A monitoring apparatus for monitoring the relative flow rates of first and second gases that are to be mixed, comprising: a first gas flow path for the first gas; a first flow-monitoring device for detecting a parameter relating to the flow of the first gas along the first gas flow path; a second gas flow path for the second gas; a second flow-monitoring device for detecting a parameter relating to the flow of the second gas along said second gas flow path; a comparator device for comparing said parameter relating to the flow of the first gas and said parameter relating to the flow of the second gas; and operable to generate a response in dependence on said comparison, characterized in that the first flow-monitoring device is a laminar flow element positioned in said first gas flow path and having an upstream side and a downstream side relative to said first gas flow path, the parameter relating to the flow of the first gas being a pressure differential between said upstream and downstream side, and that the second flow-monitoring device is a laminar flow element positioned in said second gas flow path and having an upstream side and a downstream side relative to said second gas flow path, the parameter relating to the flow of the second gas being a pressure differential between said upstream and downstream sides, and further comprising a device for impeding generation of a said response during starting-up and/or switching-off of the flow of the second gas, wherein said device for impeding generation of a said response comprises a first restriction in a first gas path communicating between a first, upstream side of the second flow-monitoring device and the comparator device and a second restriction in a second gas path communicating between a second, downstream side of the second flow-monitoring device, the diameter of said first restriction being smaller than the diameter of said second restriction. a first gas flow path for the first gas; a first flow-monitoring device for detecting a parameter relating to the flow of the first gas along the first gas flow path; a second gas flow path for the second gas; a second flow-monitoring device for detecting a parameter relating to the flow of the second gas along said second gas flow path; a comparator device for comparing said parameter relating to the flow of the first gas and said parameter relating to the flow of the second gas; and operable to generate a response in dependence on said comparison, characterized in that the first flow-monitoring device is a laminar flow element positioned in said first gas flow path and having an upstream side and a downstream side relative to said first gas flow path, the parameter relating to the flow of the first gas being a pressure differential between said upstream and downstream side, and that the second flow-monitoring device is a laminar flow element positioned in said second gas flow path and having an upstream side and a downstream side relative to said second gas flow path, the parameter relating to the flow of the second gas being a pressure differential between said upstream and downstream sides, and further comprising a device for impeding generation of a said response during starting-up and/or switching-off of the flow of the second gas, wherein said device for impeding generation of a said response comprises a first restriction in a first gas path communicating between a first, upstream side of the second flow-monitoring device and the comparator device and a second restriction in a second gas path communicating between a second, downstream side of the second flow-monitoring device, the diameter of said first restriction being smaller than the diameter of said second restriction.