Subcooling = bubble saturation temperature − measured liquid-line temperature. Use the refrigerant's PT data at the pressure measured at the same location.
Choose the measurement location
| Task | Paired pressure and temperature location | PT reference |
|---|---|---|
| Evaporator superheat | Near the coil outlet / expansion-valve sensing bulb | Saturated vapor (dew) |
| Compressor inlet superheat | Compressor suction service point and nearby suction pipe, per compressor instructions | Saturated vapor (dew) |
| Condenser-outlet subcooling | Liquid at the condenser outlet | Saturated liquid (bubble) |
| Liquid available at metering device | Liquid line near the device inlet | Saturated liquid (bubble) |
Copeland's measurement guidance distinguishes evaporator and compressor superheat and calls for subcooling measurements near the metering-device inlet. Identify the point in your worksheet: a compressor reading is not an evaporator-outlet reading. Temperature pickup and pressure loss along a line can change the result.
Read the correct PT column
Identify the refrigerant from the equipment record and match pressure units and reference: psig is gauge pressure; psia is absolute pressure. For a zeotropic blend, use dew/vapor for superheat and bubble/liquid for subcooling. Do not use the average of those temperatures. Single-component refrigerants and azeotropes have one saturation temperature at a given pressure. Schneider Electric's two-column PT explanation describes these conventions.
- Run the equipment in the test mode and conditions specified by its manual; record airflow/load and operating mode.
- Record a stabilized pressure and line-temperature pair at the selected point, following the equipment and instrument measurement procedure.
- Find saturation temperature at that pressure in the correct refrigerant and phase column. Use source-approved interpolation if necessary; do not extrapolate beyond its range.
- Subtract in the order shown above and record location, refrigerant, pressure, line temperature, PT source and calculated difference.
Two worked examples: R-407C
These are published teaching examples from Copeland bulletin 95-14, page 2, not site field measurements or targets for another system. The source supplies the pressure-to-temperature values.
| Quantity | Superheat at TEV bulb | Subcooling at condenser outlet |
|---|---|---|
| Measured pressure | 79 psig | 250 psig |
| PT saturation temperature | 51°F, dew | 108°F, bubble |
| Measured refrigerant temperature | 60°F | 98°F |
| Calculation | 60 − 51 = 9°F superheat | 108 − 98 = 10°F subcooling |
| Temperature difference in SI | 9 × 5/9 = 5 K | 10 × 5/9 ≈ 5.56 K |
Read Copeland bulletin 95-14. For temperature differences, multiply °F by 5/9 to get K or °C difference; do not subtract 32.
Compare the result with the equipment target
Find the exact equipment model's installation/service manual or charging label. Record its required operating mode, ambient/load range, measurement point, metering-device procedure and target/tolerance. A calculated 10°F of subcooling has meaning only against that procedure. A compressor operating limit and a unit charging target answer different questions.
A pressure reading alone cannot determine refrigerant charge. If the prescribed conditions are not met, retain the measurements and resolve that missing condition before comparing with the target. An unexpected difference calls for checking the refrigerant/column, units, sensor contact, pressure location, airflow/load and metering-device operation. It does not by itself prescribe adding or removing refrigerant.
Use the PT tables for the next lookup
R-407C pressure-temperature chart provides the bubble and dew lookup; check the labeled column before calculating. For a single-component comparison, see the R-134a PT chart. This guide supplies the calculation method; the tables supply refrigerant-specific saturation data.