However, the same underlying principle holds for all tables and
curves. Figure 1 is an example of a superheat-charging curve
instead of a table. The curve is based on 400-cfm/ton airflow at
50% relative humidity across the evaporator coil. The steps to
charge a system according to this curve are listed below. 1.
Measure indoor drybulb temperature (DBT). This is the return air
at the air handler. Note: Use wetbulb temperature (WBT) if the
percentage of relative humidity is above 70% or below 20%. 2.
Measure outdoor DBT at the outdoor unit. This is the condenser
in air temperature. 3. Measure suction pressure at the
compressor and convert it to a temperature using a pressure-
temperature chart. 4. Measure compressor in temperature on the
suction line near the compressor. 5. Calculate the amount of
compressor superheat. Compressor superheat is all the superheat
the compressor “sees.” It is a combination of evaporator and
suction line superheat. Compressor superheat is sometimes
referred to as total superheat. 6.