Pressure-Temperature Relationships and PT Chart Use

What Is the Pressure-Temperature Relationship?

Every refrigerant has a predictable relationship between its pressure and its temperature when it is in a saturated state — that is, when liquid and vapor exist together at the same time inside a container or component. At saturation, if you know the pressure, you automatically know the temperature, and vice versa. This is not a coincidence; it is a physical law that applies to all pure refrigerants and many blends.

When a refrigerant is saturated, raising the pressure raises the saturation temperature. Lowering the pressure lowers the saturation temperature. This principle is what makes the refrigeration cycle work: the system deliberately creates low pressure on the suction side so the refrigerant boils (evaporates) at a cold temperature and absorbs heat from the space being cooled, then raises pressure on the discharge side so the refrigerant condenses at a warm temperature and rejects that heat outdoors.

The Pressure-Temperature (PT) Chart

A PT chart is a published reference table that lists saturation pressures and corresponding saturation temperatures for specific refrigerants. Technicians use it constantly — on startup, during charging, when diagnosing a problem, and when verifying normal operation.

PT charts are printed on laminated pocket cards, included in service manuals, and built into digital apps on phones and tablets. No matter the format, they all contain the same core data for each refrigerant: a column of temperatures (usually in °F) paired with a column of pressures (usually in psig — pounds per square inch gauge).

Gauge pressure (psig) is what your manifold gauge reads in the field. It is absolute pressure minus atmospheric pressure (approximately 14.7 psi at sea level). PT charts are formatted in psig because that is what technicians measure directly.

Common Refrigerants You Will Look Up

• R-22 – An older HCFC refrigerant still found in existing equipment. It is no longer manufactured for new systems in the U.S. but remains in service in legacy equipment.
• R-410A – A common HFC blend used in residential and light commercial air conditioning. It operates at significantly higher pressures than R-22.
• R-134a – An HFC used in automotive air conditioning and some commercial refrigeration.
• R-404A – An HFC blend widely used in commercial and low-temperature refrigeration.

Each refrigerant has its own column or its own separate PT chart. Never cross-reference the wrong refrigerant column — pressures at any given temperature differ significantly between refrigerants, and using the wrong data will lead to incorrect diagnosis and possible equipment damage.

How to Read a PT Chart: Psig to °F

1. Locate the correct refrigerant section of the chart.

2. Find your gauge pressure reading in the pressure column.

3. Read across to the corresponding temperature column — that is the saturation temperature at that pressure.

This tells you what temperature the refrigerant should be changing state (boiling or condensing) at that pressure. On the low side, this is the evaporator saturation temperature. On the high side, this is the condensing saturation temperature.

How to Read a PT Chart: °F to Psig

1. Locate the correct refrigerant section.

2. Find the target temperature in the temperature column.

3. Read across to the pressure column — that is the pressure you would expect at that saturation condition.

This direction is used when you know the design conditions (for example, a 40 °F evaporator target) and need to know what suction pressure to expect.

Superheat and Subcooling: Why Saturation Matters

Saturation temperature is the baseline for two key measurements technicians make on every service call:

• Superheat — the number of degrees the suction-line vapor is above saturation temperature at the measured suction pressure. A properly fed evaporator has superheat; zero superheat means liquid refrigerant may be returning to the compressor.
• Subcooling — the number of degrees the liquid line is below saturation temperature at the measured discharge (or liquid-line) pressure. Subcooling confirms the refrigerant has fully condensed and has some margin before flashing.

You cannot calculate either value without first finding saturation temperature from the PT chart.

Safety Notes

• Refrigerant handling requires EPA 608 certification. Never release refrigerant to the atmosphere — this is illegal under Section 608 of the Clean Air Act and harms the environment.
• Always wear safety glasses and insulated gloves when connecting or disconnecting manifold gauges. Refrigerant under pressure can cause frostbite on skin contact and eye injury.
• High-pressure refrigerants like R-410A can reach pressures well above 400 psig on the high side in warm ambient conditions. Use gauges and hoses rated for the refrigerant and pressure range you are working with.
• When in doubt about which refrigerant is in a system, check the data plate and verify with the PT chart before assuming. Mixing refrigerants or misidentifying them creates serious hazards.
• Lock out / tag out the system before opening any refrigerant circuit for repairs. Even after lockout, residual pressure remains — always attach gauges slowly and observe pressure before opening valves.