If a size D oxygen tank shows a reading of 1000 psi, what does this indicate about the tank's status?

A reading of 1000 psi on a size D oxygen tank indicates that the tank is likely at least half-full but not completely full. Oxygen tanks are filled to a typical maximum pressure of around 2200 psi when full, so a reading of 1000 psi signifies that while the tank is not empty, it is also not fully stocked.

In general, for a size D tank, you would typically start considering it usable until the pressure drops significantly below 500 psi, at which point the volume of oxygen becomes insufficient for most emergency needs. Therefore, a reading of 1000 psi suggests that the tank possesses a reasonable amount of oxygen for continued use, aligning with the indication that it is at least half-full.

Other options have different implications about the tank's condition, such as being empty, completely full, or needing replacement, but these do not apply to the situation described since a reading of 1000 psi confirms usable oxygen remains in the tank.