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Unit 2 — Introduction to Refrigerants & Handling Practices
Section 2 — Leak Testing, Evacuation and Charging

Section 2 Overview

Procedures, tools, and safe work practices required to pressure test, leak test, evacuate, and charge refrigeration and air conditioning systems — ensuring systems are free of contaminants, moisture, and leaks.

General Learning Outcomes

Upon successful completion of this section, apprentices will be able to accurately describe and demonstrate the procedures used to:

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Lesson 2.1

Terminology, hazards, codes, and regulations for pressure testing, evacuation, and charging.

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Lesson 2.2

Tools and equipment for pressure and leak testing, evacuation, and charging.

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Lesson 2.3

Gases and liquids for pressure testing — acceptable test media, volumes, and hydrostatic testing of secondary systems.

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Lesson 2.4

Evacuation and dehydration procedures — deep vacuum and triple evacuation methods.

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Lesson 2.5

Charging procedures and verification — liquid, vapour, and oil charging; subcooling, superheat, and sight glass.

Key Terminology

Technicians must be familiar with standard terms used for testing and servicing refrigeration and air conditioning systems.

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Pressure Test

The process of applying a controlled pressure to a sealed system or section to verify its strength and leak tightness.

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Leak Test

Procedures used to identify the presence and location of leaks using gases, solutions, electronic detectors, or other methods.

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Evacuation

The removal of air, moisture, and non-condensable gases from a system using a vacuum pump.

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Dehydration

The process of removing water vapour and moisture from a system's internal surfaces through deep evacuation — sometimes using triple evacuation methods.

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Holding Charge

A small pressure of dry refrigerant or inert gas introduced into an idle or newly assembled system to keep it dry and indicate leaks during storage or transport.

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Charging

Adding refrigerant or oil to a system to the proper amount specified by the manufacturer or calculated for the application.

Hazards, Safe Work Practices and Regulations

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Hazards and Safe Work Practices

Pressure and leak testing, evacuation, and charging involve risks beyond normal system operation — including over-pressurization and exposure to high concentrations of gases.

  • Never exceed the maximum allowable test pressure for equipment, as specified by the manufacturer or relevant codes.
  • Use only approved test gases. Oxygen must never be used for pressure testing due to extreme explosion risk.
  • Avoid compressed air in primary refrigerant circuits — it introduces moisture and can create an explosive mixture with some refrigerants.
  • Monitor gauges during testing, evacuation, and charging; stand clear of potential failure points such as hoses and fittings.
  • Use appropriate PPE — eye and hand protection and hearing protection in high-noise environments.
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Codes and Regulations

Codes and standards specify minimum safety and performance requirements for testing and charging procedures.

  • Mechanical and building codes define allowable test pressures, test media, duration, and acceptance criteria for piping and equipment.
  • Environmental regulations govern the use of refrigerant as a leak check gas — deliberate venting must be minimized or is prohibited, and any refrigerant used must be recovered.
  • Manufacturer installation and service instructions are considered part of code requirements when referenced in product certification.

Tools and Equipment Overview

Effective testing, evacuation, and charging depend on selecting and maintaining the right instruments. The tables below summarize the key tools in each category.

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Pressure and Leak Testing Tools

  • Electronic leak detectors — heated diode, infrared, or corona discharge; must be calibrated regularly against a known reference leak.
  • Soap and leak detection solutions — applied to suspected leak sites; bubbles indicate escaping gas. Use specialized solutions, not dish soap.
  • Ultrasonic detectors — listen for high-frequency sound from escaping gas; useful when pressurized with nitrogen.
  • Certified pressure gauges — rated for maximum test pressure and calibrated periodically; selected for good resolution near the test pressure.
  • Fluorescent dyes and UV light — dye added to the refrigerant circuit; leaks glow under UV with filter glasses. Follow manufacturer limits on dye quantity.
  • Regulators — control test gas pressure from cylinders. Never connect a high-pressure cylinder directly to a system without a regulator.
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Evacuation Tools and Equipment

  • Vacuum pumps — two-stage rotary vane pumps reach deep vacuum levels; oil must be changed frequently to maintain performance.
  • Micron (vacuum) gauges — measure absolute pressure in microns of mercury; far more accurate than compound gauges in the vacuum range. Place away from the pump connection.
  • Compound gauges — read above and below atmospheric pressure; used for general monitoring but not accurate enough for deep vacuum verification.
  • Core removal tools and vacuum manifolds — remove Schrader cores under pressure; large-bore connections reduce flow restriction for faster evacuation.
  • Cold traps — condense vapours before they reach the vacuum pump; protect pump oil in specialized applications.
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Holding Charge and Charging Tools

  • Charging scales — measure refrigerant mass added or removed; critical for critically charged systems that require a precise charge amount.
  • Gauge manifold — used for testing, charging, and recovery; connects to system service ports and recovery or charging equipment.
  • Service valve wrenches — operate service valves correctly without damaging stems and seals.
  • Transfer pumps — move refrigerant between cylinders in large-scale service operations; matched to the refrigerant type.

Gases and Liquids for System Pressure Tests

Only specific test media are acceptable for pressure testing — their use must comply with applicable codes and manufacturer instructions.

Acceptable Test Gases
  • Nitrogen — most common field test gas; dry, inert, used alone or with a trace of refrigerant for leak detection
  • Helium — very low leak rate testing with mass spectrometer detectors
  • Carbon dioxide — used in specific high-pressure systems
  • Argon — inert; sometimes used where welding is associated with testing
  • Air — acceptable in some secondary systems only; avoid in primary refrigerant circuits
  • Inert gas blends — non-reactive mixtures that do not combust or react with system materials
Liquids for Secondary Systems
  • Water — most common for hydrostatic testing at ambient temperatures, provided freezing is not a risk
  • Glycol — ethylene or propylene glycol mixtures for closed hydronic loops
  • Brine — salt-water solutions for low-temperature secondary circuits operating below the freezing point of water
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Calculating Test Gas Volume

Estimate system volume from piping dimensions, component sizes, and manufacturer data. Apply gas laws to relate pressure, volume, and temperature. Always increase pressure in controlled steps, monitoring gauge readings closely to avoid over-pressurizing equipment or exhausting the gas supply.

Evacuation, Dehydration and Charging — Procedure Summaries

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Deep Vacuum Evacuation

The primary method used to remove air and moisture from internal system surfaces after passing a pressure test.

  • Connect a high-capacity vacuum pump with clean oil using large-diameter hoses and core removal tools.
  • Evacuate until the system reaches the target micron level recommended by codes or the manufacturer.
  • Isolate from the pump and monitor for pressure rise — indicating leaks or residual moisture.
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Triple Evacuation

Used when systems have high moisture content or after severe contamination.

  • Evacuate to a specified vacuum level; break the vacuum with dry nitrogen to sweep out moisture.
  • Repeat the cycle multiple times — typically three — each pass removing more moisture and non-condensables.
  • Final evacuation is taken to deep vacuum and verified with a micron gauge and standing vacuum test.
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Charging Methods

  • Liquid charging — common for blends and large systems; refrigerant cylinder positioned to deliver liquid to prevent fractionation. May be introduced into liquid line or receiver.
  • Vapour charging — refrigerant drawn as vapour into the low-pressure side with the system running; used to fine-tune charge on smaller systems.
  • R-744 (CO₂) charging — performed in stages due to high operating pressures; specialized valves and over-pressure protection are mandatory.
  • Oil charging — introduced via oil ports using oil pumps, or drawn in by system vacuum through an oil service valve.
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Verifying the System Charge

Always Required

Once charging is complete, verify correct refrigerant charge using a combination of methods:

  • Charge charts / critical charge — manufacturer data relating ambient conditions to expected system parameters; critically charged systems require weight-based verification.
  • P/T charts — relate refrigerant pressure to saturation temperature; confirm measured pressures and temperatures are consistent with normal operation.
  • Sight glass — installed in the liquid line; clear flow indicates adequate charge; bubbles may suggest undercharge or flashing.
  • Subcooling — difference between saturated condensing temperature and actual liquid line temperature; indicates liquid refrigerant is fully condensed and informs charge level.
  • Superheat — difference between saturated evaporating temperature and actual suction line temperature; confirms the evaporator is fully utilized and protects the compressor from liquid slugging.

Key Principles for This Section

Throughout this section, keep the following principles in mind:

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