Pressure-Temperature Chart

For homeowners

For any pure refrigerant, when liquid and vapor are coexisting (boiling or condensing), the pressure and temperature are locked together by a fixed relationship. Know either one and you know the other. This relationship is what every refrigerant gauge in the world uses — the pressure scale on the gauge is the actual physical measurement, and the temperature scales printed alongside it just translate pressure into the corresponding saturation temperature.

The pressure-temperature chart (often called a “PT chart”) plots this relationship. For R-410A:

• At 40°F, the saturation pressure is about 119 PSIG. So a low-side gauge reading 119 PSIG tells you the evaporator coil is operating at 40°F.
• At 110°F, the saturation pressure is about 365 PSIG. A high-side gauge reading 365 PSIG tells you the condenser coil is operating at 110°F.

This is how a tech measures coil temperatures without sticking a thermometer inside the coil — they read the pressure and look up the temperature.

Three regions on the chart:

Subcooled liquid — temperature is below the saturation point at that pressure. This is fully liquid refrigerant cooler than its boiling point. Happens at the bottom of the condenser and in the liquid line. Measured as subcooling.

Saturated — on the curve. Refrigerant is a mixture of liquid and vapor. Pressure and temperature are locked together. Happens in the evaporator and condenser themselves.

Superheated vapor — temperature is above saturation at that pressure. This is fully gaseous refrigerant hotter than its boiling point. Happens after the evaporator and at the compressor discharge. Measured as superheat.

Understanding these three regions is the foundation of everything else in refrigerant work.

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For technicians

The physics behind it. When you have a pure substance in a closed container with both liquid and vapor present at the same time (called a “saturated” state), the pressure inside the container depends only on the temperature. Heat the container slightly and some liquid boils to vapor, raising the pressure. Cool it slightly and some vapor condenses to liquid, dropping the pressure. The system is constrained: at any given temperature there is exactly one possible pressure, and vice versa.

This is fundamentally different from a gas-only system where pressure and temperature are independent variables (PV=nRT, the ideal gas law). In a saturated mixture, the phase change provides the constraint that links P and T.

For HVAC work this means: in the evaporator where refrigerant is boiling, and in the condenser where refrigerant is condensing, pressure and temperature are inseparable. Read one with a manifold gauge, look up the other on a chart.

Why this matters. A tech can’t easily put a thermometer inside an evaporator coil to measure its temperature directly — the coil is sealed and pressurized. But the suction line just outside the coil is accessible, and the suction pressure gauge reading tells the tech exactly what’s happening inside.

• Suction pressure → evaporator saturation temperature
• Discharge pressure (head pressure) → condenser saturation temperature

These two numbers describe the operating envelope of the system. Almost every diagnostic conclusion starts here.

Refrigerant blends and “glide.” Pure refrigerants like R-22 follow the PT relationship cleanly. Refrigerant blends are mixtures of two or more refrigerants, and they have a temperature glide of a few degrees, where the saturation temperature varies with the proportion of liquid vs vapor at constant pressure.

• R-410A is technically a blend (R-32 + R-125) but it’s a “near-azeotropic” blend with negligible glide — usually treated as zero-glide for practical purposes.
• R-407C has about 10°F of glide.
• R-454B (newer, low-GWP replacement for R-410A) has about 1.5°F of glide.
• R-32 (single-component) has zero glide.

For high-glide blends, the chart shows two values: “bubble point” (saturation temperature for 100% liquid) and “dew point” (saturation temperature for 100% vapor). For superheat calculations, use the dew point. For subcooling, use the bubble point.

Typical operating points (R-410A):

Location	Typical pressure	Saturation temp	What’s happening
Evaporator (low side)	110–145 PSIG	38–50°F	Refrigerant boils, absorbs heat
Suction line	Same as evap	Above sat (superheated)	Vapor heading to compressor
Compressor discharge	350–450 PSIG	105–125°F	Hot superheated vapor
Condenser	Same as discharge	105–125°F	Refrigerant condenses, releases heat
Liquid line	Same as condenser	Below sat (subcooled)	Liquid heading to expansion device

Common reference points to memorize.

For R-410A in cooling operation on a typical 95°F day:

• Low side: ~125 PSIG = 42°F saturation
• High side: ~400 PSIG = 117°F saturation

For R-22 in same conditions:

• Low side: ~70 PSIG = 41°F saturation
• High side: ~275 PSIG = 122°F saturation

These rules of thumb help a tech spot abnormal operation at a glance. Low side reading 90 PSIG on R-410A when it should be 125? That’s a 30°F evaporator temperature — abnormally low, indicating low charge or restricted airflow.

Saturation temperature applications:

1. Superheat measurement — measure suction line temperature, look up suction pressure on PT chart to get evaporator saturation temperature, subtract. Difference is superheat.

2. Subcooling measurement — measure liquid line temperature, look up discharge pressure on PT chart to get condenser saturation temperature, subtract liquid line from that. Difference is subcooling.

3. Evaluating condenser performance — condenser saturation temperature should be 15–25°F above outdoor air temperature on a typical cooling day. Higher indicates dirty condenser coil, fan problem, overcharge, or non-condensable gas.

4. Refrigerant identification — if a system’s pressure-temperature behavior doesn’t match the labeled refrigerant, contamination or wrong refrigerant has occurred.

Limits of the PT relationship.

• Only applies in the saturated region. In the subcooled or superheated regions, pressure and temperature are independent. Knowing the suction pressure tells you only the saturation temperature, not the actual suction line temperature (which is higher, by the amount of superheat).
• Requires a pure refrigerant. Contaminated systems (mixed refrigerants, air infiltration, moisture) behave unpredictably.

Modern context. With R-22 phased out and R-410A on its way out (mandated phase-down through 2030+), techs increasingly encounter R-32, R-454B, R-466A, and other new refrigerants. Each has its own PT relationship. Older gauges with only R-22/R-410A scales become less useful. Digital manifolds and phone apps that support newer refrigerants are becoming standard equipment.