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