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newSchoolWorks.com › VoTechWorks › Business & Cross-Cutting › Appliance Repair › Sealed Systems: Refrigerant Handling, Diagnosis, and Repair › Lesson 3
BUS-APPL73-06 · Lesson 3 · 12 clock hours · lab

Leak Testing, Nitrogen Pressure Testing, and System Integrity

Objective. Students will be able to pressure-test a sealed system with dry nitrogen to the manufacturer's specified test pressure, locate all intentionally introduced leak points using an electronic leak detector and bubble solution, and document findings on a standard service report.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a credentialed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Leak Testing Matters

A sealed refrigeration system must hold its charge without loss. Even a small leak wastes refrigerant, reduces cooling capacity, and — under current EPA regulations — releasing refrigerants intentionally to the atmosphere is illegal. Before you introduce refrigerant into any system you have opened or repaired, you must confirm the system is leak-free. Dry nitrogen pressure testing is the industry-standard method for that confirmation because nitrogen is inert, non-flammable, inexpensive, and carries no environmental penalty if it escapes.

Dry Nitrogen: What It Is and Why You Use It

Dry nitrogen is nitrogen gas with moisture removed to a very low dew point. Moisture is the enemy of sealed systems: it reacts with refrigerant and compressor oil to form acids and sludge. "Dry" nitrogen protects the system interior while you test it. Never substitute compressed air — air contains oxygen and moisture that contaminate the system, and oxygen mixed with refrigerant vapor creates a flammable or explosive hazard.

Nitrogen cylinders are stored and transported upright and chained to a fixed support. Always use a dedicated nitrogen regulator; never use a refrigerant regulator on a nitrogen cylinder. Nitrogen is stored at very high pressure — a regulator failure can be catastrophic.

Finding the Manufacturer's Test Pressure

Every sealed system has a maximum allowable working pressure stamped on the nameplate or listed in the service manual. Your test pressure must not exceed that limit. Check the service data sheet, the unit's data plate, or the manufacturer's service documentation before you open the nitrogen cylinder valve. Exceeding the rated pressure can rupture a component, blow a fitting, or injure you or a bystander. Your instructor will show you exactly where to find this value for your trainer unit.

Nitrogen Pressure-Testing Procedure

1. Evacuate or confirm the system is empty. Do not pressure-test a system that still contains refrigerant under pressure — you could over-pressurize it.

2. Connect the equipment. Attach the manifold gauge set to the system's Schrader valve service ports using the appropriate adapters. Connect the nitrogen cylinder (with regulator) to the center (utility) hose of the manifold.

3. Set the regulator. Adjust the regulator to the target test pressure before opening any valves. Keep bystanders clear.

4. Pressurize slowly. Open the nitrogen cylinder valve, then the manifold valve, and allow pressure to rise gradually to the test value. Sudden pressure spikes can stress fittings.

5. Isolate the system. Close both the manifold valve and the nitrogen cylinder valve once you reach the target pressure.

6. Monitor for 15 minutes. Watch the manifold gauge. A pressure drop indicates a leak. Temperature changes can also affect pressure, so note ambient conditions.

7. Record the result. Log start pressure, end pressure, elapsed time, and ambient temperature on your service report.

Safety: Wear safety glasses and gloves throughout. Never stand directly in front of a fitting you are pressurizing. Never use an open flame near pressurized fittings.

Locating Leaks: Electronic Leak Detectors

Electronic leak detectors — heated-diode and infrared types are most common in appliance service — sense halogenated refrigerant vapors. Heated-diode detectors burn a sensing element that changes electrical resistance in the presence of halogen-containing compounds; infrared detectors use optical absorption. Both are sensitive enough to find very small leaks.

Correct technique matters:

False positives: Electronic detectors respond to any halogenated compound, not just refrigerants. Residual compressor oil that contains chlorinated additives, certain cleaning solvents, and even some plastics can trigger an alarm. If you get an alarm in an unexpected location, confirm with bubble solution before recording a leak.

Bubble-Solution Leak Detection

Bubble solution — a specially formulated soap — provides a visual confirmation. Apply it generously around the fitting or joint and watch for bubbles forming or growing. Bubble solution works best as a confirmation tool after the electronic detector has identified a suspect location. It is less useful as a primary search method on a large system because you must know where to look.

Dispose of bubble solution properly; do not let it drain into drains connected to refrigerant recovery equipment.

Documenting Findings: The Service Report

A complete service report for a leak test includes:

Accurate documentation protects you legally, helps the next technician, and is required by EPA record-keeping practices for refrigerant work.

Summary of Safety Rules for This Lab

Key terms10

Dry nitrogenNitrogen gas with moisture removed to a very low dew point, used for pressure testing sealed systems because it is inert, non-flammable, and will not contaminate the system interior.
Schrader valve adapterA fitting that connects a manifold gauge hose or charging equipment to a Schrader-type service port on a refrigeration system, allowing access without releasing system pressure.
Manifold gauge setA set of gauges, valves, and hoses used to measure system pressures and to connect service equipment such as vacuum pumps, refrigerant cylinders, or nitrogen cylinders to a sealed system.
Test pressureThe specific pressure, not to exceed the manufacturer's rated maximum, to which a system is pressurized during a leak test in order to stress-test all joints and fittings.
Heated-diode leak detectorAn electronic instrument that detects halogenated refrigerant vapors by passing air over a heated sensing element; the element's electrical resistance changes in the presence of halogen compounds, triggering an alarm.
Infrared leak detectorAn electronic instrument that identifies refrigerant vapor by measuring the absorption of infrared light at wavelengths characteristic of halogenated compounds.
Bubble solutionA specially formulated soap-based fluid applied to fittings and joints to visually confirm a leak; escaping gas forms visible bubbles.
False positiveAn alarm triggered by a substance other than the target refrigerant — such as compressor oil residue or a halogenated solvent — that causes an electronic detector to indicate a leak where none exists.
System integrityThe condition of a sealed refrigeration system in which all joints, fittings, and components are leak-free and capable of holding the design pressure without loss.
Maximum allowable working pressure (MAWP)The highest pressure at which a component or system is rated to operate safely, as specified by the manufacturer; test pressure must not exceed this value.

Worked examples4

1. You pressurize a trainer sealed system to 150 psig at 8:00 AM and record the gauge reading. Fifteen minutes later, the gauge reads 144 psig. The ambient temperature has not changed. How many psi of pressure were lost, and what does this result tell you?
  1. Step 1: Identify the start and end pressures. Start = 150 psig; End = 144 psig.
  2. Step 2: Calculate the pressure drop. 150 psig − 144 psig = 6 psig lost.
  3. Step 3: Evaluate the result. The ambient temperature did not change, so the pressure drop cannot be explained by a temperature effect. A 6 psig drop over 15 minutes indicates one or more leaks in the system.
  4. Step 4: Record the finding on the service report: start pressure 150 psig, end pressure 144 psig, elapsed time 15 minutes, pressure drop 6 psig, conclusion: system fails leak test — proceed to locate leak(s).
Answer: 6 psig was lost. Because temperature was stable, this pressure drop indicates the system has one or more leaks and fails the integrity test.
2. A manufacturer's service data sheet lists the high-side MAWP as 400 psig and the low-side MAWP as 150 psig. You plan to pressure-test both sides with nitrogen. What is the maximum test pressure you may apply to each side?
  1. Step 1: The test pressure must not exceed the manufacturer's MAWP for each side.
  2. Step 2: For the high side, the maximum allowable test pressure is 400 psig.
  3. Step 3: For the low side, the maximum allowable test pressure is 150 psig.
  4. Step 4: Set your nitrogen regulator to no more than these values before pressurizing each respective side. In practice, many technicians test at a pressure slightly below the MAWP to provide a safety margin; always follow the specific manufacturer's instructions.
Answer: High side: do not exceed 400 psig. Low side: do not exceed 150 psig. These are the manufacturer's maximum limits and must not be surpassed.
3. During a leak search, your electronic detector alarms at a joint on the suction line accumulator. You sweep the probe again slowly, and the alarm repeats. A classmate says the alarm is probably a false positive because someone recently sprayed solvent cleaner in that area. Describe the steps you would take to determine whether the alarm represents a real leak.
  1. Step 1: Move the detector probe away from the area and allow it to clear in fresh air for several seconds until the alarm silences and the baseline reading returns to normal.
  2. Step 2: Return to the suspect joint and sweep again slowly — probe tip below the fitting — to see whether the alarm repeats consistently in the same location.
  3. Step 3: Apply bubble solution generously around the joint and all surrounding fittings. Watch for at least 30 seconds.
  4. Step 4: If bubbles form and grow at the joint, you have visual confirmation of a real leak. Record it as a confirmed leak on your service report with a description of location and bubble rate.
  5. Step 5: If no bubbles form, the alarm may be a false positive caused by solvent residue. Note both findings on your service report, recommend wiping the area with a clean, dry cloth, re-testing after the solvent has dissipated, and re-inspecting.
Answer: Clear the detector, re-sweep the area, and apply bubble solution. Bubbles confirm a real leak; no bubbles with a persistent detector alarm suggests a false positive from the solvent residue — document both findings and re-test after the area is cleaned and dry.
4. You begin pressurizing a sealed system trainer at room temperature (70 °F). After isolating the system at the target pressure, the gauge drops 3 psig over 15 minutes while the room temperature drops 2 °F. Is this pressure drop necessarily evidence of a leak?
  1. Step 1: Recall that gas pressure changes with temperature. A temperature decrease causes pressure to decrease even with no leak.
  2. Step 2: A rough rule for sealed gas systems: pressure changes approximately in proportion to absolute temperature (Rankine). Absolute temperature at 70 °F = 70 + 460 = 530 °R. At 68 °F = 68 + 460 = 528 °R.
  3. Step 3: Calculate the expected pressure ratio. If starting pressure is, for example, 150 psig (absolute = 150 + 14.7 = 164.7 psia), the expected new absolute pressure = 164.7 × (528 ÷ 530) = 164.7 × 0.9962 ≈ 164.1 psia, or about 149.4 psig — a drop of roughly 0.6 psig due to temperature alone.
  4. Step 4: Compare: the observed drop is 3 psig; the temperature-related drop is approximately 0.6 psig. The remaining ~2.4 psig drop is not explained by temperature change and is suspicious for a leak.
  5. Step 5: Conclusion: the pressure drop is larger than what temperature change alone accounts for. Proceed to locate the leak with electronic detector and bubble solution; document the temperature readings alongside the pressure readings on the service report.
Answer: A 2 °F drop accounts for only about 0.6 psig of the 3 psig pressure loss. The remaining approximately 2.4 psig drop is not explained by temperature and is suspicious for a leak — further investigation with electronic detector and bubble solution is required.

The math in this lesson1

Learners compute a 6 psig drop from 150 to 144 psig during a nitrogen standing pressure test.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates nitrogen pressure-testing procedure on a open-system trainer: connect nitrogen cylinder with a regulator and a Schrader valve adapter, pressurize system to a specified test pressure (following the instructor's demonstration of how to find the manufacturer's limit), isolate, and monitor pressure on the manifold gauge for 15 minutes; students observe, then each student performs the full sequence independently on their assigned trainer unit with instructor sign-off at each valve-open step. · Instructor seeds trainer units with three pinhole leaks at pre-drilled locations (covered with tape) and students must find all three using an electronic leak detector and bubble-soap solution; students record each leak location, leak severity (pass/fail bubble rate), and recommended repair action on a service report form. · Class debrief: instructor reviews common false-positive triggers for electronic detectors (oil residue, other halogenated compounds) and the correct sweeping technique (probe below the fitting, slow movement); students correct and resubmit service reports based on debrief feedback.
Materials: Dry nitrogen cylinders with regulators, Schrader valve adapters, manifold gauge sets, electronic refrigerant leak detectors (heated-diode or infrared type), bubble-solution leak-detection fluid, trainer sealed-system units (with pre-seeded leaks), service report forms, PPE (safety glasses, gloves), instructor demonstration unit
Check: Students must locate all three seeded leaks and complete an accurate service report; 100% leak detection required to pass (missing any seeded leak is a fail and the student repeats the activity). Service report is reviewed by instructor for accuracy of location description and repair recommendation; report must earn 80% of available points.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Sealed System Service and Repair
1. Why is compressed air prohibited for pressure-testing a sealed refrigeration system?
Sealed System Diagnosis
2. Where should a technician find the maximum allowable test pressure for a specific sealed-system unit?
Refrigerant Handling and Leak Detection
3. When using an electronic refrigerant leak detector, the probe tip should be held:
Refrigerant Handling and Leak Detection
4. An electronic leak detector alarms repeatedly at a suction-line fitting, but the technician knows that penetrating oil was recently sprayed in the area. The best next step is to:
Sealed System Diagnosis
5. During a 15-minute nitrogen pressure hold test with stable ambient temperature, the manifold gauge pressure drops from 150 psig to 143 psig. What should the technician conclude?
Safety and Environmental Practices
6. Which personal protective equipment is required at minimum during nitrogen pressure testing in this lab?
Safety and Environmental Practices
7. A technician pressurizes a sealed system to the manufacturer's test pressure, isolates it, and then leaves the room for 30 minutes before reading the gauge. Why is this practice a problem?
EPA 608 Regulatory Requirements
8. Under EPA Section 608 of the Clean Air Act, knowingly venting refrigerant to the atmosphere during service:
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