Air conditioner systems use refrigerant in order transfer heat back and forth between the evaporator and compressor. This allows the unit to pull heat from inside air, release it outdoors, and cycle the cooled air throughout the house. The refrigerant gas used in the AC lines must be maintained at precise levels so the system pumps at the right pressure. Proper refrigerant charging can be aided by measuring the pressure of the system while filling.
The suction side of the unit consists of the lines that lead into the compressor, often called the low side of the refrigerant system, since the pressure in these lines is much lower to the lines on the compressor output. The lines rarely have dials that automatically show pressure. There are gauges and charging systems that track refrigerant pressure for you, but you should always let your system run for a while without temperature fluctuations before taking readings.
Low-Pressure Suction Side
The low-pressure side, when connected to the gauge, should read less than 100 psi. PSI is the common measurement for pressure used in terms of pounds per square inch. By the time the refrigerant reaches these lines it has lost most of its heat and has collapsed into a liquid state, taking up less volume. This leads to a low-pressure environment aided by the compressor, which is working to draw the refrigerant in to heat it back to a gas.
While "less than 100 psi" may seem general, it is difficult to be more specific for every AC unit. For example, in a refrigeration unit, the refrigerant R-22 should operate at 75 psi on the suction side. But different refrigerants and different systems have different requirements, making it difficult to specify a particular pressure reading. For this reason, charts are published in manufacturer manuals or in local building codes showing what the pressure should be for a number of refrigerants, based on the system, lines and current temperature.
Causes of Problems
If the refrigerant is reading a low pressure, it may indicate a leak exists somewhere in your system. Refrigerant does not wear down over time, so it must be escaping from somewhere. You may need to replace pipes or valves to return the system to normal operation. If the pressure reading is too high, the unit may have been overcharged in the past, and some of the refrigerant may need to be drained out.