A Microprocessor-Based Novel Instrument for Temperature and Thermal Conductivity Measurements

Transcription

1 TECHNICAL ARTICLE A Microprocessor-Based Novel Instrument for Temperature and Thermal Conductivity Measurements M. Rehman 1, M. Abdul Mujeebu 2, T.B. Kheng 1, and B.A.J.A. Abu Izneid 1 1 School of Electrical and Electronics Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia 2 School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia Keywords Thermal Conductivity, PT 100 Sensor, PIC Microcontroller, OrCAD Layout Plus Correspondence M.A. Mujeebu, School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia mamujeeb5@yahoo.com Received: July 3, 2010; accepted: September 6, 2010 doi: /j x Abstract This article presents the details of design and development of a mobile and low-cost measuring device that can directly measure the thermal conductivity of a material. The device does not need an AC power source and needs only a battery to operate. In its operation, the output of a platinum resistance thermometer (PT100 sensor) is amplified by LM324 Quad differential input operational amplifier and fed to Microchip PIC16F877A microcontroller through an analog-to-digital converter. The microcontroller calculates the thermal conductivity of the material by a given set of temperature difference and finally the result is displayed on a liquid crystal display. PIC Basic programming language is used to program the PIC microcontroller. The analog circuit and printed circuit board layout are designed using OrCAD software. It has been observed that the proposed device can measure the temperature with an accuracy of 0.5 C and thermal conductivity with an accuracy of 0.01 W/mm C. The power consumption of the device is found to be mw. Introduction Thermal conductivity is a key thermal transport property of materials. Knowledge of thermal conductivity and its accurate measurement is crucial for a wide range of applications, including polymer injection molding, home insulation using various building materials, insulation for space shuttle, design of heat exchangers and fins, thermal management of electronic packages in semiconductor industry, and so on. Thermal conductivity measurement techniques are broadly classified under steady state methods and transient methods. The radial heat flow method and the guarded hotplate method are examples of steady state method. 1 3 Hot wire 4 and laser flash 5 methods are examples of the transient method. The comparison of various transient techniques is available in literature. 6 Other techniques include the 3ω (three omega) method, 7 9 the differential photoacoustic method, 10 the photothermal deflection method or the probe-beam deflection method, 11,12 the thermal-wave technique, 13 the transient plane source method, 14 the thermorflectance or the Nano-TheMS method, 15 and differential scanning calorimetry. 16 Apart from these, many techniques have been developed by previous researchers for specific applications. For instance, Banaszkiewicz et al. 17 developed a sensor to perform in situ thermal conductivity determination of cometary and asteroid materials. The sensor was made of platinum wire (resistance thermometer) and isotan wire (heating element) that could operate independently. A steady state, bi-substrate technique for measurement of the through-thickness thermal conductivity of ceramic coatings was developed by Tan et al. 18 A thin-film thermal conductivity meter was designed and built by Subramanian et al. 19 to measure the thermal conductivity of the thermal sensitive paints. A stationary absolute method for the thermal conductivity measurement of semiconductor materials was developed by Vahanyan. 20 While reviewing the previous works, it can be observed that most of the methods have been developed for specific applications and operating 62 Experimental Techniques 36 (2012) , Society for Experimental Mechanics

2 M. Rehman et al. Thermal Conductivity Measuring Device conditions. Many methods involve measurement of temperature by using a sensor followed by manual calculation of thermal conductivity which is time consuming. Some of them are very costly to manufacture. Hence the present work is aimed at developing a lowcost, reliable, portable, and user-friendly system for direct measurement of thermal conductivity of solids under wide range of operating conditions. The device is successfully fabricated and performance test shows satisfactory results. LM324 Quad differential input operational amplifier The LM324 series, the Op-amp used in this study are low-cost, quad operational amplifiers with true differential inputs. They have several distinct advantages over standard operational amplifier types in single supply applications. The quad amplifier can operate at supply voltages as low as 3.0 V or as high as 32 V. The output voltage of the sensor is amplified (signal conditioning) in the Op-amp. The schematic of the LM324 Op-amp is shown in Fig. 3. Basic Components of the System The proposed device has been developed based on the layout as shown in Fig. 1. The output of a platinum resistance thermometer (PT100 sensor) is amplified by LM324 Quad differential input operational amplifier and fed to Microchip PIC16F877A microcontroller through an analog-to-digital converter (ADC). The microcontroller calculates the thermal conductivity of the material by a given set of temperature difference and finally the result is displayed on a liquid crystal display (LCD). The description of each component is given as follows: Platinum resistance thermometer (PT100-RTD) Figure 2 shows the structure of the thin-film platinum resistance thermometer PT100. The platinum resistance temperature detector is used today as an interpolation standard from the oxygen point ( C) to the antimony point ( C). The main advantage of platinum is that it can withstand high temperatures while maintaining excellent stability. As a noble metal, it has limited susceptibility to contamination. MAX232 line driver The purpose of MAX232 in this system is to convert a +5 V source input voltage to +10 V and 10 V voltage. This +10 V and 10 V is used as a source voltage for the LM324 Op-amp, hence requiring only one voltage source for the whole device. MAX232 can provide +10 V and 10 V because it has voltage doubler and voltage inverter circuits inside the integrated circuit. The pin diagram for MAX232 is shown in Fig. 4. Microchip PIC16F877A microcontroller It is a CMOS FLASH-based 8-bit microcontroller. It features 200 ns instruction execution, 256 bytes of EEPROM data memory, self-programming, an in-circuit debugger (ICD), two comparators, eight channels of 10-bit ADC, two capture/compare/pulse width modulation functions, a synchronous serial port that can be configured as either three-wire serial peripheral interface or two-wire I 2 C bus, a universal synchronous/asynchronous receiver/transmitter, and Input Signal from thermal sensor Signal conditioner Circuit Interface with micro Controller Calculation In Micro Controller Display Using LCD Output signal Figure 1 General layout of the proposed system. Figure 2 Thin-film platinum resistance thermometer (PT100). Figure 3 LM324 pin diagram. Experimental Techniques 36 (2012) , Society for Experimental Mechanics 63

3 Thermal Conductivity Measuring Device M. Rehman et al. Figure 4 MAX 232 pin diagram. Liquid crystal display (16 characters 2 lines) Liquid crystal display is a very common display panel and it is quite versatile compared to a seven-segment display. Although it is more expensive than sevensegment displays, it is convenient to use in microcontroller application and suitable to be mounted on printed circuit board (PCB) due to less number of pins. Powertip 1602-D LCD panel as shown in Fig. 6 is used in the present system. This LCD display has 16 characters 2 lines display capability and also has an light emitting diode backlight, so suitable for use at night. 4 3 lines keypad A4 3 lines keypad as shown in Fig. 7 is used as input device for this thermal conductivity measurement Figure 5 PIC16F877A pin diagram. a parallel slave port. In addition, this microcontroller is a high-performance reduced instruction set computer CPU. Besides this, it has an internal oscillator of 4 MHz and can support up to 20 MHz crystal oscillator. The PIC16F877A is very popular and supports programming languages such as Microchip PIC assembly language, PIC Basic language, and PIC C language. The pin diagram of the PIC16F877A microcontroller is shown in Fig. 5. Figure 6 Powertip LCD display. 64 Experimental Techniques 36 (2012) , Society for Experimental Mechanics

4 M. Rehman et al. Thermal Conductivity Measuring Device simulation (PSpice), and PCB layout (OrCAD layout). This software is used to design analog circuit and PCB layout. OrCAD Capture Component Information System OrCAD Component Information System (CIS) is an electronic part management system that is available as an option for use with OrCAD Capture. OrCAD CIS helps manage part properties (including part information required at each step in the printed circuit board design process, from implementation through manufacturing) within schematic designs. Capture CIS provides access to local (preferred parts database) and remote part databases that contain all relevant information for the parts used in designs. This information may include company part numbers, part descriptions, PCB layout footprints, technical parameters (such as speeds, tolerances, and ratings), and purchasing information. The interface of the OrCAD Capture CIS is shown in Fig. 8. Figure 7 Typical keypad. device. The function of this keypad is to supply a high (+5 V) voltage to the row input and then get the output voltage from column output and vice versa. OrCAD Layout Plus OrCAD Layout Plus is one of the software that is included in the OrCAD software package and is used to do the PCB layout. OrCAD Layout Plus has the capability of Auto route for the layout of PCB. It is very user-friendly and easy to learn. The user just needs to export the schematic design from OrCAD PIC Basic pro programming language PIC Basic programming language is a compiler language that is used to compile the BASIC code to assembly code and even machine code (HEX code). This is the programming language that is used to program the PIC microcontroller for this study. This language is a high level language and it is more convenient to use compared to PIC assembly language. PIC has the syntax like C language, so it is easier to learn. In addition, the PIC Basic pro compiler gives direct access to all the PIC microcontroller registers I/O ports, A/D converters, hardware serial ports, and so on. It can automatically take care of the page boundaries and random access memory banks. Moreover, it has built-in commands to control intelligent LCD modules. So with PIC Basic pro, it is easy to control the LCD display. OrCAD software OrCAD is a software that was developed by Cadence Inc. OrCAD has the main capability of doing electronic circuit schematic design (Capture CIS), circuit Figure 8 Interface of the OrCAD Capture CIS. Experimental Techniques 36 (2012) , Society for Experimental Mechanics 65

5 Thermal Conductivity Measuring Device M. Rehman et al. Figure 9 Interface of OrCAD Layout Plus. Capture CIS to OrCAD Layout Plus, then put the component in the desired place, and click the Auto route function. The software will automatically route the circuitry. The interface of the OrCAD Layout Plus is shown in Fig. 9. Methodology Software development The tools used in software development are ICD2 programmer board, MPLAB IDE, and PIC Basic Pro compiler. Basically, the firmware of the proposed device is constructed from many subroutines namely the main routine, then welcome screen, keypad reading, temperature display, key-in, thermal conductivity calculation, and thermal conductivity display routines. The main routine combines all the subroutines and produces a fully functional firmware based on the design specification. Welcome screen routine is used to display the welcome message welcome to thermal conductivity measurement device. Keypad reading routine is used to detect which key is pressed by the user. A normal keypad is organized in a matrix of rows and columns. The microcontroller accesses both rows and columns through its I/O port. The present system needs only seven I/O ports to control the 4 rows 3 columns keypad, which is shown in Fig. 7. In the temperature display routine, the temperature read by the microcontroller (in volts) will be converted to digital by the ADC of the microcontroller. The digital value of the reading will be converted to the temperature that can be displayed in the LCD display. Besides, this routine also has a sequential technique to display the temperature nicely. For instance, if +050 C needs to be displayed, the routine will display +0 first and then display 50. For value less than 10, the routine will display +00 first and then display the value. The Key-in routine is used to store the dimensions of the tested material. The dimensions are area (A), length (L), and power (P). These three variables are used to calculate the thermal conductivity of the material. The storing technique uses multiplication and addition of the input key. For example, 1234 needs to be stored in a variable. So the value of the variable is equal to the first keyed value multiplied by 1000 plus second keyed value multiplied by 100 plus third keyed value multiplied by 10 plus forth keyed value. The thermal conductivity calculation routine 66 Experimental Techniques 36 (2012) , Society for Experimental Mechanics

6 M. Rehman et al. Thermal Conductivity Measuring Device Figure 10 Schematic of the axial heat flow method. calculates the thermal conductivity by Eq. 1 and is displayed in the display routine. Thermal conductivity measurement The procedure used to measure the thermal conductivity was the modification of the absolute axial heat flow method as shown schematically in Fig. 10. This is the simplest method to determine the thermal conductivity of an object, based on Eq. (1): Figure 11 The experimental setup for thermal conductivity measurement. 1, multimeter; 2, trainer board; 3, DC laboratory power supply; 4, thermal conductivity measurement device; and 5, thermal conductivity measurement section. device in measuring thermal conductivity. The overall setup is shown in Fig. 11. The zoomed view of the thermal conductivity measurement unit is given in Fig. 12. The aluminum block (specimen) has 38 mm diameter and 40 mm length as shown in Fig. 13. The experiment is carried out at 22.5 C ambient temperature and the initial temperature of the cold side of the Peltier cooler was 0 C. Thermal conductivity = K = Q/A (1) T/L where L and A are the length and area of cross section (perpendicular to heat flow), respectively, and Q is the rate of heat flow through the material which is assumed to be equal to the rate of heat energy pumped out (P) by the Peltier cooler, by neglecting the radial heat loss. Normally, one end of the specimen is heated by means of a heater to create a temperature difference ( T) within the test length (L) and this temperature difference is used to calculate K using Eq. 1. However, in the axial heat flow method, better accuracy is assured if the experiment is conducted at subambient temperatures. Accordingly, in the present system the heater is replaced by a Peltier cooler to cool one end (to temperature T 2 ), while the other end is kept at the ambient temperature (T 1 ). The rate of heat removal by the cooler is estimated as Q (equal to power P in Fig. 10). The experimental setup and procedure An experimental setup based on the above methodology has been built to study the accuracy of the Figure 12 Thermal conductivity measurement section. i, Peltier effect cooler; ii, aluminum (Aluminum 5052 Temper-O) block; and iii, PT100 sensor. Experimental Techniques 36 (2012) , Society for Experimental Mechanics 67

7 Thermal Conductivity Measuring Device M. Rehman et al. PCB through the IDE multicore cable which is easy to bend and is more wearable. The whole assembly has been accommodated in a compact, durable, and attractive casing. Performance Testing and Results Transient response test The device is tested for its transient response which is the response of temperature reading over tested time. This is done using the PT100 sensor to measure a known temperature and noting the time taken to give the temperature reading. The test was conducted for two known temperatures of water 64 and 57 Cat a room temperature of 31 C. The results are shown in Figs. 15 and 16. The results show that the response of the device is quite good and need only about 20 s to reach the actual temperature. Figure 13 Dimensions of the aluminum block. Reliability and stability test This test was done to evaluate whether the device is stable and reliable in terms of its temperature reading. The temperature reading is recorded through a period of time and the measured temperature is compared with actual temperature to find its error of reading. The actual temperature was found by measuring the Figure 14 Fabricated thermal conductivity measurement device. Temperature ( C) Temperature Vs Time Time (seconds) The Complete Assembly The complete assembly of the proposed measurement system is shown in Fig. 14. It consists of (1) platinum resistance thermometer probe; (2) reset switch; (2) variable resistor; (4) LCD display; (5) keypad; and (6) PCB. The thermometer probe is 60 cm in length and is fabricated using a multicore wire. The multicore wire can be got from the IDE cable of the personal computer. The reset switch is used to reset the system. The variable resistor is used to change the contrast of the LCD display. The reset switch and variable resistor are put outside the PCB because it is convenient for user when the PCB is fixed in a perspex box in future. The LCD display and keypad are connected to Figure 15 Transient response for 64 C. Temperature Vs Time 60 Temperature ( C) Time (Seconds) Figure 16 Transient response for 57 C. 68 Experimental Techniques 36 (2012) , Society for Experimental Mechanics

8 M. Rehman et al. Thermal Conductivity Measuring Device resistance of the PT100 and taking the equivalent temperature from the data table. The percentage of error was also calculated using the formula: Percentage of error = Measured Temperature Actual Temperature Actual Temperature 100% (2) Figure 17 shows the comparison between the measured and actual temperatures. It can be observed that the temperature reading of the device is fluctuated between 32 and 31 C. So the tolerance is ±1.899% ±2%, which is acceptable. Thermal conductivity The experiment is carried out at 22.5 C ambient temperature and the initial temperature of the cold side of the Peltier cooler was 0 C. The power P is measured as W assuming no radial heat loss. A and L of the specimen are m 2 and 0.4 m, respectively. Accordingly, the thermal conductivity is obtained based on Eq. 1. The result is shown in Table 1, which shows that the thermal conductivity, Temperature ( C) Temperature Vs Time Time (seconds) Temperature Measured Ideal Temperature Figure 17 Measured temperature and actual temperature comparison. Table 1 Thermal conductivity measurement data Time (min) T 1 ( C) T 2 ( C) K = T (W/m C) measured for 10 min, at 1-min interval remains steady at W/m C. This value is fairly close to the actual thermal conductivity of the specimen, that is, 144 W/m C, the error being 7% which is acceptable. Conclusion A low-cost, reliable, and user-friendly electronic device is being developed, which can measure both thermal conductivity and temperature directly with good accuracy. The overall cost of fabrication was only within 150 USD (United States dollars), which would be extremely cheaper than any other similar instruments commercially available. The performance of the device is observed satisfactory in terms of accuracy. There is enough scope to minimize the error of thermal conductivity measurement and to develop a durable and compact device based on the proposed methodology. We selected only one sample; however, the device may be tested further for different materials and geometries. Theoretically there should not be any limiting value or range for the accuracy of the device. The upper or lower limit of thermal conductivity that could be measured by the proposed instrument depends on the size, thickness, and material of the sample. A detailed comparative study may also be carried out to ensure the excellence of the proposed method compared to the existing techniques. The following modifications are recommended for improved performance of the system. 1. The accuracy of the temperature measurement may be improved using higher bits conversion ADC like 10 bits or 12 bits. 2. Make the device as small as possible using surface mount technology component. 3. Use constant current source to drive the platinum resistance thermometer so as to keep its output consistent which is possible using Op-amp. 4. Data acquisition system (using MATLAB or Visual Basic) may be added to the device so that is can be used to record data and plot graph. 5. For thermal conductivity measurement the comparative cut bar method may be used to produce more accurate result. References 1. Speyer, R.F., Thermal Analysis of Materials, Marcel Dekker, New York, Xamán, J., Lira, L., and Arce, J., Analysis of the temperature distribution in a guarded hot plate apparatus for measuring thermal conductivity, Applied Thermal Engineering, 29(4): (2009). Experimental Techniques 36 (2012) , Society for Experimental Mechanics 69

9 Thermal Conductivity Measuring Device M. Rehman et al. 3. Huang, B.L., Ni, Z., Millward, A., McGaughey, A.J.H., et al., Thermal Conductivity of a Metal-Organic Framework (MOF-5): Part II. Measurement, International Journal of Heat and Mass Transfer, 50: (2007). 4. Assael, M.J., Antoniadis, K.D., and Tzetzis, D., The Use of the Transient Hot-Wire Technique for Measurement of the Thermal Conductivity of an Epoxy-Resin Reinforced with Glass Fibers and/or Carbon Multi-Walled Nanotubes, Composites Science and Technology, 68(15 16): (2008). 5. Parker, W.J., Jenkins, R.J., Butler, C.P., and Abbot, G.L., FLASH Method of Determining Thermal Diffusivity, Heat Capacity and Thermal Conductivity, Journal of Applied Physics, 32: (1961). 6. Mathis, N., Transient Thermal Conductivity Measurements: Comparison of Destructive and Nondestructive Techniques, High Temperatures High Pressures, 32: (2000). 7. Cahill, D.G., Thermal Conductivity Measurement from 30 to 750 K: The 3ω Method, Review of Scientific Instruments, 61: (1990). 8. Cahill, D.G., Katiyar, M., and Abelson, J.R., Thermal Conductivity of a-si:h Thin Films, Physical Review B, 50: (1994). 9. Oh, D.-W., Jain, A., Eaton, J.K., Goodson, K.E., and Lee, J.S., Thermal conductivity Measurement and Sedimentation Detection of Aluminum Oxide Nanofluids by Using the 3-ω Method, International. Journal of Heat and Fluid Flow, 29: (2008). 10. Govorkov, S., Ruderman, W., Horn, M.W., Goodman, R.B. and Rothschild, M., A New Method for Measuring Thermal Conductivity of Thin Films, Review of Scientific Instruments, 68: (1997). 11. Jackson, W.B., Amer, N.M., Boccara, A.C., and Fournier, D., Photothermal Deflection Spectroscopy and Detection, Applied Optics, 20: (1981). 12. Jeona, P.S., Kima, J.H., Kimb, H.J., and Yoob, J., Thermal Conductivity Measurement of Anisotropic Material Using Photothermal Deflection Method, Thermochimica Acta, 477:32 37 (2008). 13. Calzona, V., Cimberle, M.R., Ferdeghini, C., Putti, M., and Siri, A.S., Fully Automated Apparatus for Thermal Diffusivity Measurements on HTSC in High Magnetic Field, Review of Scientific Instruments, 64: (1993). 14. Solórzano, E., Reglero, J.A., Rodríguez-Pérez, M.A., Lehmhus, D., Wichmann, M., and De Saja, J.A., An Experimental Study on the Thermal Conductivity of Aluminium Foams by Using the Transient Plane Source Method, International Journal of Heat and Mass Transfer, 51(25 26): (2008). 15. Kuwahara, M., Suzuki, O., Takada, S., Hata N., Fons, P., and Tominaga, J., Thermal conductivity Measurements of Low-k Films Using Thermoreflectance Phenomenon, Microelectronic Engineering, 85: (2008). 16. Hu, M., Yu, D., and Wei, J., Thermal Conductivity Determination of Small Polymer Samples by Differential Scanning Calorimetry, Polymer Testing, 26: (2007). 17. Banaszkiewicz, M., Seweryn, K., and Wawrzaszek, R., A Sensor to Perform In-Situ Thermal Conductivity Determination of Cometary and Asteroid Material, Advances in Space Research, 40: (2007). 18. Tan, J.C., Tsipas, S.A., Golosnoy, I.O., Curran, J.A., Paul, S., and Clyne, T.W., A Steady-State Bi-Substrate Technique for Measurement of the Thermal Conductivity of Ceramic Coatings, Surface & Coatings Technology, 201: (2006). 19. Subramanian, C.S., Amer, T., UpChurch, B.T., Alderfer, D.W., Burkett, C., and Sealey, B., New Device and Method for Measuring Thermal Conductivity of Thin-Films, ISA Transactions, 45(3): (2006). 20. Vahanyan, A.I., A Method for the Thermal Conductivity Measurement of Semiconductors, Measurement 39: (2006). 70 Experimental Techniques 36 (2012) , Society for Experimental Mechanics

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