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The output for O2 refers to dissolved oxygen potential. This not directly measured, but is calculated, using an industry standard formula, from the other parameters that are directly taken. For this reason it only shows the maximum potential, not a true value in your pond or aquarium. The output is shown in PPM.
The graph is included as a guide so that your can see how the other parameters affect the potential of dissolved oxygen in the water. The biggest factor to the level is temperature and if the aquarium or pond get very warm in the summer, extra airation may need to be considered.
There are some factors, such as atmospheric pressure, that are 'assumed' and not taken into account in the calculation. However, these do not have a very large influence on the output.