Eme

EME 2066 Measurement and Instrumentation, Trim.3, 2014-2015

Tutorial 5 * Q1-5 to discuss in the class. Q6-10, take home and submit one week later. 1) A resistance-temperature detector (RTD) has (20C) = 0.004/C satisfying the equation: R(T) = R(To)[1 + (T – To)] i. If the RTD has a resistance of 106 at 20C, find its resistance at 25C. [108.12] ii. Assume the RTD has a dissipation constant of 25mW/C and is used in a circuit that puts 8mA through the sensor. If the RTD is placed in a bath at 100C, a. what resistance will the RTD have? [139.92] b. what then is the indicated temperature? [100.36C] 2) For a certain thermistor  = 3420K and the resistance at 200F is known to be 1010. The thermistor is used for a temperature measurement and the resistance is measured as 2315. Calculate the temperature. [146.14oF] 3) A radiant energy measurement is made to determine the temperature of a hot block of metal. The emitted energy from the surface of the metal is measured as 28 kW/m 2 and the surface emissivity is estimated as  = 0.9. Calculate the surface temperature of the metal. What is the emissive power of a blackbody at the same temperature? Find the error in the temperature if an apparent blackbody temperature is taken as the measured value. [860.7K, 31.11 kW/m2, 2.6%] 4) A thermometer having an emissivity of 0.9 is placed in a large room and indicates a temperature of 25C. The walls of the room are at 35C and the convection heat transfer coefficient is 5.0 W/m2.C. Calculate the true temperature of the air. [13.63C] 5)

A Known Unknown

Insulation

Thermocouples

Heat Source

An apparatus shown in Fig. 1 is to be used to measure the thermal conductivity of a sample B having an approximate value of k = 15 W/m.C. The standard metal A used for comparison has k = 69 W/m.C. If sample and standard both have nominal lengths of 10cm, calculate the heat flux for an overall temperature difference of 25C. If another sample is connected in between, what is the thermal conductivity k of the sample if the heat flux reduces by 20%.

B

[3080W/m2, 49.26 W/m.C] Heat Sink

Fig. 1

6) A copper sphere 2.5cm in diameter is used as a slug type heat flux meter. A thermocouple is attached to the indicator and the transient temperature recorded when the sphere is suddenly exposed to a convection environment with h = 570 W/m2.C. Assume the sphere is initially at 95C and the environment temperature remains constant at 35C. What is the time constant

EME 2066 Measurement and Instrumentation, Trim.3, 2014-2015

of this system? Find the temperature after 15s. (copper = 8890 kg/m3, specific heat = 398 J/kg.C) [25.86s, 68.59oC] 7) The thermal conductivity of an insulating material is to be determined using a guarded hot plate apparatus. A 30  30 cm sample is used and a differential-thermocouple arrangement is used to measure the temperature drop across the sample. A heat input of 5kW is supplied, with a resulting temperature drop of 55C. If the sample thickness is 2.0mm, what is the thermal conductivity? Assume no heat losses. [2.02 W/m.C] 8) A capillary tube apparatus is to be used for measuring viscosities of oils with =0.001 kg/m.s and =890 kg/m3. The design Reynolds number is 500. Calculate the length of a 1 mm diameter tube that produces a pressure drop of 40kPa. [2.22m] 9) A Saybolt viscometer is used to measure the viscosity of certain oil. The time for drainage of the standard 60 ml sample is 1001 s. Calculate the kinematic and dynamic viscosities of oil if the density is 880 kg/m3. Find the % uncertainties of the viscosities. [ 2.022 105 m2 / s , 1.18%, 0.018 kg/m.s, 1.16% ] 10) A slug type heat-flux sensor has U = 0.1 W/m2.C. The slug is a copper cube 5 mm on a side embedded in a wall. What heat flux would be necessary to produce a rise in temperature of the sensor of 100C in a time of 30s? Given copper = 8890 kg/m3 and ccopper = 398 J/kg.C. [58965.3 W/m2] Review questions? 1.

What is temperature? Conversion between different temperature scales.

2.

What are the heat transfer modes?

3.

What are the possible ways of measuring temperature?

4.

What is temperature measurement by mechanical effect? Name a few.

5.

What is temperature measurement by electrical effect? Name a few.

6.

What is temperature measurement by radiation effect? Name a few.

7.

What are the differences between thermocouples, thermistors and RTDs?

8.

What is thermal radiation? What is a black body? Calculation of emissive power, total thermal radiation of a blackbody and emissivity.

9.

States the equations for conduction, convection and radiation heat transfers.

10. What is thermal conductivity and how does one measure thermal conductivity in metals and fluids? 11. What is dynamic viscosity? What is kinematic viscosity? How is it measured? 12. What is gas diffusion? 13. What is calorimetry and calorimeter? 14. What are free- and forced- convection? Calculation of free convection and forced convection. 15. What is heat flux and how do we measure it? 16. What is pH measurement?