Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Electrochemical Sensing Elements Muhajir Ab Rahim School of Mechatronics Northern Malaysia College University of Engineering Flashback… 4 types of element in measurement system 1. 2. 3. 4. Sensing element Signal Conditioning element Signal Processing element Data Presentation element Sensing element 1st element in measurement system , which in contact with, and draw energy from, the process or system being measured Classified by physical principle involved (refer Table 8.1 in text book) Electrochemical Sensing Element What kind of input we want to measure? How it works? Advantages and disadvantages? Type of input measured by electrochemical sensing element 1. 2. Ionic concentration Gas composition Application Medical- to measure O2, glucose, CO2 Water Treatment- to measure pH, Cl2, O2 Industrial safety- to detect toxic gases Military- to detect explosive agent 1. Ionic Concentration Measurement Use Ion Selective Electrodes (ISEs) as a sensor Reaction between the charged species (selective single ion) in the immersed solution and those on sensor surface will establish equilibrium potential difference. The measurement of e.m.f. of ISEs is based on Nernst equation Ion- Selective Electrodes (ISEs) High input impedance meter or buffer amplifier RISE Ion- selective electrode Test Solution Reference electrode ZIN RTH RREF Meter or amplifier 2. Gas Composition Measurement The detection method varies depending on the type of sensor used Zirconia is a solid state material which gives electrochemical response to oxygen Semiconductor based gas sensor is the most popular gas sensor applied in industry TGS-813 Gas Sensor The working principle is based on the properties of the elements Iridium (Ir) and Palladium (Pd) The sensor element is essentially an assembly of a heater coil buried in SnO2. The heater coil is made of Ir and Pd. When the current flows in the heater coil at a given temperature, the oxygen takes away electrons from the donor at the surface of the semiconductor element, creating a potential barrier. TGS-813 Gas Sensor (cont…) Because the potential barrier makes the movement of the free electrons more difficult, the electrical resistance of the element increases. However, the sensitivity of the sensor can be affected by the ambient temperature and humidity. Normally, requires 2-3 minutes of warm-up time, and the resistance of element rises by 20% after the warm-up time. Chemically Sensitive Field Effect Transistors (CHEMFET) The basic principle underlying the MOSFET is the control of a current flowing between two semiconductor electrodes. Drain and source are placed on the same element, with a third electrode, the gate, between them. The metallic gate electrode is insulated against drain and source by means of silicon dioxide and can only influence the drainsource current electrostatically. The extremely high input resistance of the gate electrode means that no large input power is required to control this current. Chemically Sensitive Field Effect Transistors (CHEMFET) cont…. The MOSFET’s gate consists of a metallic coating and is used as an electrode to control the drainsource current through the external potential Vgs. In the case of the ISFET the metallic gate is replaced by a special oxide-coated gate, that is sensitive to certain ion concentration. When immersed in a liquid, the electrical circuit Vgs is closed with the reference electrode and the certain ion concentration in solution can influence the drain-source current Useful Interactive Link http://www.sensedu.com/