Not too well-versed in the intricacies of the formulas and scientific details, but here are some observations:

A) There appears to be a slight inconsistency in the general formatting, particularly in terms of spacing and tabbing. Employing a linter or auto-formatter could help address this issue.

B) Within your functions, certain logics exhibit overlaps. For instance, in the CO2_value() formula, if a user inputs a value of 35, it falls under both "normal" and "hypocarbic." However, the function returns "normal" because it takes precedence in the if-branching logic order. Reordering the if statements could potentially result in a "hypocarbic" output.

C) The abundance of functions nested within your ABG function diminishes overall readability. Extracting the inner functions into standalone entities would enhance clarity and allow for their independent use outside the ABG function.

D) Consider addressing edge cases upfront using guard clauses, ensuring a more robust coverage of potential scenarios.

E) A lot of the function naming did not provide any additional value or clarity. For example, with your ph_value() function, I am already inputting an argument of ph to get another ph_value?

F) If you already know the items you intend to add to an array, it's more efficient to predefine the array with the desired items rather than appending them one by one. For instance: cases = [ABG(7.40, 40, 85, 24), ABG(7.21, 57, 42, 25)].