Leak Check and Evacuation) Nitrogen Test Procedure

 

1. Introduction to Charles's Law

• Charles's Law:

  • States that the volume of a fixed amount of gas at constant pressure is directly proportional to its absolute temperature.
  • Implication: When the temperature of the gas increases, the pressure increases; when the temperature decreases, the pressure decreases.

• Practical Understanding:

  • Temperature Increase: Pressure gauge goes up.
  • Temperature Decrease: Pressure gauge goes down.
  • Observation: The gauge should not move if there's no temperature change. A change in gauge reading due to temperature change doesn't necessarily indicate a leak.

2. Example Scenario: Temperature-Induced Pressure Change

• Scenario:

  • Begin a pressure test at 4 PM with an R22 system at 150 PSIG and 95°F outside.
  • Return the next morning at 75°F.
  • Observation: Pressure gauge shows a drop to 144 PSIG.

• Explanation:

  • This is not a leak but a result of the temperature drop.
  • Calculation Using Charles's Law:
    • Formula: $\frac{P1}{T1} = \frac{P2}{T2}$
    • P1: Initial pressure (150 PSIG + 14.7 PSI atmospheric pressure = 164.7 PSI absolute pressure).
    • T1: Initial temperature (95°F + 460 = 555°R).
    • P2: Final pressure we need to find.
    • T2: Final temperature (75°F + 460 = 535°R).
    • Calculation: $\frac{164.7}{555} = \frac{P2}{535}$
    • Solving for $P2$ gives approximately 144 PSIG.

• Conclusion:

  • The pressure drop from 150 PSIG to 144 PSIG is due to the temperature drop, not a leak.

3. Practical Application: Standing Pressure Test with Nitrogen

• Setup:

  • Equipment Needed: Gauges, nitrogen cylinder, regulator, hoses, and manifold.
  • Procedure:
    • Connect the charging hose (yellow) to the regulator.
    • Set the regulator to just over 150 PSIG for an R22 system.
    • Open the tank and manifold valves to fill the system with nitrogen.
    • Adjust to exactly 150 PSIG, then close the gauges and tank valve.

• Steps:

  • Step 1: Charge the system with nitrogen.
  • Step 2: Ensure the system is at the correct pressure.
  • Step 3: Close all valves to prevent leaks from the gauge manifold.
  • Step 4: Mark the exact gauge pressure (optional if using the same pressure each time).
  • Step 5: Leave the system for a set period (e.g., overnight).

• Evaluation:

  • No Pressure Drop: Indicates no leaks.
  • Pressure Drop: Indicates a possible leak, unless explained by temperature changes as per Charles's Law.