Thermocouple and thermistor - Essay Example

?Literature Review on Thermocouple and Thermistor Introduction Awareness of heat and attempting to measure the intensity can be traced to ancient history in the development of humankind. The initial success in measuring heat or temperature was based on the phenomenon of thermal expansion. It has remained one of the simplest methods and the most popular means for temperature sensing and is the basis of the liquid-in-glass thermometers that find applications ranging from home to the science laboratories. (1). Advances in the field of science required more accurate thermal sensing then was possible with the best liquid-in-glass thermometers, which led to development of thermal sensing devices using different methods. The range of modern thermal sensing devices includes thermoelectric, resistive, semi-conductive, optical, acoustic, and piezo-electric detectors. (1). Thermoelectric and resistive devices are based on the transmission of a fraction of the quantity thermal energy of the object to a contact sensor that has the ability to convert the heat energy into an electric signal that can be used for measuring the temperature of the object. However, there arises an error issue in this method that requires the sensor to be placed on or in the object. The error issue pertains to the sensor, irrespective of its size, disturbing the point of contact or the measurement site, and thereby causing inaccuracy in the measurement of the temperature. These inaccuracies are applicable to any method of sensing, be it conductive, convective, or radiative. A key goal in engineering has been to minimize these inaccuracies in temperature measurements through appropriate design of the sensors and use of correct measurement techniques. (1). 2. Principle and Design of the Thermocouple Thermocouples are based on the discovery by Thomas Johann Seebeck of thermoelectric currents during his studies of electromagnetic effects using bismuth-copper and bismuth-antimony circuits. The observation of Seebeck in 1821 that small currents flow in circuits made up of two different conductors, when their junctions are maintained at dissimilar temperatures is called the Seebeck Effect and is the principle on which thermocouples are founded. (2). In case one junction of a circuit consisting of dissimilar metals is maintained at a hotter temperature than the other junction, an electro-motive force (emf) is generated, which is in proportion to the difference in temperature between the hot junction or the measuring junction and the cold junction or the reference junction. The emf produced in this condition is called the Seebeck emf, which is measured in millivolts, and the pair of conductors that make up the thermoelectric circuit is called the thermocouple. (2). Measuring of the Seebeck emf can be effected either through a closed circuit current or an open circuit current. The Seebeck voltage constitutes the net conversion of thermal energy to electric energy, which is seen in the electric current generated. There are two factors on which the direction and magnitude of the Seebeck voltage is dependent, and they are the temperatures of the two junctions and the materials that constituter the thermo couple. When the temperature of the references junction is available, it is possible to measure the temperature of the measuring junction through an accurate measurement of the Seebeck emf generated by the thermocouple. There are three possible emf-measuring instruments that can be used for this purpose, namely deflection meters, digital voltmeters, and potentiometers. Digital voltmeters and potentiometers are the preferred emf-measuring instruments, because of their higher accuracy. (2). There are four main types of thermocouple junctions. These are exposed or bare-wire junction, grounded junction, ungrounded or isolated junction, and reduced diameter junction. In the exposed or bare-wire junction the sheath and insulating material are removed causing exposure of the thermocouple wires and are joined to create a measuring junction. This type of junction carries the advantage of quick response, but has the disadvantage of mechanical damage from the environment. In the grounded junction a closure is created through welding in an inert environment. This junction reduces the disadvantage of mechanical damage from environment, but has slow response time. In the ungrounded or isolated junction there is similarity to the grounded junction, with the difference coming from the creation of a thermocouple junction initially and then insulating it from the sheath ad sheath enclosure. This junction enhances the protection from damage from the environment, but at the same rime increases the slowness in response. The reduced diameter junction is employed, when the chosen wires or sheath of the wires of the thermocouple are thick and heavy. It may be constructed as a grounded or insulated junction, but delivers much quicker response. (2). Wires for thermocouples to measure temperatures from -190 to 2000 degrees Celsius are available in the form of matched pairs that are consistent with the published standards for the industry. Each of the wires used in the thermocouple are calibrated separately. Based on this calibration wires from different materials are selected for the making of thermo couple in such a manner the temperature – emf relationship for the selected pair is not more deviant than the established standard tolerances. (2). The calibration of a thermocouple includes the determination of the emf values in adequate number of temperatures, so that the thermocouple becomes suitable for measuring temperatures on a particular scale at the desired levels of accuracy. This process involves annealing, test junction assembly, emf measurement and the development of emf-temperature tables or equations. (2). 3. Principle and Design of the Thermistor The principle on which thermistor’s work is based on the well known Ohm’s Law, where v = Ri, in which v is the potential difference between two points, i is the current between the two pints, and R the resistance between the two pints. It is a generally accepted rule that the resistance value R corresponds to a specific temperature. Thus, there is a resistance-temperature (R-T) characteristic in almost all two terminal resistors. These characteristic can be classified into three types. In the first type the resistance shows little or no variance with change in temperature. Examples of materials that demonstrate this characteristic include manganin and constantan. The second type consists of materials that show a positive temperature coefficient of resistance. Materials that demonstrate this characteristic include platinum, copper, and nickel. The third type is made up of materials that demonstrate a negative temperature coefficient of resistance. Materials that demonstrate this characteristic include semiconductor materials like oxides of manganese, nickel, and cobalt. Thermistors belong to this type of materials with a negative temperature coefficient of resistance. The name thermistor is comes from the thermally sensitive nature of these materials. (3). The difference in thermal dependence of resistivity of materials like metals and semiconductors can be explained on the basis of the resistivity or conductivity of the materials. The value of the resistance of a material R is influenced by the physical dimensions of the resistor and the material constitution. Changes in resistance due to temperature mainly occur because of change in resistivity p, or due to changes in its reciprocal conductivity a. When the evaluation of the atomic model of a material shows that there is no energy gap between its valence and conduction bands, then the material is taken as a conductor. Evaluation of semiconductors shows that there is an energy gap that is present in the range from 0.1eV and 0.3eV between these two bands. (3). The value of the conductivity is reliant on the quantum of charge carriers n that are present in the conduction band and the velocity of these carriers under the influence of an electric field. This velocity of the carriers is dependent on the available mobility in the charge carriers u. Evaluation of metals show that the number of charge carriers does not vary much with temperature and the variance in resistivity is the result of changes in mobility in the charge carriers. Increase in temperature results in enhanced thermal agitation of the atoms in the crystal lattice, which causes reduction in the mean free path of a charge carrier between two successive collisions. This leads to reduction in the mobility of the charge carriers and the increase in resistivity observed with metals. On the other hand, in the case of semiconductors, increase in temperature causes several charge carriers to cross into the conduction band increasing the number of charge carriers present, which more than neutralizes the reduction in mobility of the charge carriers. Semiconductors thus show high negative temperature coefficient of resistance. The negative temperature coefficients of resistance of semiconductors are between -1% and -5%. (3). Intrinsic semiconductors like germanium or silicon have a high temperature coefficient of resistance, but the problem is that their conductivity at normal temperatures is too low, making them unsuitable materials for resistors. Thermistors for practical use are thus made from compound semiconductors, which are usually oxides of cobalt, copper, manganese, nickel, tin, and titanium that do not demonstrate the conductivity weakness associated with the intrinsic semiconductors like germanium and silicon. (3). 4. Advantages and Disadvantages of the Thermocouple Thermo couples present several advantages for measurement of temperature. They can be used across wide ranges of temperatures based on the kind of metal wires used in them, and are capable of measuring temperatures ranging from -200 degrees Celsius to 2,500 degrees Celsius. Thus, they find a wide range of applications that include temperature measurements in cryogenics and jet engine exhausts. The ruggedness of thermocouples makes them invulnerable to shocks and vibrations. The robust nature of thermocouples makes them suitable for temperature measurement applications in environments that are hazardous. The small size and low thermal capacity give thermocouples the capability to respond quickly to rapid changes in the temperature, particularly when the measurement junction is exposed. The response time is within a few hundred milliseconds, making them well suited for applications where sensitivity to quick changes in temperatures is required in the measurement of temperature. The working of the thermocouple does not cause any self heating and this makes them an intrinsically safe tool for measuring temperatures. (4). The thermocouple suffers from several inherent disadvantages. The first such disadvantage relates to the complex signalling requirement. The thermocouple voltage has to be converted into an appropriate signal that can be used to read the temperature, which results in the complex signalling requirement. Consequently, a lot of time has to be spent on its design to ensure that errors that can affect its accuracy are not present. The metallurgical properties of the metals used in the construction of a thermocouple lead to inherent inaccuracies in the measurement of temperature with thermocouples. Added to this is the fact that only the temperature at the reference junction is measured with a thermocouple, and that too normally within one to two degrees Celsius of the actual temperature, making it unsuitable for applications, where accuracy requirements fall below this range. Metals are used in the construction of thermocouples, and in corrosive environments deterioration of the metals lead t inaccuracies in the measurements of temperatures. This means that thermocouples need to be protected, cared for, and maintained in such applications. Another disadvantage of thermocouples is their susceptibility to noise. Noises in the form of stray electrical or magnetic fields can effect the measurement of the micro-volt level changes induced in thermocouples from changes in temperature. Susceptibility to noise can be reduced in many ways. Twisting the wires of the thermocouple reduces interference from magnetic fields. Employing shielded cable or running the wires of the thermocouple through a metal conduit and guarding reduces interference from electrical field pick up. The measuring device used for reading the signals from the thermocouple has to have signal filtering either in the hardware or the software that is capable of strongly rejecting line frequency of 50Hz or 60Hz and its harmonics. (4). 5. Advantages and Disadvantages of the Thermistor In comparison to the thermocouple the thermistor gives a larger output signal that makes the thermistor more precise in its reading of the temperature. In addition, the thermistor has better stability properties that result in more accurate performance over longer durations of time without care and maintenance. Thermistors give more precise temperature readings than thermocouples in mid range temperatures, and are more suited for such applications than the thermocouples. (5). The factors that make the thermistors popular for applications in temperature measurement are easier calibration, ease in wiring, and the direct acceptance of many automatic panels. (6). The features of the thermistor that give it an advantage as a temperature measuring device are its high sensitivity, availability in small sizes that allow point measurements with quick response time, wide range of resistance values, and accuracy in measuring temperatures in the range from 120K to 470K. These advantages of thermistors have led to their extensive use in applications that have electrical and non-electrical variables. Thermistors are commonly seen in the process industry for measuring and controlling temperature. Design of high performance electronic circuits suffer from the problem of the sensitivity of their performance to ambient temperatures. The high negative temperature coefficient of thermistors makes them useful to overcome this problem. This solution of thermistors has led to their popular use in millivoltmeters, biasing circuits of bipolar transistors, and logantilog amplifiers. (3). There are disadvantages in the use of thermistors for measuring temperatures. One of the serious disadvantages is the problem of inter changeability. Even when thermistors are fabricated by the same technique with strict control over their fabrication they demonstrate a spread in their R-T characteristics. Another serious problem with thermistors is the lack of stability that is a feature of the resistance in a thermistor drifting with age and continued use. Bead type resistors are more stable than other configurations of thermistors and offer some solution to the stability problem. In addition, there is the issue of high temperatures causing change in composition of the materials used in the thermistor and the sensor. Re-calibration is a solution for this issue. Finally, there is the limitation in the temperature range for which thermistors can be used. An upper limit of approximately 590K is usually applied to the use of thermistors due to the impact high temperatures have on the composition of the materials used in a thermistor. There is a lower limit also in the use of thermistors that stem from the largest resistance value beyond which measurement is not easy. This lower limit is usually about 120K. Thus, thermistors are most suitable in the temperature ranges of 120K to 590K, and other temperature measuring devices like thermocouples are more suitable for the measurement of other temperature ranges. (3). Conclusion Many of the activities of humankind require the accurate measurement of temperature, for which a range of temperature measuring devices are now available. The thermocouple and the thermistor are two of such devices. Small currents flow in circuits made up of two different conductors, when their junctions are maintained at dissimilar temperatures is called the Seebeck Effect and is the principle on which thermocouples are founded. Ohm’s Law is the basis of thermistors, by which there is a resistance-temperature (R-T) characteristic in almost all two terminal resistors. The different principles that are the basis of the thermocouple and the thermistor give them several advantages and also disadvantages. Standout strengths of the thermocouple include robustness, quick response to rapid changes in temperature, lack of self-heating making them intrinsically safe, and operability over a wide range of temperatures. The disadvantages associated with thermocouples include the complex signalling requirement leading to time and effort in the design and fabrication of the thermocouple, the inherent metallic properties of thermocouples that contribute to inaccuracies, degrading of metals used in the thermocouple in corrosive environments, and susceptibility to stray electrical and magnetic noise. Thermistors offer the advantages of high sensitivity, availability in small sizes that allow point measurements with quick response time, wide range of resistance values, accuracy in measuring temperatures, and ease in installation. However, thermistors suffer from disadvantages that include inter changeability, lack of stability, the limitation in the temperature range for which thermistors can be used. The advantages and disadvantages of temperature measuring devices make it necessary for evaluation of the temperature measurement application requirements and environment and correlating it to the benefits and demerits of temperature measuring devices before arriving at the decision of the type of temperature measurement device to be used. Works Cited 1. Fraden, Jacob. Handbook of Modern Sensors: Physics, Designs, and Applications. Fourth Edition. New York: Springer, 2010. 2. McDowell, Mark. “Thermocouples”. Survey of Instrumentation and Measurement. Ed. Stephen A. Dyer. New York: John Wiley & Sons, Inc., 2001. 129-135. 3. Sankaran, P., Kaliyugavaradan, S. & Murti, V. G. K. “Thermistors”. Survey of Instrumentation and Measurement. Ed. Stephen A. Dyer. New York: John Wiley & Sons, Inc., 2001. 122-129. 4. Duff, Mathew & Towey, Joseph. “Two Ways to Measure Temperature Using Thermocouples Feature Simplicity, Accuracy, and Flexibility”. Analog Dialog 44.10 (2010): 1-6. 5. “Temperature Measurement: Thermocouples vs. Thermistors”. 2008. VERITEQ. 28 May 2011. . 6. “Comparison of Thermistors, Thermocouples and RTD’s”. 2008. ENERCORP Instruments Ltd. 28 May 2011. . Read More