Which two readings are used to assess whether non-condensables are present in a recovery cylinder?

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The key idea here is that non-condensables disrupt the expected relationship between pressure and temperature for a refrigerant in a cylinder. In a cylinder containing mainly refrigerant, the vapor pressure at a given temperature should match the refrigerant’s known pressure–temperature curve. If you measure the temperature and find the pressure is higher than what the curve predicts for that temperature, it indicates non-condensables (like air or other gases) are present in the headspace because they don’t condense and push the total pressure up. Volume doesn’t reliably reveal this, since the cylinder’s fixed volume doesn’t reflect how much non-condensable gas is mixed with the refrigerant, and humidity isn’t the factor being tested in this context. By using both pressure and temperature readings together, you can determine whether the cylinder’s gas is in the expected equilibrium or contaminated with non-condensables.