The absolute humidity is measured in gram of water vapor per m^3. What are the units of H2 in "a 80:20 H2O:H2 gas mixture?"

Thank you for clearing this. In this particular case I want to attain an 80% water vapor content by volume.

Have you heard about the relative humidity? "to attain an 80% water vapor content by volume" may require to keep all your set-up at temperature higher 80C (rough estimations) to avoid water condensation

Dear Freddy,

To be very clear, you can setup a bubbler and bubble your desired flowrate into boiling water in this case you want to saturate your H2 flow (which means you need to see condensation at the outlet). Then using a thermocouple at the point of exit, measure the cooled down steam + H2 mixture. Then using the saturation vapor pressure, https://www.engineeringtoolbox.com/water-vapor-saturation-pressure-d_599.html , you know that the temperature of your mixture at the exit of the bubbler should be around 90 to 96 °C (look at the vapor pressure in column 3. you can calculate the mole or volume fraction using Dalton's law.

I use the same method for my steam and gas mixtures but make sure you supper heat the steam after exiting the bubbler so the volume fraction is the same but you wont have condensation.