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Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 3 — Pressure and Temperature Relationship

3.2 — The Pressure–Temperature Chart

The P–T chart is the technician’s translation dictionary: it converts a gauge pressure reading into a saturation temperature in seconds. Mastering it is the first step toward calculating superheat and diagnosing charge.

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3.2.1 — What Is the P–T Chart?

The Pressure–Temperature (P–T) chart lists the saturation temperature of a specific refrigerant at each pressure. Saturation temperature is the temperature at which the refrigerant is actively boiling or condensing — existing as both liquid and vapour at the same time.

Different refrigerants have completely different P–T relationships. A chart for R‑410A cannot be used for R‑22 or R‑134a. Every refrigerant has its own curve. Manufacturers publish P–T charts in paper card format (often laminated) and as apps on smartphones.

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Saturation Temperature

The temperature at which a refrigerant changes phase at a given pressure. Below this temperature it is liquid; above it, vapour. It is not a single fixed value — it shifts with every change in pressure.

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What the Chart Tells You

If you know the pressure, the chart gives you the saturation temperature. If you then measure the actual pipe temperature, the difference between the two is either superheat (vapour side) or subcooling (liquid side).

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Gauge pressure vs. absolute pressure

Manifold gauges read in psig (gauge pressure). P–T charts also list values in psig, so no conversion is needed for lookups. Gas law calculations, however, require psia (add 14.696 to psig).

3.2.2 — How to Read and Use the P–T Chart

Using the chart in the field is a three-step process. Practice until it is automatic.

  1. Identify the refrigerant. Confirm the refrigerant type from the equipment nameplate or the system data sheet. Select the matching P–T chart — never guess or substitute.
  2. Read the gauge pressure. Connect the manifold gauge set. Read the low-side (suction) pressure for evaporator analysis, or the high-side (discharge) pressure for condenser analysis.
  3. Look up the saturation temperature. Find your gauge pressure in the pressure column. The number beside it in the temperature column is the saturation temperature at that point in the system.
  4. Measure the actual pipe temperature. Use an accurate clamp-on or contact thermometer on the suction line near the evaporator outlet, or on the liquid line near the condenser outlet.
  5. Calculate superheat or subcooling. Subtract saturation temperature from pipe temperature (suction side = superheat). Or subtract pipe temperature from saturation temperature (liquid line = subcooling).
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Always insulate your temperature probe

A temperature probe measuring suction line temperature in a warm mechanical room will read ambient air temperature if not insulated. Wrap the probe with insulation tape for an accurate reading.

3.2.3 — R‑410A — Saturation Pressure–Temperature Values

R‑410A is a near-azeotropic blend (R‑32 / R‑125) that behaves like a pure refrigerant for P–T purposes — a single saturation temperature at each pressure. It operates at roughly 60% higher pressures than legacy R‑22, so equipment and service tools must be rated for the higher working pressures.

Pressure (psig) Saturation Temp (°F) Typical Location in System
5720°FLow-side / cold evaporator (winter heating mode)
8330°FLow-side / typical heat-pump cooling evaporator
11840°FLow-side / typical A/C evaporator target
15850°FLow-side / warmer evaporator condition
21065°FHigh-side / low ambient condensing start
27880°FHigh-side / moderate ambient condensing
36095°FHigh-side / typical summer condensing
450110°FHigh-side / hot day / reduced airflow
493117°FHigh-side / high-pressure cutout territory
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Rule of thumb for R‑410A

On a residential A/C system, the low-side target is often 118 psig ≈ 40°F saturation. This gives a comfortable margin above freezing on the evaporator coil while still achieving efficient heat absorption.

3.2.4 — Calculating Superheat — Step-by-Step Example

Superheat tells you how much the refrigerant vapour has warmed above its boiling point by the time it reaches the end of the evaporator. It confirms the evaporator is fully used and that only vapour (no liquid) is heading back to the compressor.

Superheat = Tsuction line − Tsat (from P–T chart)

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Worked Example — R‑410A System

Step 1 — Read low-side pressure: Manifold gauge reads 118 psig.

Step 2 — Look up saturation temperature: R‑410A P–T chart → 118 psig = 40°F.

Step 3 — Measure suction line temperature: Thermometer on insulated suction line at evaporator outlet reads 52°F.

Step 4 — Calculate superheat:

Superheat = 52°F − 40°F = 12°F superheat

Interpretation: Most residential A/C manufacturers target 10–15°F superheat at the evaporator outlet. A reading of 12°F falls squarely in range — the system charge appears correct. Higher superheat (>20°F) may indicate low charge or a restricted metering device. Lower superheat (<5°F) risks liquid returning to the compressor.

High Superheat Causes (>20°F)
  • Low refrigerant charge (leak)
  • Restricted or undersized metering device
  • Low evaporator airflow / dirty filter
  • Liquid line restriction
Low Superheat Causes (<5°F)
  • Overcharged system
  • Oversized or stuck-open TXV
  • Excessive evaporator airflow
  • Suction line flooding (risk of slugging)
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