MEC201 Theory of Measurements and Sensors - Chapter 4 Temperature Measuring Instruments PDF

# Chapter (4): Measuring Instruments: Temperature Measuring Instruments ## **Mechanical Engineering Department** ## **Theory of Measurements and Sensors** ## **Code: MEC201** ## **Dr. Ashraf Elsayed** # Chapter (4) ## **Measuring Instruments: Temperature Measuring Instruments** ## **Dr. Ashraf Elsayed** # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Lecture Detailed Contents:** ### **Topics to be Covered in this Lecture** - Basic Concepts of Temperature - Measuring Temperature (Mechanical Methods) - Liquid-in-Glass Thermometer - Bimetallic Strip Thermometer - Filled Bulb Thermometer - Measuring Temperature (Electrical Methods) - Thermocouple - Resistance Temperature Detector (RTD) - Thermistor - Radiation Temperature Measurements - Optical Pyrometer - Total Radiation Pyrometer # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Basic Concepts of Temperature** ## **Assoc. Prof. Mohamed Reda Salem & Dr. Ashraf Elsayed** # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Temperature Scales** - Heat Is a form of energy transferred from a hotter subset to a colder one. - Temperature is a measure of the molecular activity of a substance, which expresses the degree of hotness or coldness or is the thermal state of a substance that determines whether it will give heat to another substance or receive. - Based on our physiological sensations, we express the level of temperature qualitatively with words like freezing cold, cold, warm, hot, and red-hot. However, we cannot assign numerical values to temperatures based on our sensations alone. - The temperature can not be measured directly but it is measured by its effect (relative expansion, change of state….) - The temperature scales enable us to use a common basis for temperature measurements, and several have been introduced throughout history. ### **Commonly used Scales** - Celsius (°C) - Absolute or Kelvin (K) - Fahrenheit (°F) - Rankine (R) ### **Temperature Relationships** - T(K) = T(°C) + 273.15 - T(°F) = 1.8 T(°C) + 32 - T(R) = 1.8 T(K) # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Temperature Scales** - The temperature scales used in the SI and in the English system today are the Celsius scale and the Fahrenheit scale, respectively. - On the Celsius (formerly called Centigrade) scale, there are 100 degrees between the ice and steam points, while on the Fahrenheit scale, there are 180 degrees. - On the Celsius scale, the ice and steam points were originally assigned the values of O and 100°C, respectively. The corresponding values on the Fahrenheit scale are 32 and 212°F. | | Fahrenheit | Celsius | Kelvin | |:----------|:-----------|:--------|:-------| | Boiling Point of water | 212°F | 100°C | 373.15 K| | Freezing Point of water | 32°F | 0°C | 273.15 K| | | -40°F | -40°C | 233.15 K| - °F = 32.0 + (9/5)°C - °C = (°F-32.0)(5/9) - K = °C + 273.15 - °C = K - 273.15 # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Temperature Scales** - Absolute temperature scale was defined with Pressure versus Temperature plots of the experimental data from a constant-volume gas thermometer using four different gases (air, hydrogen, helium and nitrogen) at different (but low) pressures. A constant-volume gas thermometer would read -273.15°C at absolute zero pressure. | T (°C) | T (K) | P (kPa) | |:-------|:-------|:--------| | -200 | 75 | 120 | | -225 | 50 | 100 | | -250 | 25 | 80 | | -275 | 0 | 60 | | -273.15°C| 0 | 0 | # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Temperature Sensors** - Temperature Measurement devises may be classified as contact and non-contact methods. - Contact methods can be divided into mechanical and electrical temperature measurements. - Non-contact method such as radiation temperature measurements. - Our objective is to present a discussion of temperature measurements and indicate the principle of operation of a number of devices that are commonly used. ## **Part One: Temperature Measurement by Contact Methods** ### **(A) Mechanical (Expansion) Methods** - Sometimes calls mechanical effect. We shall be concerned with those devices operating based on a change in mechanical dimension with a change in temperature. - Temperature Measurement by direct effect may be classified as follows: - Liquid-in-glass thermometer. - Bimetallic strip thermometer. - Filled bulb (Fluid expansion) thermometer. # **Part One: Temperature Measurement by Contact Methods** ### **(A) Mechanical (Expansion) Methods** - Liquid-in-Glass Thermometer - Bimetallic Strip Thermometer - Filled Bulb (Fluid Expansion) Thermometer. ## **Assoc. Prof. Mohamed Reda Salem & Dr. Ashraf Elsayed** # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Liquid-in-Glass Thermometer** - The liquid in glass thermometer is one of the most common types of temperature measurements devices. The construction details of such an instrument is shown in fig. Mercury in glass thermometer are generally applicable up to about 315 °C, but their range may be extended to 538 °C by filling the space above the mercury with a gas like nitrogen, this increases the pressure on the mercury, raises its boiling point, and there by permits the use of the thermometer at higher temperatures. - **Working principle**: the liquids expands on heating and contracts on cooling; Temperature is indicated on scale as liquid expands. - **Basis**: Glass thermometers contain liquid (usually mercury or alcohol) that expands or contracts with temperature changes. ### **The important characteristics of liquid in glass thermometers are :-** #### **Advantages:** - Power source not required. - Simple (Easy to use). - Cheap. - Easily portable. #### **Disadvantages:** - Fragile. - Relatively high heat capacity causing serious lag between change in temp. and thermometer response (Slow response). - Not suitable for surface temperature Measurements. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Bimetallic Expansion Thermometer** - **Basis**: Bimetallic thermometers use two different metals joined into one strip. As the temperature changes, the two metals expand at different rates causing the strip to bend. - Bimetallic strip is a very widely used method of temperature measurement. Two pieces of metal with different coefficient of thermal expansion shown in fig. are subjected to a temperature higher than the bonding temperature, it will bend in one direction; when it is subjected to a temperature lower than the bonding temperature, it will bend in the other direction. - Bimetallic strip mainly used in industries in temperature control devices. - Most bimetallic strips use one of steel or stainless steel coupled with Invar (nickel-iron alloy) or use one of brass coupled with iron. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Bimetallic Expansion Thermometer** - **BULB** - **INDICATING DEVICE** - **INSTRUMENT CASE** - A pointer is attached to the rotating coil which indicates the temperature on the dial. - **Bimetal Coil** rotation is caused by the difference in thermal expansions of the two metals. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Bimetallic Expansion Thermometer** - The important characteristics of bimetallic strip are :- #### **Advantages:** - Power source is not required. - Simple (Easy to use). - Robust. - Cheap. - Compact. - Close linearity throughout the temperature range - Range of application: -30 to 550 °C, - Accuracy: 1≈2% of the scale range. #### **Disadvantages:** - Poor accuracy, - Not suitable for very low temperatures because the expansion of metals tend to be too similar, so the device becomes a rather insensitive thermometer. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Filled Bulb (Fluid Expansion) Thermometer** - **Basis**: The filling fluid (liquid or gas) expands as temperature increases. - This causes the tube to uncoil and indicate the temperature on a calibrated dial. ### **The important characteristics of Filled Bulb Thermometer are :-** #### **Advantages:** - Power source is not required. - Simple (Easy to use). - Suitable for surface temperature Measurements. #### **Disadvantages:** - Entire system has to be replaced in case of damage - Lower accuracy, sensitivity & temperature span compared to electrical temperature instruments # **Part One: Temperature Measurement by Contact Methods** ### **(B) Electrical Methods** - Thermocouple (T/C) - Resistance Temperature Detector (RTD) - Thermistor ## **Assoc. Prof. Mohamed Reda Salem & Dr. Ashraf Elsayed** # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermocouple** - **Basis**: Thermocouple is based on the Seebeck effect: generation of a potential difference as a function of temperature. - **Thermocouple** (الإزدواج الحراري) is a junction between two different metals that produces a voltage difference related to a temperature difference. - **The Electromotive Force:** $E = K(T_h - T_c)$ Where: K is the thermocouple sensitivity. - **Cold Junction:** Needs to be held constant to give a fixed reference. (early methods held cold junction at 0°C using ice or refrigeration unit). - Two wires of different metal alloys. - Converts thermal energy into electrical energy. - Requires a temperature difference between measuring junction and reference junction. - Easy to use and obtain. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermocouple** - **Thermocouple** - **Thermocouple (Animation)** # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermocouples Types and Range** | Type | Positive wire | Negative wire | Seebeck Coeff. (μV/°C) | Range, °C | |:------|:--------------------------|:---------------------------|:--------------------------|:-----------| | E | Chromel | Constantan | 58.7 | -270 to 1000 | | G | Tungsten | Tungsten and 26% Rhodium | 19.7 | 0 to 2320 | | C | Tungsten and 5% Rhodium | Tungsten and 26% Rhodium | 19.7 | 0 to 2320 | | D | Tungsten and 3% Rhodium | Tungsten and 26% Rhodium | 19.7 | 0 to 2320 | | J | Iron | Constantan | 50.4 | -210 to 760 | | K | Chromel | Alumel | 39.4 | -270 to 1372| | N(AWG 14) | Nicrosil (84.3% Ni, 14% Cr, 1.4% Si, 0.1% Mg) | Nisil (95.5% Ni, 4.4% Si, 0.1% Mg) | 39 | -270 to 400 | | N(AWG 28) | Nicrosil (84.3% Ni, 14% Cr, 1.4% Si, 0.1% Mg) | Nisil (95.5% Ni, 4.4% Si, 0.1% Mg) | 26.2 | 0 to 1300 | | B | Platinum and 6% Rhodium | Platinum and 30% Rhodium | 1.2 | -50 to 1768 | | R | Platinum and 13% Rhodium | Platinum | 5.8 | -50 to 1768 | | S | Platinum and 10% Rhodium | Platinum | 5.9 | -50 to 1768 | | T | Copper | Constantan | 38.7 | -270 to 400 | # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermocouple Construction** - Sheath (normally stainless steel) - Arc Welded Junction (some are earthed at tip For improved response time) - Conductors insulated by Magnesium Oxide Powder - Normally element is in a thermowell - Commonly element is 1/4" outside Diameter - Sheath material, normally Stainless steel but can be special material such as Inconel, Incoloy, Hastelloy etc. - Duplex thermocouples have 2 elements inside one sheath. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermocouple Tip Types** 1. **(A)** Thermocouple element is detached from the probe wall - **Ungrounded** - For use in corrosive and pressurized apps. - Slow response time. - Offers electrical isolation. 2. **(B)** Thermocouple element is attached to the probe wall - **Grounded** – For use in corrosive and pressurized apps. - Quicker response time than ungrounded due to improved heat transfer. 3. **(C)** Thermocouple junction protrudes outside of the probe sheath - **Exposed** – For use in dry, non-corrosive, non-pressurized apps. - Quickest response time of all three. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Multiple-Junction Thermocouple Circuits** ### **Thermocouples in Series** - **Thermopiles** - Thermopile provides an amplified out put signal; in this case the out put voltage Would be N times the single thermocouple out put, where N is the number of Measuring junctions in the circuit. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermocouples in Parallel** - When a spatially averaged temperature is desired, multiple thermocouple junctions can be arranged in parallel. - In such an arrangement of N junctions, a mean emf (electromotive force) is produced, given by: $emf = \frac{1}{N}\sum_{i=1}^{N}(emf)_i$ - The mean emf is indicative of a mean temperature, $T = \frac{1}{N}\sum_{i=1}^{N}T_i$ # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermocouple** - Advantages: - Point temperature sensing - Wide temperature range - Cheap - Simple (Easy to use), Rugged - Robust - Power source is not required - Fastest response to temperature changes - Disadvantages: - Low sensitivity to small temperature changes - Least stable, least repeatable - Output is a non-linear function - Extension wire must be of the same thermocouple type - Wire may pick up radiated electrical noise if not shielded - Lowest accuracy # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Example** - **Problem**: An Iron-Constantan thermocouple (T/C) is used to measure temperature between 0 °C and 300°C at which it generates 5268 µV. If the thermal emf is 12500µV relative to the reference junction at 20 °C. Estimate the measured junction temperature. - **Solution**: - **Given**: - Thermocouple: 0°C to 300 °C 5268 μν - 20 °C to Th °C 12500 μν Th = ?? - *The Electromotive Force*: $E = K(T_h - T_c)$ - *Where*: K is the thermocouple sensitivity. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Example** Problem: A K type thermo-couple is assumed to have a linear operation range up to 1100 °C, with emf (0 °C reference) equal to 45.14 mV at this temperature. The thermocouple is exposed to a temperature of 840 °C. The meter used as a cold junction and its temperature is kept at 25 °C. Calculate the true thermal emf. - **Given**: - thermocouple linear operation up to 1100 °C - 0 ÷ 1100°C 45.14 mv E= ?? - *The Electromotive Force*: $E = K(T_h - T_c)$ - *Where*: K is the thermocouple sensitivity - **Solution**: - E = K (Tĥ – Tc) → E (45.14 mv) = K (1100 - 0) → *K* = $\frac{45.14}{1100}$ = 0.04104 mv/°C - E = $\frac{45.14}{1100}$ (840° − 25°) = 33.44 mv - E = 33.44 mv # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Resistance Temperature Detector (RTD)** - Resistance Temperature Detectors (RTDs) operate under the principle that the electrical resistance of certain metals increases and decreases in a repeatable and predictable manner with a temperature change. - Wire wound and thin film devices. - Nearly linear over a wide range of temperatures. - Can be made small enough to have response times of a fraction of a second. - Require an electrical current to produce a voltage drop across the sensor. ### **RTD Applications** - Air conditioning and refrigeration servicing - Furnace servicing - Foodservice processing - Medical research - Textile production # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Resistance Temperature Detector (RTD)** - RTD is a temperature-sensing element composed of semiconductor material that changes its resistance with temperature. - RTD usually have positive temperature coefficients (PTC) which means the resistance of the RTD increases as the temperature increases. - The most common method for measuring the resistance of an RTD is to use a Wheatstone bridge circuit. In a Wheatstone bridge, electrical excitation current is passed through the bridge, and the bridge output current is an indication of the RTD resistance. - *R* = *R0* [1 + *α*(T – T0)] = *R1* *$\frac{R3}{R2}$ - *R* = RTD resistance at temperature T - *R0* = RTD resistance at temperature T0 - *α* = Material constant (°C-1) - *R2* & *R3*= Fixed resistances - *R1*= Variable resistance, adjusted to balance the bridge circuit # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Example** - **Problem**: An RTD forms one arm of an equal-arm Wheatstone bridge as shown. The fixed resistances, R₂ and R3 are equal to 25 Ω. The RTD has a resistance of 25 Q at a temperature of O°C and is used to measure a temperature that is steady in time. Suppose the coefficient of resistance for this RTD is 0.003925°C-1 If the value of the variable resistance, R₁, must be set to 37.36 Ω to balance the bridge circuit, determine the temperature of the RTD. Also, if the required uncertainty in the measured temperature is ≤ ±0.5°C, would a ±1% total uncertainty in each of the resistors that make up the bridge be acceptable? Neglect the effects of lead wire resistances for this example. - **Given**: - RTD - R0 = 25 Ω - α = 0.003925°C-1 - T0 = 0°C - R1 = 37.36 Ω - ωTRTD = ±0.5°C - TRTD = ?? # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Example** - RRTD = R0 [1 +α(T - T0)] = R1 * R3 - .. 25[1 + 0.003925 (T-0)]= 37.36 * 25 - ... TRTD = 126°C # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Resistance Temperature Detector (RTD)** - Disadvantages: - Slowest response time - Less Robust than thermocouples - Self heating - Current source required - Expensive - Sensitive to vibration (strains the platinum element wire) - Low sensitivity to small temperature changes - Advantages: - Most repeatable temperature measurement - Change in resistance is linear - Very accurate - Very resistant to contamination - Most stable over time # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **RTD Platinum Thermometers** - **RTD Platinum Thermometer Pt 100, Pt 1000** (nominal resistance 100/1000 ohms respectively) - *R* = *R0*(1 + 0.0039083T – 5.77x10-7T2) - A typical platinum RTD has a resistance of 100 ohms at 0°C, although other values such as 200, 500 and 1000 ohms are also used. - Therefore, coefficient of relative temperature change is approximately given by: $α =$$\frac{1}{R_0}$$\frac{dR}{dT}$ - (This value slightly depends upon platinum purity, for example typical US standards α = 0.00392, European standard α = 0.00385) - The temperature coefficient of resistance (*α*) for a platinum RTD is 0.00385 ohms/ohm/°C, which means that the resistance of the RTD changes by 0.00385 ohms for every 1°C change in temperature. - Platinum RTDs are commonly used in industrial applications due to their accuracy, stability, and durability. - Platinum RTDs can operate in a temperature range of -200°C to 1,000°C, making them suitable for a wide range of applications. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Resistance Temperature Detector (RTD)** ## **Difference between RTD and Thermocouple** | | RTD | Thermocouple | |:--------------|:--------------------------|:---------------------------| | Temperature Range | -328°F to 1562°F | -310°F to 3308°F | | Accuracy | ±0.001°F to 0.1°F | ±1°F to 10°F | | Response Time | Moderate | Fast | | Stability | Stable over long periods | Not as stable | | | <0.1% error / 5 yr. | 1°F error / 1yr. | | Linearity | Best | Moderate | | Sensitivity | High | Low | | Vibration applications | Poor | Good | # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermistor** - A thermistor is a temperature-sensing element composed of semiconductor material that changes its resistance with temperature. (Semiconductor used as a temperature sensor). - Mixture of metal oxides pressed into a bead, wafer or other shape. - Beads can be very small, less than 1mm in some cases. - The typical temperature range for a thermistor is -100°C to 400°C, although some specialized thermistors can measure temperatures as low as -200°C or as high as 1000°C. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermistor** - Thermistors usually have two types negative temperature coefficients (NTC) which means the resistance of the thermistor decreases as the temperature increases, and Positive temperature coefficient (PTC) in which the temperature increases the resistance also increases exponentially, and when temperature decreases resistance decreases. - The resistance temperature relation for the thermistor may be given as: $R = R_0 exp [\beta (\frac{1}{T} - \frac{1}{T_0})$ - *R*: Resistor resistance at temperature T in Kelvin - *R0*: Resistor resistance at temperature T0 in Kelvin - *β*: Material constant (Kelvin) # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermistor Applications** - Most are seen in medical equipment markets. - Thermistors are also used are for engine coolant, oil, and air temperature measurement in the transportation industry. - Thermistors are available in different sizes and shapes as per their value and use. - Bead types, - Disk type, - Rod type. - Probe type and Washer type. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermistor** - **Advantages:** - Temperature measurements become more stable with use - More accurate than RTD and thermocouples - Copper or nickel extension wires can be used - High sensitivity to small temperature changes - Quick response - **Disadvantages:** - Self heating - Current source required - Limited temperature range - Output is a non-linear function - Fragile - Some initial accuracy “drift” - Lack of standards for replacement - Decalibration if used beyond the sensor's temperature ratings # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Example** - **Problem**: A thermistor is placed in a 100°C environment, and its resistance measured as 20 ΚΩ. The material constant, *β*, for this thermistor is 3650°C. If the thermistor is then used to measure a particular temperature, and its resistance is measured as 500 Ω, determine the thermistor temperature. - **Given**: - Thermistor - R0 = 20000 Ω - R = 500 Ω - T0 = 100°C = 373 K - *β* = 3650°C = 3923 K - T = ?? - $R = R_0 exp [\beta (\frac{1}{T} - \frac{1}{T_0})$ - .. 500 = 20000 e3923(1/373) - .. T≈ 574.5 K = 301.5°C # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Example** - Thermocouple - RTD - Thermistor # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Optical Pyrometer** ## **Part Two: Temperature Measurement by Non-Contact Methods** ### **Radiation** - In addition to the methods in the preceding section, it is possible to determine the temperature of a body though a measurement of the thermal radiation emitted by the body. The advantage of this type is that they are not in contact with the hot body. Two methods are commonly employed for measurements: - Optical Pyrometer - Total Radiation Pyrometer # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Optical Pyrometer** - This method refers to the identification of the temperature of a surface with the color of the radiation emitted. As a surface is heated, it becomes dark red, orange and finally white on color. Optical pyrometers fig., are based on comparison of the energy emitted by a hot body at agiven wavelength with that of a black body calibrated lamp. The optical pyrometer are used mainly in the range 600 - 3000 °C # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Optical Pyrometer** - To use the instrument the point where temperature is required to be known is viewed through the instrument. - The current through the lamp filament is adjusted so the filament disappears. - The temperature of filament is known from its electrical resistance. - Since their operation requires the eye and judgment of an operator, they are not suitable for recording or control. - Temperature accuracy is +5 °C (800 °C -1300 °C) and ± 10 °C (1300 °C -2000 °C). # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Total Radiation Pyrometer** - **Total Radiation Pyrometer** - Total Radiation Pyrometer, or Pyrometers. Make use of the fact that all emit thermal radiation, as seen when looking at the bars of an electric fire or a light bulb. The amount of radiation emitted can be measured and related to temperature. Temperatures can be measured remotely using this technique, with the sensor situated some distance away from the object. Hence it is useful for objects that are very hot, moving or in hazardous environments. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Total Radiation Pyrometer** - “Temperature of a body can be measured by measuring radiant energy emitted by that hot body” - Qrad = σT4 - σ = Stefan Boltzmann's constantin (5.67*108 W/m².K4) - T = Absolute Temperature in Kelvin - Radiation pyrometers are used to measure the temperature of very hot objects without beinging contact with them. - Molten glass and molten metals during casting and shaping operations are typical of the objects they measure. - In some instruments, a telescopic eye magnifies radiant energy to measure smaller objects at longer distances. - On some instruments, hot objects up to 1/16 inch in diameter can be measured. The construction of the instrument components, such as the lens and the curved mirrors, control the route of the view. # **Appendix** # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Thermowell** - A thermowell is a cylindrical fitting used to protect temperature sensors installed when measuring liquid, the thermowell protects the thermometer when measuring the temperature as well as protects the liquid from contamination. The thermowell acts as a barrier between the device measuring temperature and the liquid as well as the liquid from the outside air. - Thermowells are recommended whenever a temperature element is to be inserted into a process where corrosion, pressure, abrasion, or shear forces may threaten the life of the element. In addition, thermowells allow for a defective instrument to be removed without shutting down or draining the process. - They fit into three broad categories: screwed, flanged, or weld-in, and can be designed to accept thermocouples, RTDs, temperature gauges or filled systems. # Ch. (4): Measuring Instruments: Temperature Measuring Instruments ## **Seebeck Effect** - The Seebeck effect, named for Thomas Johann Seebeck (1770-1831), refers to the generation of a voltage potential, or emf, in an open thermocouple circuit due to a difference in temperature between junctions in the circuit. - The Seebeck effect refers to the case when there is no current flow in the circuit, as for an open circuit. There is affixed, reproducible relationship between the emf and the junction temperatures *T1* and *T2*. - This relationship is expressed by the Seebeck coefficient, *αAB*, define das: $α_{AB} = [\frac{∂(emf)}{∂T}]_{open circuit}$ - Where A and B refer to the two materials that comprise the thermocouple. Since the Seebeck coefficient specifies the rate of change of voltage with temperature for the materials A and B, it is equal to the static sensitivity of the open-circuit thermocouple. # **Thank You** # **QUESTIONS?**

MEC201 Theory of Measurements and Sensors - Chapter 4 Temperature Measuring Instruments PDF

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