Temperature measurments
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Description of Temperature Measurments
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Temperature measurments
- 1. PRESENTED BY PREM BABOOM.Sc.,B.Tech(Chemical Engineering),M.Phil, M.B.A Fellow of Institution of Engineers India An Expert for www.ureaknowhow.com
- 2. WHAT IS TEMPERATURE? • Qualitatively, the temperature of an object determines the sensation of warmth or coldness felt by touching it. More specifically, temperature is a measure of the average kinetic energy of the particles in a sample of matter, expressed in units of degrees on a standard scale.
- 3. • Temperature is the degree of "hotness" (or "coldness"), a measure the of the heat intensity. When two objects of different temperature are in contact, the warmer object becomes colder while the colder object becomes warmer. It means that heat flows from the warmer object to the colder one.
- 4. • Thermometer helps us determine how cold or how hot a substance is. Temperatures in science (and in most of the world) are measured and reported in degrees Celsius (o C). In the US, temperatures are commonly reported in degrees Fahrenheit (o F). On both Celsius and Fahrenheit scales, the temperature at which ice melts (water freezes) and the temperature at which water boils are used as reference points.
- 5. • On the Celsius scale, freezing point of water is defined as 0 o C, and the boiling point is defined as 100 o C. • On the Fahrenheit scale, water freezes at 32 o F and boils at 212 o F. • On the Celsius scale there are 100 degrees between freezing and boiling of water, compared to 180 degrees on the Fahrenheit scale. This means that 1 oC= 1.8 oF. • Thus the following formulas can be used to convert temperature between the two scales: • (1) F = 1.8 C + 32 = 9/5 C + 32 (2) C = 0.56( F -32) =5/9( F-32)
- 6. DEG F DEG C DEG RDEG K ABSOLUTE ZERO ICE POINT STEAM POINT 00-273-459 491273032 671373100212
- 8. Types Of Measurement • There are four basic types of temperature measuring devices, each of which uses a different principle: – Mechanical (liquid-in-glass thermometers, bimetallic strips, etc.). – Thermojunctive (thermocouples). – Thermoresistive (RTDs and thermistors). – Radiative (infrared and optical pyrometers).
- 9. Mechanical temperature measuring devices • Principle of operation: – A change in temperature causes some kind of mechanical motion, typically due to the fact that most materials expand with a rise in temperature. Mechanical thermometers can be constructed which use liquids, solids, or even gases as the temperature-sensitive material. – The mechanical motion is read on a physical scale to infer the temperature.
- 10. Liquid-in-glass thermometer – The most common and well-known thermometer is the liquid-in-glass thermometer. – As the temperature rises, the liquid expands, moving up the tube. The scale is calibrated to read temperature directly. • Usually, mercury or some kind of alcohol is used for the liquid
- 11. Bimetallic strip thermometer – Two dissimilar metals are bonded together into what is called a bimetallic strip, as sketched. – Suppose metal A has a smaller coefficient of thermal expansion than does metal B. – As temperature increases, metal B expands more than does metal A, causing the bimetallic strip to curl upwards as sketched.
- 12. Working of bimetallic strip
- 13. Pressure thermometer • considered mechanical, operates by the expansion of a gas instead of a liquid or solid. (Note: There are also pressure thermometers which use a liquid instead of a gas.) • Suppose the gas inside the bulb and tube can be considered an ideal gas. The ideal gas law is PV = mRT
- 14. o R is a constant. The bulb and tube are of constant volume, so V is a constant. Also, the mass, m, of gas in the sealed bulb and tube must be constant. Hence, the above equation reduces to P = constant times T. – A pressure thermometer therefore measures temperature indirectly by measuring pressure. • The gage is a pressure gage, but is typically calibrated in units of temperature instead.
- 16. MERCURY IN STEEL THERMOMETER • MERCURY - IN - STEEL thermometer works on the principle of expansion of mercury due to rise in temperature. The whole system is filled with mercury under pressure. The definite volume of mercury contained in the bulb expands under effect of temperature to be measured. The increaase in volume of confined mercury is transmitted through the capillary to the coiled burdon tube which uncoils proportionally to volume increase resulting an indication of temperature by the pointer.
- 17. GAS FILLED DIAL THERMOMETERS • Gas Filled Thermometers deploy Nitrogen Gas at high pressure as an expansion gas filled into a closed system comprising of a bulb (of Steel or Chrome – Moly Steel), a microbore capillary (of Steel or Stainless Steel) and a spiral or ‘C’ shaped Bourdon Tube (of MS or SS). This system when heated at bulb end the Gas in the bulb expands and a pressure is generated within which moves the spiral / ‘C’ Shaped bourden as it is the only the elastic element. This movement is transmitted to a rack and pinion movement which drives a pointer thus showing temperature on a calibrated dial.
- 18. THERMOCOUPLE • The Thermocouple is a thermoelectric temperature sensor which consists of two dissimilar metallic wires, e.g., one chromel and one constantan. These two wires are connected at two different junctions, one for temperature measurement and the other for reference. The temperature difference between the two junctions is detected by measuring the change in voltage (electromotive force, EMF) across the dissimilar metals at the temperature measurement junction.
- 26. EXPOSED JUNCTION
- 29. THERMOCOUPLE
- 30. THERMOCOUPLE
- 34. RTD ELEMENTS
- 37. LEAD EFFECTS
- 38. 3 WIRE RTD
- 41. Thermistors – A thermistor is similar to an RTD, but a semiconductor material is used instead of a metal. A thermistor is a solid state device. – A thermistor has larger sensitivity than does an RTD, but the resistance change with temperature is nonlinear. – Furthermore, unlike RTDs, the resistance of a thermistor decreases with increasing temperature. – Thermistors cannot be used to measure high temperatures either, compared to RTDs. In fact, the maximum temperature of operation is sometimes only 100 or 200 oC.
- 42. From the circuit diagram, it is clear that this is a simple voltage divider. Rs is some fixed (supply) resistor. Rs and the supply voltage, Vs, can be adjusted to obtain the desired range of output voltage Vo for a given range of temperature.
- 43. Radiative temp.measuring devices (radiative pyrometry) • Principle of operation: – Radioactive properties of an object change with temperature. – So, radioactive properties are measured to infer the temperature of the object. – The advantages of radioactive pyrometry are: • There is no physical contact with the object whose temperature is being measured. • Very high temperatures can be measured.
- 44. • The fundamental equation for radiation from a body is the Stefan-Boltzmann equation,
- 45. – where • E is the emissive power radiated per unit area (units of W/m2). • is the emissivity, defined as the fraction of blackbody radiation emitted by an actual surface. The emissivity must lie between 0 and 1, and is dimensionless. Its value depends greatly on the type of surface. A blackbody has an emissivity of exactly 1. • is the Stefan-Boltzmann constant, • T is the absolute temperature of the surface of the object (units of K).
- 46. Infrared Pyrometer • An infrared pyrometer infers the temperature of a hot surface by measuring the temperature of a detector inside a detector chamber as shown below:
- 47. IDENTIFICATION BY COLOUR • DARK RED :540 DEG C • MEDIUM CHERRY RED :680 DEG C • ORANGE :900 DEG C • YELLOW :1010 DEG C • WHITE :1250 DEG C
- 48. Features of Pyrometer – An optical pyrometer is useful for measuring very high temperatures (even flames). – An optical pyrometer works by comparing a glowing wire of known temperature to the glow (optical radiation) from a hot object. – When the internal wire and the glow of the object are the same color, the temperatures are assumed to be equal. • The temperature of the internal wire is controlled and known, and thus the temperature of the object can be inferred
- 49. Attribute Thermocouple RTD Thermistor Cost Low High Low Temperature Range Very wide -450ºF +4200ºF Wide -400ºF +1200ºF Short to medium -100ºF +500ºF Interchange ability Good Excellent Poor to fair Long-term Stability Poor to fair Good Poor Accuracy Medium High Medium Repeatability Poor to fair Excellent Fair to good Sensitivity (output) Low Medium Very high Response Medium to fast Medium Medium to fast Linearity Fair Good Poor Self Heating No Very low to low High Point (end) Sensitive Excellent Fair Good Lead Effect High Medium Low Size/Packaging Small to large Medium to small Small to medium
Hinweis der Redaktion
- The Thermocouple is a thermoelectric temperature sensor which consists of two dissimilar metallic wires, e.g., one chromel and one constantan. These two wires are connected at two different junctions, one for temperature measurement and the other for reference. The temperature difference between the two junctions is detected by measuring the change in voltage (electromotive force, EMF) across the dissimilar metals at the temperature measurement junction. Thermocouples manipulate the fact that the electromotive force (EMF) between two dissimilar metals is a function of their temperature difference (gradient). However, three major effects are involved in a thermocouple circuit: the Seebeck, Peltier, and Thomson effects. The Seebeck effect describes the electromotive force (EMF) existing between two dissimilar metallic materials. The change in material EMF with respect to a change in temperature is called the Seebeck coefficient or thermoelectric sensitivity. This coefficient is usually a nonlinear function of temperature. EMF that is reversible and associated with changes in temperature is called the Peltier effect. Finally, the Thomson effect relates the reversible thermal gradient and EMF in a homogeneous conductor.
- Grounded When assembling the thermocouple into a protective metal sheath, we can weld the thermocouple junction directly to the inside tip of the sheath (Figure 16). Why attach the junction to the sheath? Can you think of any reasons? Attaching the junction to the sheath ensures rapid heat transfer from the sheath to the junction. Thus, the sheath protects the thermocouple junction while minimizing any heat transfer delays to it.
- Ungrounded The ungrounded junction is similar to the grounded junction, except it is isolated (insulated) from the metal sheath (Figure 17). Why insulate the junction from the metal sheath? Insulating the thermocouple junction electrically isolates it from the sheath metal. This is done to prevent stray voltages on a machine from inducing a measuring error in the thermocouple. Ungrounded junctions are also more shock resistant and better survive under rapid temperature change conditions. Unfortunately, the insulation slows down heat transfer to the thermocouple junction. An ungrounded junction in a MgO insulated, metal-sheathed thermocouple requires 2 to 3 times as long to respond to temperature changes as a grounded thermocouple.
- Exposed This type of junction protrudes from the end of the sheath, but is insulated from it (Figure 18). Because the junction is directly exposed to the material being heated, the junction responds very quickly to temperature changes. There is no sheath or insulation to slow down heat transfer. The disadvantage, though, is that an exposed junction is not protected from mechanical damage and chemical attack. If the junction is damaged or chemically attacked, a measuring error will result.
- Life “How long will this thermocouple last?” This is one of the most frequently asked questions about thermocouples. How should you answer? Try, “It depends.” Diplomatic, heh? Seriously, the fact is that it DOES depend. Life depends on many factors. Among them: operating temperature, thermocouple wire size, thermocouple protection, operating environment, accuracy required, etc. Instead of examining each point in detail, let’s focus on “life” and how it relates to thermocouple accuracy. Normally, thermocouples don’t “fail” like electric heaters. When an electric heater “fails”, it burns out - no more heat. A thermocouple, on the other hand, gets more and more inaccurate as time goes on. At some point, the accuracy is so “bad” that the thermocouple is said to have “failed.” Here’s how it works. When thermocouple wires are heated and cooled, physical and chemical changes take place. Physically, the molecular structure of the thermocouple metal changes. Chemically, the thermocouple wires react with oxygen or other substances. These chemical reactions change the chemical composition of the thermocouple wire. The chemical reactions are accelerated at higher temperatures. As these chemical (and physical) changes take place over time, a thermocouple‘s millivolt signal “drifts” (Figure 10). The result is that the thermocouple will no longer measure temperature within its stated accuracy band. “Drifting” may start within a few minutes or may take many months - it all depends on how a thermocouple is used. This is also the reason why tolerances are called initial calibration tolerances. The tolerances are only valid for the very first use. After the first use, there is no guarantee that the tolerance will hold.
- RTDs are precision temperature sensors. They are used in industrial applications as well as laboratories. RTD elements are typically more accurate than thermocouple elements and maintain that accuracy over a longer period of time. They are generally used up to 1200°F (650°C). How does an RTD work? How does it compare to a thermocouple? Let’s pursue the answers to these questions and many more! A RTD can take many forms. The most often used RTD elements are shown below. Figure 19a shows a fine platinum element wire coiled around a very small diameter ceramic cylinder. Platinum resistance elements are most often used, but nickel, copper and nickel-iron are also used. Small lead wires are welded on to the resistance element. The assembly is then encapsulated in glass to seal it and prevent contamination. Figure
- The detector itself is usually a thermopile. It measures Tdet, the temperature of the detector inside the chamber. Tind is the indicated temperature, which is calculated from Tdet, from the known geometry and the radiation equations. Tind is calibrated as a function of TH for a body of some assumed emissivity. The instrument is set up such that Tind is a function of the voltage output. The instrument typically displays a temperature, i.e. Tind, rather than voltage Vdet. Tind can be thought of as an uncorrected estimate of TH, since the emissivity of the object may not be the same as that assumed by the infrared pyrometer. In other words, if the actual emissivity of the object is not the same as the assumed emissivity, Tind will be incorrect. To correct for the actual emissivity of the object, In the above equations, absolute temperatures must be used!