Level 1, Unit 3, Section 3 – Pressure & Temperature Relationship

Unit 3 — Refrigeration System Fundamentals & Maintenance

Section 3 — Pressure and Temperature Relationship

Section 3 Overview

Every reading a technician takes on a manifold gauge set connects directly to a temperature. Understanding why pressure and temperature are linked — through the gas laws and the P–T chart — transforms guesswork into confident diagnosis.

3.0.1 — General Learning Outcomes

Upon successful completion of this section, the apprentice will be able to:

State Boyle’s, Charles’, Gay-Lussac’s, and Dalton’s laws and explain how each applies to refrigerant vapour behaviour in an HVAC/R system.
Explain the concept of absolute temperature (Rankine and Kelvin) and explain why it must be used in gas-law calculations.
Use a Pressure–Temperature (P–T) chart to find the saturation temperature of a refrigerant at a given pressure and calculate superheat.
Describe temperature glide in blended refrigerants, distinguish between bubble point and dew point, and explain when each is used for subcooling and superheat calculations.
Define saturated, subcooled, and superheated states; calculate subcooling and superheat from field measurements; and explain what each value tells a technician about system charge.

3.0.2 — Section 3 — Lessons at a Glance

Section 3 builds from fundamental gas behaviour, through the tools technicians use every day, to the three refrigerant states that define system performance. Each lesson builds on the previous one.

3.1

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The Gas Laws

Boyle’s, Charles’, Gay-Lussac’s, and Dalton’s laws. Absolute temperature scales. Why ideal gas laws matter for refrigerant analysis and troubleshooting.

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3.2

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The P–T Chart

What the Pressure–Temperature chart is, how to read it, and how to use it with a manifold gauge set to find saturation temperature and calculate superheat on a pure refrigerant (R‑410A example).

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3.3

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Blended Refrigerants & Temperature Glide

Why blended refrigerants have two saturation temperatures at any given pressure. Bubble point vs. dew point. Subcooling and superheat calculations for R‑407C.

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3.4

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Saturated, Subcooled & Superheated States

Defining the three refrigerant states. Calculating subcooling and superheat from field measurements. What target values tell a technician about refrigerant charge and system health.

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3.0.3 — Key Terms — Section 3 Preview

These terms appear throughout Section 3 and connect directly to Section 4 (Vapour Compression Cycle) and real-world service tasks.

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Boyle’s Law

At constant temperature, pressure and volume are inversely proportional: P V = constant. Explains compression and expansion.

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Charles’ Law

At constant pressure, volume is directly proportional to absolute temperature. Heated gas expands; cooled gas contracts.

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Gay-Lussac’s Law

At constant volume, pressure is directly proportional to absolute temperature. Explains why sealed refrigerant cylinders build pressure in the sun.

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Dalton’s Law

Total pressure of a gas mixture equals the sum of partial pressures. Used in psychrometrics and non-condensable gas diagnosis.

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Saturation Temperature

The temperature at which a refrigerant boils or condenses at a given pressure. Found directly from the P–T chart.

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Temperature Glide

The range between bubble point and dew point in a blended refrigerant. R‑407C has a glide of about 5–7°F (3–4°C).

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Superheat

T_vapour − T_{sat (dew)}. Indicates the evaporator is fully used and protects the compressor from liquid slugging.

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Subcooling

T_{sat (bubble)} − T_{liquid line}. Confirms solid liquid flow to the metering device and indicates charge level at the condenser.

3.0.4 — Why P–T Relationship Matters in HVAC/R

A refrigerant does not have a single boiling point — it has a pressure-dependent boiling point. The P–T relationship is the engine behind every refrigeration cycle: by controlling pressure, the system engineer controls where the refrigerant boils (to absorb heat) and where it condenses (to reject heat).

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The Technician’s Most-Used Tool

A manifold gauge set measures pressure — but technicians think in temperatures. The P–T chart bridges that gap. Within seconds, a technician can convert a low-side pressure reading into a saturation temperature, compare it to a measured pipe temperature, and calculate superheat to verify charge.

Gas laws explain why those numbers behave the way they do. Understanding the physics turns the P–T chart from a lookup table into a diagnostic tool — and that understanding separates a competent technician from an exceptional one.