In my experience, most facilities that have oxigen flow also have compressed air flow with cheap controllable fluxometers, but a blender is often lacking or too expensive.

In those cases we use a Y piece connecting air and oxigen flows to the inspirarory limb.

The role of the blender is to control the percentage of oxigen offered. With the Y piece we can control that by the equation:

%O2 = (O2flow + Airflow * 0.21) / Total flow

If you can use Total flow of 8L/min (reasonable in most cases) the approximations table below is of easy memorization and precise enough:

  O2 flow - Air flow - %O2

    8 - 0 - 100%
    7 - 1 - 90%
    6 - 2 - 80%
    5 - 3 - 70%
    4 - 4 - 60%
    3 - 5 - 50%
    2 - 6 - 40%
    1 - 7 - 30%
    0 - 8 - 21%

Edited:formatting