Controlling % oxygen in medium

[Stephen G. Ernst]

   We would like to control and monitor the oxygen concentration in the
culture medium (plant cell culture).  Can anyone suggest a simple yet accurate
system for mixing oxygen and inert gas (e.g., argon) at prescribed
concentrations?  Any recommendations for flow valves, splitters, oxygen
detectors, etc.?We need such a system to study the effects of hypoxia on
specific physiological parameters in vitro.
 
Any info would be most appreciated!
  Steve
  sernst at unl.edu