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(1) A cold junction of a thermocouple is at room temperature of 15°C. A voltmeter connected to the thermocouple circuit indicates 10 mV. If the voltmeter is calibrated as 20°C/mV, find the temperature of the hot source by using the law of thermocouple. (ii) A bimetallic strip is constructed of strips of Yellow brass and Invar bonded together at 30°C. Each has a thickness of 0.3 mm. Calculate the radius of curvature when a 6 cm strip is subjected to a temperature of 100°C. One end of the bimetallic strip is fixed. The thermal-coefficient of expansion of yellow brass and Invar are respectively 20.2 x 10$^{-6}$/°C and 1.7 x 10$^{-6}$/°C and their modulus of elasticity are respectively 96.5 GN/m² and 147 GN/m².
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- Given: - Cold junction temperature (\(T_c\)) = 15°C - Voltmeter reading = 10 mV - Calibration = 20°C/mV - Calculate the temperature difference (\(\Delta T\)) indicated by the voltmeter: \[ \Delta T = \text{Voltmeter reading} \times \text{Calibration} Show more…
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'A thermocouple is temperature-measurement device that consists of two dissimilar metal wires joined at one end: An oversimplified diagram follows_ Metal POTENTIOMETER Metal 2 voltage generated at the metal junction is read on potentiometer millivoltmeter. When certain metals are used. the voltage varies linearly with the temperature at the junction of the two metals: V(mV) = aTCC) +b An iron-constantan thermocouple (constantan is an alloy of copper and nickel) is calibrated by inserting its junction in boiling water and measuring voltage V = 5.27 mV, and then inserting the junction in silver chloride at its melting point and measuring V = 24.88 mV Derive the lineur = equation for V(mV) in terms fT(C) Then convert it to an equation for T in terms of V. (b) If the thermocouple is mounted in chemical reactor and the voltage is observed to g0 from 10.0 mV to 13.6 mV in 20 $ what is the average value of the rate of change of temperature. dTldt during the measurement period? c) State the principal benefits and disadvantages of thermocouples_'
Sri K.
A thermocouple is a temperature-measurement device that consists of two dissimilar metal wires joined at one end. An oversimplified diagram follows. A voltage generated at the metal junction is read on a potentiometer or millivoltmeter. When certain metals are used, the voltage varies linearly with the temperature at the junction of the two metals: $$V(\mathrm{mV})=a T\left(^{\circ} \mathrm{C}\right)+b$$ An iron-constantan thermocouple (constantan is an alloy of copper and nickel) is calibrated by inserting its junction in boiling water and measuring a voltage $V=5.27 \mathrm{mV}$, and then inserting the junction in silver chloride at its melting point and measuring $V=24.88 \mathrm{mV}$. (a) Derivethelinear equation for $V(\mathrm{mV})$ in terms of $T\left(^{\circ} \mathrm{C}\right)$. Then convert it to an equation for $T$ in terms of $V$. (b) If the thermocouple is mounted in a chemical reactor and the voltage is observed to go from 10.0 mV to $13.6 \mathrm{mV}$ in $20 \mathrm{s}$, what is the average value of the rate of change of temperature, the the the terme the are a renter $d T / d t,$ during the measurement period? (c) State the principal benefits and disadvantages of thermocouples.
Extension of thermocouple wires. A K-type thermocouple measures temperature T, and has reference T, as shown in Figure 3.33. Calculate the reading of the voltmeter under the following conditions: a. T = 100°C, T, = 0°C, and the extension section is absent. The y-y' junctions are in a temperature zone. (c = chromel, a = alumel). The extension section is not present (i.e., x and w are the same point and x' and y' are the same point). b. T, = 100°C, T. = 0°C, and the extension section is present with the alumel wire on top and the chromel wire on the bottom. The junctions x-x', y-y', and z-z' are each in their own temperature zones, all three at 25°C. The junction w is at 20°C. Calculate the error in the reading of temperature T. c. In an attempt to reduce the error, the extension is flipped so that now the chromel wire is on top and the alumel wire is on the bottom. Does that resolve the issue?
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