What should the ullage be when measuring oxygen content in cargo tanks?

The correct approach for measuring oxygen content in cargo tanks requires ensuring that the sample truly reflects the conditions within the tank. When the measurement is taken one meter below the tank top, it is situated in a part of the tank that is less influenced by surface effects, such as layering of products or tank top turbulence. This allows for a more accurate and representative reading of the oxygen concentration in the cargo, as it is less likely to be affected by the atmosphere and any fluctuations at the surface.

Other measurements might not provide the same reliability. For instance, measuring one half the ullage could lead to results that are heavily influenced by both the vapor space and any residual vapors that may not represent the overall condition of the tank. Always opting for a location that maintains enough distance from surface influences, such as taking the measurement a distance below the tank top, assures that the reading accurately depicts the oxygen levels that are critical for the safe handling of cargo and preventing flammable atmospheres.