I'm checking charge on a straight-cool split with a fixed-orifice (piston) metering device.

For a fixed-orifice system, you're checking charge by comparing superheat at the evaporator.

What to measure:

• Suction line temperature (at the service valve or 6" from the compressor)
• Suction line pressure (at the service port)
• Outdoor ambient temperature
• Indoor wet-bulb or dry-bulb temperature (for entering air)

What you're looking for:

• Target superheat from the manufacturer's charging chart (based on outdoor ambient and indoor wet-bulb). If no chart, use a superheat calculator.
• Actual superheat = (suction line temperature) − (evaporator saturation temperature from pressure).

Charge adjustment:

• If actual superheat is too high → undercharged (add refrigerant).
• If actual superheat is too low → overcharged (recover refrigerant).

Key constraints:

• Measure under stable conditions (10+ minutes of steady operation).
• Ensure airflow is normal (clean coils, proper fan operation).
• Fixed orifice systems are very sensitive to airflow and load — verify these before adjusting charge.

Safety note: Overcharging a fixed orifice system causes liquid floodback — damaging to the compressor. Be conservative. If unsure, consult the manufacturer's data or a licensed technician.

Code reference: ACCA R-410A Fixed Orifice Charging Procedures (Section 6.3) and ASHRAE Standard 15 for safe handling.

For a fixed-orifice (piston) system, you cannot use subcooling to verify charge. You must use Superheat.

What to Measure

1. Suction Line Pressure (at the condensing unit service port).
2. Suction Line Temperature (on the copper tubing near the service port, insulated probe).
3. Return Air Wet-Bulb (WB) Temperature (at the indoor coil inlet).
4. Outdoor Dry-Bulb (DB) Temperature (at the condenser coil inlet, in shade).

The Procedure

1. Run system ≥15 mins (stable operation).
2. Convert suction pressure to Saturation Temperature using your PT chart/app for the specific refrigerant (R-410A, R-22, etc.).
3. Measured Superheat = Suction Line Temp – Saturation Temp.
4. Plot Target Superheat on the manufacturer’s charging chart (usually on the unit panel) using Outdoor DB and Return WB. No chart?* Use a generic fixed-orifice target superheat chart or calculator app.

What You’re Looking For

• Measured Superheat ≈ Target Superheat (± 2–3°F).
• Measured > Target: Undercharged. Add refrigerant.
• Measured < Target: Overcharged. Recover refrigerant.
• Critical: If measured superheat is < 5°F, stop. You risk liquid floodback (compressor damage). Recover charge until ≥ 5°F and re-evaluate.

Common Pitfalls

• Airflow issues first: Verify clean filter, correct blower speed, clean coil, proper duct static. Bad airflow makes superheat readings lie. Fix airflow before adjusting charge.
• Wrong refrigerant selection on gauges/app.
• Probe on wrong line (liquid vs. suction) or uninsulated probe reading ambient.

Code/Standard: ACCA Standard 5 (QI) / Manufacturer’s Installation Instructions.