Sensors

Sensors

Sensors detect the presence of energy, changes in or the transfer of energy. Sensors detect by receiving a signal from a device such as a transducer, then responding to that signal by converting it into an output that can easily be read and understood. Typically sensors convert a recognized signal into an electrical – analog or digital – output that is readable. In other words, a transducer converts one form of energy into another while the sensor that the transducer is part of converts the output of the transducer to a readable format. Consider the previous examples of transducers. They convert one form of energy to another, but they do not quantify the conversions. The light bulb converts electrical energy into light and heat; however, it does not quantify how much light or heat. A battery converts chemical energy into electrical energy but it does not quantify exactly how much electrical energy is being converted. If the purpose of a device is to quantify an energy level, it is a sensor.

Thermal Sensors

· Thermometer – measures absolute temperature (discussed in the previous section)

· Thermocouple gauge– measures temperature by its effect on two dissimilar metals

· Calorimeter – measures the heat of chemical reactions or physical changes and heat capacity

 A thermocouple is a device that directly converts thermal energy into electrical energy. When two dissimilar metal wires are connected at one end forming a junction, and that junction is heated, a voltage is generated across the junction (see the figure below). If the opposite ends of the wires are connected to a meter, the amount of generated voltage can be measured. This effect was discovered by Thomas Seebeck, and thus named the Seebeck Effect or Seebeck coefficient. The voltage created in this situation is proportional to the temperature of the junction

See beck effect

Chemical sensor ( O2 analyser)

Oxygen analysers determine oxygen concentration using the conductivity of a ceramic cell.  Ceramic cells only allow oxygen ions to pass through at high temperatures. With reference gas on one side and sample gas on the other, oxygen ions move from the side with the highest concentration of oxygen to that with the lowest concentration. The movement of ions generates an EMF (Electro Motive Force) which can be measured to determine the oxygen content.

O2 analysers