To determine the Target Superheat for an air conditioning system with a fixed orifice (such as a piston or capillary tube) measure the indoor WB (wet bulb) temperature with a digital psychrometer and the outdoor DB (dry bulb) temperature with a standard digital temperature reader. Input these temperatures in a superheat chart, calculation, app, or digital manifold set in order to determine the Target Superheat at that moment. Remember that the target superheat will change as the building lowers in WB and while charging refrigerant. The outdoor DB will general stay the same while checking the charge but it may fluctuate some.
Set the Actual Superheat as close to the Target Superheat as possible to have an accurate refrigerant charge.
Measure the indoor WB as close as possible to the inlet of the evaporator coil, preferably in the return duct a few feet prior to the coil. Measure the outdoor DB roughly one foot away from the inlet of the outdoor coil in the shade.
Example 1: Indoor WB 66° F, Outdoor DB 90° F= Target Superheat of 13 ° F
Example 2: Target Superheat Calculation (This calculation will get you close to manufacturer superheat chart results)
WB is 64° F, DB is 96° F
Target Superheat Formula = [(3 x WB) – 80 – DB] /2
[(3 x 64) – 80 – 96] /2 = Target Superheat
3 x 64=192, 192-80=112, 112-96=16 16/2=8° F of Target Superheat
8° F of Target Superheat
If you want to see a video on finding target superheat using the chart above- https://youtu.be/bIL9BrKCujc
If you want to see a video finding target superheat using a digital manifold see this video- https://youtu.be/YPitQHSgvog
If you want to learn more about refrigerant charging methods, procedures, and troubleshooting, read the “Refrigerant Charging and Service Procedures” book by Craig Migliaccio.
Author: Craig Migliaccio, © 2019 AC Service Tech LLC
About the Author: Craig is a Licensed HVACR Teacher and Master Licensed Contractor in the State of New Jersey.