AEE 407 -002 Temperature Instruments PDF

Temperature Measurement  Temperature (sometimes called thermodynamic temperature) is a measure of how hot or cold something is: specifically, a measure of the average kinetic energy of the particles in a system. While there is no maximum theoretically reachable temperature, there is a minimum...

Temperature Measurement  Temperature (sometimes called thermodynamic temperature) is a measure of how hot or cold something is: specifically, a measure of the average kinetic energy of the particles in a system. While there is no maximum theoretically reachable temperature, there is a minimum temperature, known as absolute zero, at which all molecular motion stops. Temperature is by far the most measured parameter. It impacts the physical, chemical and biological world in numerous ways.  All matter is made of particles - atoms or molecules - that are in constant motion. Because the particles are in motion, they have kinetic energy. The faster the particles are moving, the more kinetic energy they have. The more kinetic energy the particles of an object have, the higher is the temperature of the object. The higher the temperature, the faster the molecules of the substance move, on the average. History about Temperature 1592 - Galileo Galilei invented the liquid-in-glass thermometer. 1643 - Athanasius Kircher invented the first mercury thermometer. 1714 - Daniel Gabriel Fahrenheit invented both the mercury and the alcohol thermometer with Fahrenheit scale. 1742 - Anders Celsius proposed a centigrade scale 1800’s - William Thomson (later Lord Kelvin) postulated the existence of an absolute zero. 1821 - Thomas Seebeck discovered the principle behind the thermocouple the existence of the thermoelectric current. 1821 - Sir Humphry Davy noted the temperature dependence of metals. 1932 - C.H. Meyers built the first Resistance Temperature Detector (RTD). 1948 – the name centigrade scale was change to Celsius 20th century - The development of temperature sensors fully developed.  Temperature measurement, also known as thermometry, describes the process of measuring a current local temperature for immediate or later evaluation. Temperature measurement can be classified into a few general categories: a) Thermometers b) Probes c) Non-contact  Conduction - the process by which heat or electricity is directly transmitted through a substance when there is a difference of temperature or of electrical potential between adjoining regions, without movement of the material. Convection - the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat. "The final transfer of energy to the surface is by convection." Radiation - the emission of energy as electromagnetic waves or as moving subatomic particles, especially high-energy particles which cause ionization International Practical Temperature Scale The International Practical Temperature Scale is the basis of most present-day temperature measurements. The scale was established by an international commission in 1948 with a text revision in 1960. A revision of the scale was formally adopted in 1990 and still being used today. Nonelectric Temperature Sensors Liquid-in-Glass Thermometers Most versions have used mercury as the liquid. The element mercury is liquid in the temperature range of about −40 to 700°F (−38.9 to 356.7°C). As a liquid, mercury expands as it gets warmer; its expansion rate is linear. Because of mercury’s toxicity and the strict governing laws, the use of the mercury-in-glass thermometer has declined.  Bimetallic Thermometers Bonding two dissimilar metals with different coefficients of expansion produces a bimetallic element. These are used in bimetallic thermometers, temperature switches, and thermostats having a range of 100 to 1000°F (−73 to 537°C). Solids tend to expand when heated. The amount that a solid sample will expand with increased temperature depends on the size of the sample, the material it is made of, and the amount of temperature rise.  One way to amplify the motion resulting from thermal expansion is to bond two strips of dissimilar metals together, such as copper and iron. If we were to take two equally-sized strips of copper and iron, lay them side-by-side, and then heat both of them to a higher temperature, we would see the copper strip lengthen slightly more than the iron strip:  If we bond these two strips of metal together, this differential growth will result in a bending motion that greatly exceeds the linear expansion. This device is called a bi- metal strip:  This bending motion is significant enough to drive a pointer mechanism, activate an electromechanical switch, or perform any number of other mechanical tasks, making this a very simple and useful primary sensing element for temperature.  Filled-bulb Systems Filled system thermometers have been used for decades. They have a useful range of -125°F to 1200°F. Filled-bulb systems exploit the principle of fluid expansion to measure temperature. If a fluid is enclosed in a sealed system and then heated, the molecules in that fluid will exert a greater pressure on the walls of the enclosing vessel. By measuring this pressure, and/or by allowing the fluid to expand under constant pressure, we may infer the temperature of the fluid.  There are basically four types of filled bulb temperature sensors in use in industrial applications They are: ✓ Liquid Filled Systems Temperature Sensors (Class I) Class I systems use a liquid fill fluid. Here, the volumetric expansion of the liquid drives an indicating mechanism to show temperature as shown. The steel bulb, stem and indicator are completely filled under pressure with a liquid. The system is totally filled to provide a constant volume. Expansion of the fluid in the tube is converted to pressure. This pressure expands the Bourdon tube which moves the pointer on the scale. The filling fluid is usually an inert hydrocarbon, such as xylene. ✓Vapor Filled Systems Temperature Sensors (Class II) The vapor filled system uses a volatile liquid/vapor combination to generate a temperature dependent fluid expansion. This form of measurement is based on the vapor- pressure curves of the fluid and measurement occurs at the transition between the liquid and vapor phases.  This transition occurs in the bulb, and will move slightly with temperature, but it is the pressure that is affected and causes the measurement. If the temperature is raised, more liquid will vaporize and the pressure will increase. A decrease in temperature will result in condensation of some of the vapor, and the pressure will decrease. ✓Gas Filled Systems Temperature Sensors (Class III) Here, the change in pressure with the temperature allows us to sense the bulb’s temperature. As the volume is kept constant, the pressure varies in direct proportion to the absolute temperature  Gas filled systems do provide a faster response than other filled devices, and as it converts temperature directly into pressure it is particularly useful in pneumatic systems. Nitrogen is quite commonly used with gas filled systems. ✓Mercury Filled Systems Temperature Sensors (Class V) Mercury expansion systems are different from other liquid filled systems because of the properties of the metal. Mercury is toxic and can affect some industrial processes and is used less in filled system. Mercury filled system provides the widest range of operation (-40 °C to 650°C)  Bistate/Phase Change Sensors These low cost nonelectric sensors are made from heat- sensitive fusible crystalline solids that change decisively from a solid to a liquid with a different color at a fixed temperature depending on the blend of ingredients. They are available as crayons, lacquers, pellets, or labels over a wide range of temperatures from 100 to 3000°F (38 to 1650°C).  All these devices undergo a change in color or appearance depending upon the temperature variations. “They are used, for instance, with steam traps – when a trap exceeds a certain temperature, a white dot on a sensor label attached to the trap will turn black. Response time typically takes minutes, so these devices often do not respond to transient temperature changes.” The major uses are where a quick check of the temperature of an object is desired, or, in the case of the temperature labels or stickers, a record of whether the object has exceeded a certain temperature. Electronic Thermometers/Sensors Thermocouples A thermocouple is an assembly of two wires of unlike metals joined at one end designated the hot end. At the other end, referred to as the cold junction, the open circuit voltage is measured. Called the Seebeck voltage, this voltage (electromotive force) depends on the difference in temperature between the hot and the cold junction and the Seebeck coefficient of the two metals. Principles of Operation 1.) Peltier Effect- If the junctions of a thermocouple are at the same temperature and a current is passed through the circuit of the thermocouple, HEAT is produced at one junction and ABSORBED at the other.  2. ) Thompson Effect- The absorption or evolution of heat when current is passed through an unequally heated conductor.  3) Seebeck Effect - When two dissimilar metals with different temperatures and they’re touching, they produce an emf or voltage.  When two dissimilar metal wires are joined together at one end, a voltage is produced at the other end that is approximately proportional to temperature. That is to say, the junction of two different metals behaves like a temperature-sensitive battery.  This phenomenon provides us with a simple and direct way to electrically infer temperature: simply measure the voltage produced by the junction, and you can tell the temperature of that junction.  3 Thermocouple junctions Grounded Junction Wires are physically attached or welded to the inside of the probe wall Good heat transfer from the outside Faster response than the ungrounded junction type Demerits : low accuracy , noise problems  3 Thermocouple junctions Ungrounded Junction Thermocouple junction is detached from the probe wall Recommended for measurements in corrosive environments Demerits : Slow response  3 Thermocouple junctions Exposed Junction Thermocouple protrudes out of the tip of the sheath Offers the best response time Limited in use to non-corrosive and non-pressurized applications Demerits: Noise problem, damage to circuit, no protection for use in harsh environment, low accuracy.  THERMOPILE Several thermocouples connected in series or parallel Passive radiation sensing voltage- generating device Does not emit any radiation and require cooling or bias As a SENSOR As a GENERATOR  Thermopile Thermopile detectors are thermal detectors that utilize the Seebeck effect in which a thermal electromotive force is generated in proportion to the incident infrared light energy. Thermopile detectors themselves have no wavelength dependence and so are used with various types of window materials for diverse applications such as temperature measurement, human body sensing, and gas analysis. Thermocouple Types Thermocouples exist in many different types, each with its own color codes for the dissimilar-metal wires.  Resistance Temperature Detectors (RTD) A Resistance Temperature Detector or simply RTD is a temperature sensor which measures temperature using the principle that the resistance of a metal changes with temperature. For most metals the change in electrical resistance is directly proportional to its change in temperature and is linear over a range of temperatures. This constant factor called the temperature coefficient of electrical resistance is the basis of RTDs.  RTDs work on a basic correlation between metals and temperature. As the temperature of a metal increases, the metal's resistance to the flow of electricity increases. Similarly, as the temperature of the RTD resistance element increases, the electrical resistance, measured in ohms (Ω), increases. RTD elements are commonly specified according to their resistance in ohms at zero degrees Celsius (0° C). The most common RTD specification is 100 Ω, which means that at 0° C the RTD element should demonstrate 100 Ω of resistance.  Thermistors Like the RTD, the thermistor is also a resistive device that changes its resistance predictably with temperature. Its benefit is a very large change in resistance per degree change in temperature, allowing very sensitive measurements over narrow spans. Due to its very large resistance, lead wire errors are not significant. Difference between RTDs and Thermistors Thermistors are devices made of metal oxide which either increase in resistance with increasing temperature (a positive temperature coefficient) or decrease in resistance with increasing temperature (a negative temperature coefficient). RTDs are devices made of pure metal (usually platinum or copper) which always increase in resistance with increasing temperature. The major difference between thermistors and RTDs is linearity: thermistors are highly sensitive and nonlinear, whereas RTDs are relatively insensitive but very linear.  Pyrometers Pyrometers also called as Radiation Thermometers was invented by Josiah Wedgwood. They are non-contact temperature sensors that measure temperature from the amount of thermal electromagnetic radiation received from a spot on the object of measurement. Pyrometers are mainly divided to two types: a.) Radiation Pyrometers b.) Optical Pyrometers  Pyrometers are used to measure the temperature which is difficult to measure. They are non-contact devices, used to measure temperature above 1500 degree Celsius, contact devices may melt at this temperature.  Radiation Pyrometers A radiation pyrometer also referred as infrared (IR) thermometer is a noncontact radiant energy detector. Every object in the world radiates IR energy. The amount of radiant energy emitted is proportional to the temperature of an object. Noncontact thermometers measure the intensity of the radiant energy and produce a signal proportional to the target temperature. The physics behind this broadcasting of energy is called Planck’s Law of Thermal Radiation.  As shown in the figure, the radiation pyrometer has an optical system, including a lens, a mirror and an adjustable eye piece. The heat energy emitted from the hot body is passed on to the optical lens, which collects it and is focused on to the detector with the help of the mirror and eye piece arrangement. The detector may either be a thermistor or photomultiplier tubes. Thus, the heat energy is converted to its corresponding electrical signal by the detector and is sent to the output temperature display device.  Optical Pyrometer Optical Pyrometers work on the basic principle of using human eye to match the brightness of the hot object to the brightness of the calibrated lamp filament inside the instrument. In an optical pyrometer, a brightness comparison is made to measure the temperature. As a measure of the reference temperature, a color change with the growth in temperature is taken. The device compares the brightness produced by the radiation of the object whose temperature is to be measured, with that of a reference temperature. The reference temperature is produced by a lamp whose brightness can be adjusted till its intensity becomes equal to the brightness of the source object. The radiation  The radiation from the source is emitted and the optical objective lens captures it. The lens helps in focusing the thermal radiation on to the reference bulb. The observer watches the process through the eye piece and corrects it in such a manner that the reference lamp filament has a sharp focus and the filament is super- imposed on the temperature source image. The observer starts changing the rheostat values and the current in the reference lamp changes. This in turn, changes its intensity.  This change in current can be observed in three different ways. 1. The filament is dark. That is, cooler than the temperature source. 2. Filament is bright. That is, hotter than the temperature source. 3. Filament disappears. Thus, there is equal brightness between the filament and temperature source. At this time, the current that flows in the reference lamp is measured, as its value is a measure of the temperature of the radiated light in the temperature source, when calibrated. Mechanical Temperature Measuring Devices are simple to use but limited in application due to the construction, temperature range and function. Quite durable and in some extent reliable. THERMOCOUPLE: Generally can measure temperatures over wide temperature ranges, inexpensively and are very rugged, but they are not as accurate or stable as RTD’s and Thermistors. RTD’s: are stable and have a fairly wide temperature range but not as rugged and inexpensive as thermocouples. Since they require the use of electric current to make measurements, RTD’s are subjective in accuracies from self heating. THERMISTOR: tend to be more accurate than RTD’s or thermocouples, but they have a much more limited temperature range. They are also subject to self heating. INFRARED SENSORS: can be used to measure temperature higher than of any other devices and without direct contact with the surface being measured. However they are not as accurate and are sensitive to surface radiation efficiency. -End of Temperature Measurement-