an increasing sensor size, the oxygen consumption increases and so does the stirring sensitivity. In large sensors there tend to also be a drift in the signal over time due to consumption of the electrolyte. However, Clark-type sensors can be made very small with a tip size of 10 µm. The oxygen consumption of such a microsensor is so small that it is practically insensitive to stirring and can be used in stagnant media such as sediments or inside plant tissue. Optodes An oxygen optode is a sensor based on optical measurement of the oxygen concentration. A chemical film is glued to the tip of an optical cable, and the fluorescence properties of this film depend on the oxygen concentration. Fluorescence is at a maximum when there is no oxygen present. The higher the concentration of oxygen, the shorter the lifetime of the fluorescence. When an O2 molecule comes along, it collides with the film, and this quenches the photoluminescence. In a given oxygen concentration there will be a specific number of O2 molecules colliding with the film at any given time, and the fluorescence properties will be stable. The signal (fluorescence) to oxygen ratio is not linear, and an optode is most sensitive at low oxygen concentration. That is, the sensitivity decreases as oxygen concentration increases, following the Stern–Volmer relationship. The optode sensors can, however, work in the whole region 0% to 100% oxygen saturation in water, and the calibration is done the same way as with the Clark-type sensor. No oxygen is consumed, and hence the sensor is insensitive to stirring, but the signal will stabilize more quickly if the sensor is stirred after being put in the sample. These type of electrode sensors can be used for in situ and real-time monitoring of oxygen production in water-splitting reactions. The platinized electrodes can accomplish the real-time monitoring of hydrogen production in water-splitting device. Planar optodes are used to detect the spatial distribution of oxygen concentrations in a platinized foil. Based on the same principle than optode probes, a digital camera is used to capture fluorescence intensities over a specific area. See also Exhaust gas analyzer Digifant engine management system Jetronic Motronic Oxygen saturation Winkler test for dissolved oxygen Pulse oximetry References Breathing gases Engine sensors Gas sensors