Argon is an inert gas that is non-reactive and non-flammable.

• Sometimes used for pressure testing where welding or TIG/MIG operations are associated with the work, as the same gas supply may be used for both.
• Like all inert gases, can displace oxygen in confined spaces — adequate ventilation is required.

Carbon dioxide is used as a test gas in specific high-pressure system applications.

• Requires pressure-rated equipment — regulators, gauges, hoses, and fittings must be rated for CO₂ service and the applicable pressures.
• Non-flammable but can cause asphyxiation in high concentrations — ensure adequate ventilation during use.

Helium is used in very low leak rate testing applications where conventional methods are not sensitive enough.

• Used in combination with mass spectrometer leak detectors for extremely sensitive leak detection.
• Small molecular size allows helium to pass through very small leaks that other gases cannot — making it ideal for precision testing of critical components.
• Not a standard field test gas; more common in factory, laboratory, or specialized industrial applications.

Compressed air must be used with caution and is generally avoided in primary refrigerant circuits.

• May be used in some secondary systems — such as hydronic loops or glycol circuits — that do not contain primary refrigerant.
• Avoid in primary refrigerant circuits — compressed air introduces moisture and can form explosive mixtures with certain refrigerants under pressure.
• Oxygen must never be used for pressure testing under any circumstances.

Water is the most common liquid used for hydrostatic pressure testing at ambient temperatures.

• Suitable for most secondary system tests provided freezing is not a risk during or after the test procedure.
• After testing, piping must be thoroughly drained and dried — especially before connecting to refrigeration circuits or where freezing temperatures could occur.
• Potable water is preferred to prevent corrosion or biological growth in the piping.

Ethylene or propylene glycol mixed with water in the appropriate concentration for the design operating temperature.

• Used to pressurize and test closed hydronic loops where the system will ultimately operate with glycol as the secondary coolant.
• Testing with the intended operating fluid avoids later contamination from residual water left after a water-only test.
• Propylene glycol is preferred over ethylene glycol in applications where incidental contact with food or potable water is possible.

Brine solutions — typically calcium chloride or sodium chloride in water — are used for testing low-temperature secondary circuits.

• Used where the circuit operates below water's freezing point and must remain liquid during and after the test.
• Brines can be corrosive — piping materials, fittings, and seals must be compatible, and corrosion inhibitors may be required.
• Spills must be cleaned up promptly to prevent corrosion of structural components and environmental contamination.

• Estimate system volume from piping dimensions, component sizes (receivers, vessels, coils), and manufacturer data sheets.
• Apply gas laws (Boyle's Law: P₁V₁ = P₂V₂ at constant temperature) to relate initial cylinder pressure and volume to the final test pressure in the system — where accurate calculations are required.
• Increase pressure in controlled steps — never open the regulator to full test pressure immediately. Step up gradually, verifying gauge stability at each intermediate pressure before continuing.