an efficient system). Checking Subcooling in the heat mode of a heat pump has to be done carefully because you have influence of the space between the indoor coil and the point of measurement. For best heating you will want most of the refrigerant to be condensing in indoor coil without backing it up with refrigerant. As a general rule Subcooling = Refrigerant charge quantity, Superheat = Refrigerant cycle performance. Check both!!! The following conditions are for R22 in the cooling mode and relate to indoor coil problems which contrary to human nature we find to be 80% of the reason for poor cooling. Human nature says that the Thermostat is "where is all comes from" and if you can just get a new enough outdoor unit with the right brand name everything will be ok, B.S! I will have to make a chart for outdoor coil problems. The goal for outside is to get a full liquid line and the head pressure as low as possible. On a newer unit 190-225 PSI of head is normal, on some older units (25+ years) 300 PSI of head on a 90 degree day is not out of the question and be normal. Anything higher wash the coil and consider the unit may be overcharged. *Warning: There are situations where due to the conditions of you system you may have lower than normal suction pressure and/or higher than normal head pressure. Usually lower than normal suction pressure due to low airflow (either intentional or due to restrictions) and any attempts to raise the suction pressure by overcharging will cause damage by flooding the compressor. The trick is to determine if this pressure is normal for this system, which is usually caused by airflow problems. For example some systems 50-55 PSI (30-35 degree evaporating temperature) is normal. The secret to being a good mechanic it to determine if this is a normal pressure and leave it alone.