When gas A is dry air and mixed with an oxygen-free dry gas or gas mixture B (e.g., N2), then the \( {p_{{{\rm{O}}_{\rm{2}}}}} \) in water equilibrated with this gas mixture is
$$
{p_{{{\rm{O}}_{\rm{2}}}}} = {(_{\rm{b}}}p - {p_{{{\rm{H}}_2}}}{\rm{o,}}{T_{\rm{s}}}) \times {\phi _{\rm{A}}} \times \phi _{{{\rm{o}}_{\rm{2}}}}^*,
$$
((1))
where bp is barometric pressure [kPa], \( {p_{{{\rm{H}}_{\rm{2}}}}}{\rm{o,}}{T_{\rm{s}}} \) is the saturation water vapor pressure at the temperature of the water sample, ΦA is the volume fraction of gas A in the gas mixture leaving the gas mixing pump,
$$
{\phi _{\rm{A}}} = {{{p_{\rm{A}}}} \over {_{\rm{b}}p}}{\rm{and1 - }}{\phi _{\rm{A}}} = {{{p_{\rm{B}}}} \over {_bp}},
$$
((2))
\( \phi _{{{\rm{o}}_{\rm{2}}}}^* \) is the volume fraction of oxygen in dry air which can be taken as constant
\( \phi _{{{\rm{o}}_{\rm{2}}}}^* = 0.20946 \).