\(\varphi = v / v_{sat}\), where \(v\) is mass concentration of water vapour, \(v_{sat}\) is its mass concentration of water vapour at saturation (at the same temperature). For normal cases, the relative partial pressure may be assumed to be equal to relative mass concentration of vapour.
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\(\varphi\)
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"Relative Mass Concentration of Vapour" is one of a number of "Relative Concentration" quantities defined by ISO 8000.