Oxygen sensor

The type of oxygen sensor used in most underwater diving applications is the electro-galvanic oxygen sensor, a type of fuel cell, which is sometimes called an oxygen analyser or ppO2 meter. They are used to measure the oxygen concentration of breathing gas mixes such as nitrox and trimix. They are also used within the oxygen control mechanisms of closed-circuit rebreathers to keep the partial pressure of oxygen within safe limits. and to monitor the oxygen content of the breathing gas in saturation diving systems and of surface supplied mixed gas. This type of sensor operates by measuring the voltage generated by a small electro-galvanic fuel cell. ==Scientific and production applications==

Soil respiration In soil respiration studies oxygen sensors can be used in conjunction with carbon dioxide sensors to help improve the characterization of soil respiration. Typically, soil oxygen sensors use a galvanic cell to produce a current flow that is proportional to the oxygen concentration being measured. These sensors are buried at various depths to monitor oxygen depletion over time, which is then used to predict soil respiration rates. Generally, these soil sensors are equipped with a built-in heater to prevent condensation from forming on the permeable membrane, as relative humidity can reach 100% in soil. Marine biology In marine biology or limnology, oxygen measurements are usually done in order to measure respiration of a community or an organism, but have also been used to measure primary production of algae. The traditional way of measuring oxygen concentration in a water sample has been to use wet chemistry techniques e.g. the Winkler titration method. There are however commercially available oxygen sensors that measure the oxygen concentration in liquids with great accuracy. There are two types of oxygen sensors available: electrodes (electrochemical sensors) and optodes (optical sensors). Brewing In breweries, dissolved oxygen is measured at multiple places within a beer production operation, from DO (dissolved oxygen) control at wort aeration, to measurement with a trace oxygen sensor (low-PPB; low-parts per billion) at the filling line. These measurements are either taken with an in-line dissolved oxygen sensor or a portable dissolved oxygen meter. Pharmaceutical production Oxygen sensors play a critical role in the production of active pharmaceutical ingredients made in a bioreactor by cell culture or fermentation. Because oxygen is important in cellular respiration, the oxygen sensor provides a critical measurement to ensure that cells in the bioreactor are getting the oxygen needed to maximize production. The accuracy of the oxygen sensor is critical, as a lack of oxygen negatively impacts productivity and excess oxygen can lead to changes in cell metabolism. In bioreactors, oxygen sensors can be installed vertically or at an angle. For vertical installations, angled-tip oxygen sensors help to provide accurate readings. ==Oxygen sensor technologies==

Electrodes The Clark-type electrode is the most used oxygen sensor for measuring oxygen dissolved in a liquid. The basic principle is that there is a cathode and an anode submersed in an electrolyte. Oxygen enters the sensor through a permeable membrane by diffusion and is reduced at the cathode, creating a measurable electric current. There is a linear relationship between the oxygen concentration and the electric current. With a two-point calibration (0% and 100% air saturation), it is possible to measure oxygen in the sample. One drawback to this approach is that oxygen is consumed during the measurement with a rate equal to the diffusion in the sensor. This means that the sensor must be stirred in order to get the correct measurement and avoid stagnant water. With 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 with 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 types 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 as optode probes, a digital camera is used to capture fluorescence intensities over a specific area. ==See also==