A monitoring apparatus for monitoring the relative flow rates of first and second gases that are to be mixed comprises a first flow-monitoring, laminar flow element in a first gas flow path for detecting a differential first gas pressure thereacross and a second flow-monitoring, laminar flow element in a second gas flow path for detecting a differential second gas pressure thereacross. A comparator device compares the differential pressures and is operable to generate a response in dependence on said comparison.
