More info about this article: http://www.ndt.net/?id=21199 Multi Level Temperature Measurement Using a single 90 bend waveguide Nishanth R 1a, Lingadurai K 1, Suresh Periyannan a and Krishnan Balasubramaniam a 1 Department of Mechanical Engineering, UCE-Dindigul Anna university, Chennai 600 036, INDIA a Centre for Non Destructive Evaluation, Indian Institute of Technology, Chennai 600 036, INDIA Email :nisanth.be@gmail.com ABSTRACT This paper reports the use of a single waveguide based sensor with 90 bend for measurement of temperatures at different locations. The variation in mechanical properties of the waveguide sensor as a function of change in temperature is examined. The time of flight (TOF) of the reflected signals were monitored in this case for temperature measurement.in this work, a 1 mm diameter solid cylindrical Stainless Steel (SS) waveguide with reflector embodiments (Bend, Notches) at predefined locations is used. Fundamental Longitudinal L(0,1) mode with low frequency (500 khz) is chosen owing to its ease of detection (high velocity) and sensitivity to transverse notches. The propagation of the guided wave mode in the waveguide and their interaction with Bend and notches are studied The interaction of the guided wave with the geometric discontinuities results in mode conversion, with energy being partitioned among the reflected and transmitted modes. The results are experimentally validated using a straight and bend waveguide. The bend waveguide is then used for multilevel temperature measurement of air inside a furnace from 30 C to 250 C. Keywords: Ultrasonic, Waveguide, Temperature Measurement. 1. Introduction Currently Thermocouples that are used in steel melting plants, atomic power plants etc have accuracy problems. This is due to sensor drift, during the long time operation and more prominently the thermocouple joint failures are of serious concern. The Frequent replacement of thermo-couples in hostile environment is not desirable and new innovative measurement technologies are essential. The use of ultrasonic waveguide has been reported extensively in the literature for much critical measurement that includes temperature, viscosity, density, and level of fluids in the process control applications. [1-5] 348 Non-Destructive Evaluation 2016
Most of the previous approaches described measurements in a single zone of interest. In order to measure at multiple points of interest using a single waveguide Suresh et al. [6-10] Developed long straight waveguide based sensors for Simultaneous moduli measurement of elastic materials at elevated temperatures and bend waveguide for distributed temperature measurement of surrounding region inside a furnace and further Developed reconfigurable Helical waveguide for multi-level temperature measurement by expanding or compressing the waveguide to achieve desired spacing in a given area/volume. In this paper, we investigate more on the physics behind the interaction of axially-symmetric guided wave mode L(0,1) with notches and bend. Further we explore the feasibility of using multiple periodic embodiments (bend, notches ) at known intervals which acts as reflectors that are positioned along the length of the waveguide and their ability in making multiple temperature measurements using a single waveguide The obtained L (0,1) results are compared and validated with conventional temperature measurement instruments (thermocouple/pyrometre) 2. Working Principle of Waveguide Sensors and apparatus 2.1. Ultrasonic Waves in Rod Waveguide The guided waves [11] can be thought of as a superposition of partial plane waves that are reflected within waveguide boundaries. The propagation of ultrasonic waves in waveguides is characterized by the variables: frequency, phase velocity, and attenuation. In a cylindrical waveguide, there are three families of modes; longitudinal (L), torsional (T) and flexural (F) that are propagating in the axial direction (z) of cylindrical coordinate system (r, ș and z). While, many wave modes can be excited in cylindrical rods, we concentrate on the fundamental longitudinal mode, L (0, 1). This mode has smaller levels of dispersion over a wide range of frequencies as shown by Pavlakovic et al. (1997) and can be easily generated in the wires (Periyannan and Balasubramaniam. 2015) made of high temperature materials. 2.2. Working Principle of Waveguide Sensors The working principle waveguide sensor depends on the fact that a velocity (Sound speed) change in a material is related to its temperature. Velocity changes are due to the variations of material properties (, E, G &ȡ) at different temperatures. Here, the pulse echo L(0, 1) mode was transmitted and received by longitudinal transducer (Panametrics-0.5MHz) on the waveguide 349 Non-Destructive Evaluation 2016
sensors for the temperature measurements. After that, the waveguide sensors were calibrated using the change in time of flight ( tof) that is directly proportional to the change in coefficient of thermal expansion of material at different temperatures. 3. Results and Discussion The Bend waveguide can be used to measure temperature at different regions if used in pulse echo mode. This technique was tested at furnaces for high temperature measurements. A stainless steel waveguide of length 850 mm and diameter 1 mm with a 90 bend and bend raddi of 2mm at 7450mm was used, a pair of notches after the bend with a gauge length of 30mm acts as a sensor and monitoring region of our interest. Figures 1 describe experimental setup used for temperature measurements of surrounding medium inside a furnace. A hole at the top of the furnace allowed the waveguide to reach outside the furnace creating a clearance distance of at least 300 mm between the oven and the fixture location. The furnace was heated from 25 C to 250 C. During heating, the temperature was acquired from pre calibrated K-type thermocouple co-located with the monitoring region of our interest for simultaneous measurement of temperature. The thermocouple data was logged every minute using NI for temperature read out. Ultrasonic signals were acquired and logged every minute during heating using Technofour Pulser/Receiver. The A-scan of the reflected signal obtained using this pulse-echo approach is shown in Figure 2. From the A-scan results the reflected L(0,1) wave mode signals from the bend region, notches and from the end of the waveguide is clearly observed. Figure 2 displays a typical A-Scan signals from region of our interest that were acquired during a typical heating cycle. A peak-tracking algorithm reported by Periyannan and Balasubramaniam [6-10] was implemented for the dynamic signal peak tracking during the heating of the waveguide. This algorithm ensured that the TOF measurements were extremely reliable and repeatable. Subsequently the δtof between each pair of notches (one sensor) was measured using Equation (1). Instantaneous time of flight difference (δtof ci ) of a waveguide is defined as follows. 350 Non-Destructive Evaluation 2016
(δtof ci ) = [TOF bi TOF ai ] [TOF b TOF a ] (1) where TOF ai, TOF bi are Instantaneous TOF at variable temperature and TOF a, TOF b are TOF at room temperature. The experiment was repeated several times and recorded. A graph that displays the measured Temperature vs δtof is shown in figure 3. It can be observed that the comparisons between Figure 1: Experimental setup for calibration Figure 2: A-Scan the ultrasonic waveguide measurements and the thermocouple measurements are comparable with a maximum error of 10 C. It is also observed from Figure 12 that the temperature gradient profile can be obtained using a single bend ultrasonic waveguides once the calibration curve has been obtained for the specific material and thickness of the waveguide. The data clearly shows excellent correlation with repeatability and the data from the waveguides fit a single calibration curve as shown in figure 12. The plots were curve fitted with 2nd order polynomial equation which is represented below in Eq. (2): 351 Non-Destructive Evaluation 2016
Monitoring point 2 nd order polymonial equation R² ax 2 + bx c Calibration -5E-06x 2 + 0.005x - 0.026 0.981 (2) Hence, these calibration curve constants (a = -5e-06, b = 0.005, c =0.026) were employed for measuring the temperature profile in a laboratory experiment(case Study-A and Case Study-B). Figure 3: A-Scan-TOF calculation Fig.4: Calibration Curve 352 Non-Destructive Evaluation 2016
4. CONCLUSIONS The ultrasonic distributed temperature sensor as described here provides a more robust and cost effective solution for measurement of temperature gradients, in applications involving elevated temperature processes, when compared to junction based thermocouples. This novel technique reported here uses a multiple gratings (sensors) in a single waveguide, functioning as ultrasonic waveguide sensor, and employs the guided L (0, 1) mode that can be reliably generated and received by using a conventional longitudinal transducer. The technique relates the δtof parameter to multi sensors of bend waveguide temperatures. This technique was demonstrated to measure the temperatures inside a furnace in the laboratory. Further the ultrasonic sensors output has been verified with thermocouple data. Hence, its is possible to measure temperature at multiple regions using a single set of ultrasonic electronics and transducer 5. References : 1. L. C. Lynnworth, Ultrasonic Measurements for Process Control: Theory,Techniques, Applications b(academic Press, New York, 1989). 2. K. Balasubramaniam, V.V. Shah, G. Boudreaux, R.D. Costley, C. Menezes, J.P. Singh, Temperature and viscosity in-situ sensor for hostile processes, Rev. Prog. Quant. Nondestr. Eval. 18B (1999)1163 1170. 3. K. Balasubramaniam, V.V. Shah, D. Costley, G. Bourdeaux, J.P. Singh,High temperature ultrasonic sensor for the simultaneous measurement of viscosity and temperature of melts, Rev. Sci. Instrum. 70 (12) (1999) 1 6. 4. K. Balasubramaniam, V.V. Shah, D. Costley, G. Bourdeaux, J.P. Singh,Viscosity and temperature measurements at very high temperature by ultrasound reflection, US Patent, 2001, p. 6296385. 5. Cawley P, Cegla FB (2010) Ultrasonic non-destructive testing. U.S patent no: 8381592 6. Periyannan, S., & Balasubramaniam, K. (2015). Multi-level temperature measurements using ultrasonic waveguides. Measurement, 61, 185-191. 7. Periyannan, S., & Balasubramaniam, K. (2015). Simultaneous moduli measurement of elastic materials at elevated temperatures using an ultrasonic waveguide method. Review of Scientific Instruments, 86(11), 114903. 8. Periyannan, S., & Balasubramaniam, K. (2016). Moduli Determination at Different Temperatures by an Ultrasonic Waveguide Method. Experimental Mechanics, 1-14. 353 Non-Destructive Evaluation 2016
9. Periyannan, S., Rajagopal, P., & Balasubramaniam, K. (2016). Torsional mode ultrasonic helical waveguide sensor for re-configurable temperature measurement. AIP Advances, 6(6), 065116. 10. Periyannan, S., Rajagopal, P., & Balasubramaniam, K. (2016). Re-configurable multilevel temperature sensing by ultrasonic įspring-like helical waveguide. Journal of Applied Physics, 119(14), 144502. 11. J. L. Rose, Ultrasonic Waves in Solid Media (Cambridge University Press,1999, pp. 143 152. 354 Non-Destructive Evaluation 2016