Oxygen sensor
An oxygen sensor is a device used to determine the concentration of gaseous or dissolved oxygen O 2 . The most important areas of application include oxygen sensors for determination in breathing air , but also the control of processes in sewage treatment plants or during fuel combustion in engines.
The determination of oxygen concentrations is also called oximetry . An oxygen sensor is a device for automatic and often continuous oximetry. In the blood, oxygen can occur as dissolved O 2 , as in water , but it is also bound to hemoglobin and transported in this bound form by the blood. In the pulse oximetry of the portion of the hemoglobin is measured, the O 2 is loaded, the different coloring of the various forms of hemoglobin is utilized.
Operating principles
There are several working principles that are used for oxygen sensors. Are especially important
- amperometric sensors, i.e. the measurement of the current strength of a galvanic cell , with oxygen being converted at a cathode and thus leading to a current.
- with solid electrolyte , see galvanic oxygen sensor .
- with liquid electrolyte, see Clark sensor .
- Resistance sensors in which an electrical resistance, for example of a semiconductor oxide , changes
- Paramagnetic sensors: Oxygen is a paramagnetic gas (see Magnetic Susceptibility ). It can align itself in a magnetic field and is then attracted by a sufficiently strong magnet. The effect depends on the oxygen concentration and disappears at higher temperatures.
- optical sensors:
- Absorption sensors that use the absorption of light by oxygen, preferably at 760 nm.
- Optical sensors that use fluorescence that can be quenched by oxygen ( fluorescence quencher )
Clark sensors
The Clark sensor was developed by the American biochemist Leland C. Clark in 1954 and is one of the oldest oxygen sensors. Originally it was specifically designed to measure blood oxygen levels, but this type of oxygen sensor can be used to determine oxygen in many different solutions, such as aquarium water or wastewater. The membrane that separates the measuring cell from the measuring water to be examined is typical of this type of sensor . Clark originally used a platinum cathode and a silver anode in a chloride-containing solution, the reaction leading to silver chloride . An important variant uses an alkaline electrolyte and a lead anode (overall reaction: 2 Pb + O 2 → 2 PbO).
Nernst probe / lambda probe
The lambda probe is used to determine the residual oxygen in exhaust gases (often engine exhaust gases) and to control and optimize combustion processes and their efficiency. In addition to the vehicle engines, other combustion processes such. B. in waste incineration , coal-fired power plants but also steelworks where such sensors are used. Many lambda probes are built as Nernst probes and use a solid, ceramic electrolyte that conducts oxygen ions (mostly made of zirconium oxide ) as a membrane , whereby a voltage is created on the membrane that is dependent on the difference in the oxygen content of the gases on both sides (exhaust gas / air) being measured.
Paramagnetic sensors
The sensor is based on the paramagnetic property of oxygen, which means that oxygen can be attracted or accelerated in a magnetic field. At higher temperatures (≈ 300 ° C) oxygen loses its paramagnetic properties.
A gas circulation occurs in the sensor, during which the gas is heated up by a heating wire (300 ° C) and cooled down again on the walls. If oxygen is present, the magnet accelerates the O 2 molecules towards the heating wire, where they lose their magnetic properties. This creates an additional flow, the intensity of which depends on the oxygen content. The flow also cools the heating wire, which leads to a change in resistance that can be detected with the aid of a bridge circuit.
Resistance probe
The sensor element of a resistance probe consists of a semiconducting titanium dioxide ceramic . The charge carriers are made available by oxygen vacancies that act as donors . With surrounding oxygen, the defects are filled and reduce the number of free charge carriers. The oxygen ions do not contribute significantly to the conductivity, but the oxygen reduces the number of free charge carriers. If the oxygen concentration is high, the sensor material has a high resistance. The electrical conductivity σ in the working range is described by an Arrhenius equation with an activation energy E A :
The signal is generated by a voltage divider with a fixed resistor .
swell
- ↑ Precise sense of oxygen . Fraunhofer Institute for Physical Measurement Techniques (IPM), May 10, 2005, accessed on September 17, 2010 (press release).