Click back and forth between buttons 1 and 2 and see how many differences in operation you can find between the two systems. Can you come to a conclusion about the operation of these two systems? When you are done click the Answers button to find out if you came to the correct conclusions.
There are some points that should be made about the preceding example. A sight glass that is not full (while the system is operating) may indicate an undercharged system. However there are other things that can cause that symptom as well. For example, if the LL Filter (Liquid Line Filter) was partially clogged but still allowing some flow, the sight glass wouldn't be full yet the system could be fully charged. That condition is called a High Side Restriction. Fortunately there is a simple way to identify this situation. A partially clogged filter/drier can cause enough pressure drop to lower the temperature of the liquid passing through it making the drier and downstream liquid line cold. This is the refrigeration effect taking place and the drier and liquid line can even get cold enough to allow frost to form. If the liquid line feeding the filter/drier is 105 °F and the shell of the filter/drier cold to the touch, the high side restriction will be obvious.
The key to diagnosing a malfunctioning system is to gather as many symptoms as you can. Several symptoms consistent with a potential cause are more likely to lead you in the right direction than a single indicator. You must also eliminate other possible causes as with the high side restriction example.
It is logical that an undercharge will cause higher than normal superheat. Normal operation is to have as much of the evaporator as possible filled with liquid in order to pick up heat when it changes state. At the same time it must not be so full that there is no room left over for superheating the vapour. With undercharged conditions there is a lack of liquid and a surplus of vapour in the evaporator. That translates to a great deal of evaporator area that is available for superheating. Hence undercharge and high superheat go hand in hand.
The refrigerant pressure in the evaporator is affected by the thermal loads imposed on the evaporator, the removal of vapour from the continual suction of the compressor as well as the continual addition of refrigerant from the metering device. With an undercharged system the metering device can not feed the evaporator at the rate it is supposed to because there is not a full column of liquid in the liquid line. There also may be lower than normal high side pressure pushing liquid through the TXV. Without the required feed rate to the evaporator, the compressor tends to empty out the low side. That's why the suction pressure becomes lower than normal. This also explains why an over sized compressor will also cause lower than normal suction pressure. (The greater suction capacity removes vapours at a faster rate than the rest of the system was designed to keep up with.)
The Discharge Pressure is the result of compressing the low side vapours. So lower suction pressure tends to lower the high side pressure. However there are other forces also affecting the high side pressure. This is especially true if the system has some type of head pressure control. For example, a water cooled system has a water regulator valve that may totally mask the high side pressure from the symptoms you would expect with an undercharge. It will reduce the water flow trying to maintain it's head pressure setting and can possibly maintain normal high side pressure with undercharged conditions. Air cooled systems that have some form of head pressure control will also mask the expected symptoms. Measuring several parameters will reduce the number of possibilities and narrow the number of possible causes of a problem.