To determine drop accross an "A" coil you simply drill a hole big enough for a temp probe between the two coils WATCH FOR TUBING, and then drill another above but close to the coil nearest the supply duct. Let the unit run in whatever mode you choose but put the t-stat on a good high setting in order to keep it from becoming satisfied to quickly. After about 10 minutes of run time measure the two temperatures and subtract to get the difference or "THE DROP" accross the coil. Keep well in mind however that you must check all air filters, all air registers, and the blower for cleanliness and no blockege anywhere. If the unit is equiped with a humidifer dont let it rn during the check. I hope this info helps you, For more info you can order a book called HVAC service procedures from either Honeywell or Carrier.
AirFlow Measurment- Pressure Drop Method
1. Drill a quarter inch hole on either side of the evap coil. If it is a slab coil then one hole goes above the coil and the other goes below the coil but, keep both in the center as much as possible, ang be careful not to drill into nearby piping or the coil. If it is an "A" coil put one hole in the middle of the A and the other above the A on the side nearest the supply duct.
2. Turn the unit on and set the t-stat so that the compressor and systm blower run continuosly during the measurement.
3. Level the inclined manometer. Insert a quarter inch tube about 2 inches in each hole. Seal the tubes at the insertion points. Connect the open ends of the tubes to the manometer. The tube from the hole at the output side of the coil should be connected to the lower end of the monometer.
4. Read and record the static pressure drop across the coil.
5. Compare the reading to the performance data supplied by the manufacturer.
Always remember to make sure that the system air filter is clean, the blower wheel is clean , the evap coil is clean, there are no loose or worn belts on belt drives and that belt tension is right, and that the blower rotates in the proper direction.