A microfluidic flow controller for receiving analyte fluid and calibration fluids wherein the flow controller is configured to switch between (i) an analysis mode in which analyte fluid is passed to an analysis module and (ii) a calibration mode in which the analyte fluid is passed to an alternative destination and calibration fluid is passed to the analysis module, thereby maintaining flow rate of analyte fluid from a source and maintaining a steady flow rate of fluid through the analysis module in both analysis mode and calibration mode. The flow controller may vary the ration of multiple calibration fluids during a calibration mode. Means for accurately positioning sensors within a flow conduit of the analysis module is also described. Sensors are also described for use with or without the microfluidic flow controller for the detection of metabolites and molecules. The sensors may or may not comprise enzymes and may be used with a sensing reagent, also described.