Universal Calibration Device for Pressure Transducer Calibration

Transcription

1 Universal Calibration Device for Pressure Transducer Calibration Trudeep N. Dave 1, Dasaka S. Murty 2 and Hemant D. Rane 3 1 Research Scholar, Department of Civil Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, India PIN PH (+91) , trudeep@iitb.ac.in 2 Assistant Professor, Department of Civil Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, India PIN PH (+91) ; dasaka@iitb.ac.in 3 Senior Technical Superintendent, Department of Civil Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, India PIN PH (+91) ; hdrane@iitb.ac.in ABSTRACT Measurement of pressure using earth pressure transducer or pore pressure transducer is crucial task in experimental geomechanics. Calibration of transducers is very important and would give an idea about actual stresses at the point of measurement. In absence of such relationship, interpretation of data obtained using transducers is questionable. Present study describes details of indigenously developed universal pressure transducer calibration device developed by modifying conventional triaxial apparatus. Calibration of earth pressure transducers with 6.5mm and 40mm and pore pressure transducer with 5mm diameter have been performed using the device. Control over applied pressure is achieved by applying cell pressure through pneumatic constant pressure system of triaxial set up with an accuracy of 0.1 kpa. The data is acquired using data acquisition system (cdaq) which showed linearity with the pressure applied for loading, unloading and repeated loading cycles. Calibration constants are obtained for earth pressure transducers and pore-pressure transducer. INTRODUCTION Measurement of pressure inside particulate media or at the interface of structure and soil is done with the help of pressure transducers. Proper information about pressure at different points gives overall idea about stresses at point of interest. Unavailability of a proper correlation between applied pressure and pressure sensed by transducers, stresses at desired location becomes unattainable. The calibration of pressure transducer involves the investigation of the unique relationship between the applied pressure and pressure cell output (Take, 1998). The output from pressure transducers is related to normal stress by applying calibration factor that converts cell s electrical output from voltage to stress (kpa). It is usually determined from calibration tests by applying a known fluid pressure to the earth pressure cell (EPC) and recording the output (Selig, 1980). One of the challenges associated with the 1

2 calibration process is to ensure that the applied pressure is both consistently applied, and precisely known. LITERATURE REVIEW Pang (1986) calibrated boundary stress transducers both by using deadweights and by using the centrifuge and the results represented about ± 2% error at full scale (FS). Clayton & Bica (1993) designed experimental setup to calibrate EPC by modifying triaxial pedestal for performing calibration with water. Preliminary calibration tests were carried out in modified Rowe-type hydraulic consolidation cell. Further calibration was performed in an automated stress path triaxial cell so as to measure small and local strain. Take (1998) performed fluid calibration and centrifuge calibration of pressure transducer and adopted fluid calibration as base line for all other calibration outputs. Bao et al. (2003) found that fluid column pressure calibration is precise, repeatable and proportional to actual pressure applied to pressure transducer. Labuz & Theroux (2004) performed EPC calibration using allaround hydrostatic loading, uniaxial loading on the active face, and radial loading around the perimeter of the EPC. Uniaxial calibration under uniform fluid pressure resembled performance of EPC supplied by cell s manufacturer. OBJECTIVES OF THE PRESENT STUDY Calibration with centrifuge suffers from fluid meniscus formation and hence non-uniform pressure application. Also, setups devised by previous authors were suitable for a pressure transducer with dimensions for which it has been designed. Obtaining pressure calibration values for transducers with different dimensions is demanding and difficult in traditional calibration methods. The aim of present study is to develop an in-house device which can be used for calibration of new as well as used pressure transducers and to obtain repeatability under several loading-unloading cycles. DEVELOPMENT OF UNIVERSAL CALIBRATION DEVICE In the present study, universal pressure transducer calibration device is developed by modifying triaxial apparatus suitable for 100 mm diameter triaxial specimen. The modification consists of 1) a dummy aluminum spacer ring with brass couplings 2) a brass pedestal with set of replaceable dummy rings. Figs.1 & 2 represent descriptive and pictorial view of modification done for universal calibration device. Dummy spacer ring of 300 mm (OD) and 185 mm (ID) and with 4- replaceable brass couplings is placed above the triaxial base to allow transducer cables of different diameters into the triaxial cell. At the same time water leak proof system is ensured by providing rubber O ring. Brass pedestal with replaceable dummy ring and inclined hole to pass cable of transducer is placed at the centre of the cell to place transducer accurately. Dummy rings of 40 mm and 6.5 mm diameter is used in the present study. The combination of dummy ring with pedestal is planned to allow transducers of different diameters to sit exactly on the pedestal. Chances of 2

3 water entering inside the cavity of pressure transducer are taken care by applying silicon rubber gel around the cables. All dimensions are in mm Fig. 1 Descriptive view of Universal Calibration Device Fig. 2 Pictorial view of Universal Calibration Device Universal calibration device is utilized for calibration of earth pressure transducer of 6.5 mm and 40 mm diameter and pore pressure transducer of 5 mm diameter. Detailed specifications of transducers calibrated are given in table 1. Table 1 Detailed specifications of transducers used for calibration Transducer Terminology Haris Earth Pressure Transducer TML PDA PA Druck PDCR 81 Pressure range 0-2 kg/cm kg/cm kg/cm 2 Sensitivity mv/v at FS +946 µv/v mv/v/bar Non-linearity & hysteresis Thermal sensitivity shift 0.5 % of FS 0.5 % RO ±0.2%/BSL 0.1 % of FS/ C 1%/ C ±0.2%/ C Dimensions 40mm Ø 10mm 6.5mm Ø 1mm 5mm Ø 10mm 3

4 CALIBRATION USING UNIVERSAL CALIBRATION DEVICE The triaxial cell was filled with de-aired water, and pressure was applied by use of an air-water bladder cylinder with an accuracy of 0.1 kpa. Data acquisition system NI cdaq 9172 had been used as source of input voltage to the pressure transducer and output from pressure transducer were monitored and logged using NI Signal Express. The schematic presentation of calibration procedure is shown in Fig. 3. S Source of pressure E Pressure standard T Transducer DL Data logger C Computer PS Power supply Fig. 3 Schematic presentation of calibration procedure The fluid pressures applied to transducer were increased in 20 kpa increments to maximum values of 180 kpa. From the maximum value, the transducer was unloaded by decreasing the fluid pressure in 20 kpa steps back to zero gage pressure. The transducer data acquired using cdaq were recorded interms of the output strain from the cell at each step of loading as well as unloading. Readings were obtained once the fluid pressure had stabilized, typically in 30 s. A total of 5 loading/unloading pressure cycles were performed for calibration test on each transducer. The relationship between applied pressure and acquired data (measured strain) for all three transducer are shown in Figs.4, 5 and 6. Calibration factors and measurement of non-linearity are obtained for all three type of transducer considered in present studies. Fig. 4 Performance under single and five loading-unloading cycles for Haris EPC 4

5 Fig. 6 Performance under single and five loading-unloading cycles for TML Transducer Fig. 5 Performance under single and five loading-unloading cycles for Druck PDCR-81 Pore Pressure Transducer RESULTS AND CONCLUSIONS Uniaxial calibration of three different type of transducers viz. Haris earth pressure cell (EPC), Druck (PDCR-81) pore pressure transducer (PPT), and TML pressure transducer using universal calibration device have been studied. Results of calibration performance of each of transducers are represented in Figs 4, 5 and 6 respectively. Results shows behavior for 1 st loading-unloading cycle as well as under five loading-unloading cycles respectively. Maximum non-linearity of 0.6% of FS, 0.31% of FS and 2.64% of FS has been observed during first loading-unloading cycle of Haris earth pressure cell, Druck PDCR-81 and TML transducers, respectively. Maximum non-linearity of 0.79% of FS, 0.4% of FS and 2.67% of FS has been observed under five loading-unloading cycles of Haris earth pressure cell, Druck PDCR-81 and TML transducers, respectively. Calibration relationship between applied pressure and measured strain are found to be 0.486, and for the above transducers, respectively. Calibration results of three different pressure transducers reveal that universal calibration device developed and presented works accurately. The developed device is a versatile set up for in-house calibration of pressure transducers. The device can be used for calibration of most of the transducers used in the experimental studies by just changing the combination of dummy ring, brass coupling and at the most brass 5

6 pedestal used in the present studies. Accuracy of results obtained depends on least count of source of pressure regulation and mode of recording the output data. ACKNOWLEDGEMENT The work reported in this paper is substantially supported by the Department of Science and Technology (DST), India (Project No. 09DST004). REFERENCES Bao, M., Sun, Y., Yang, H. and Wang, J. (2003). A fast and accurate calibration method for high sensitivity pressure transducers, Sensors and Actuators A: Physical, 108(1-3): Clayton, C.R.I. and Bica, A.V.D (1993). The design of diaphragm-type boundary total stress cells, Geotechnique, 43(4): Labuz, J.F and Theroux, B. (2004). Laboratory calibration of earth pressure cells, Geotechnical Testing Journal, 28(2): 1-9. Pang, P.L.R. (1986). A new boundary stress transducer for small soil models in the centrifuge, Geotechnical Testing Journal, 9(2): Selig, E.T. (1980). Soil stress gage calibration, Geotechnical Testing Journal, 3(4): Take, W.A. (1998). Lateral earth pressure behind rigid fascia retaining wall, M.S. Thesis, Dept. of Civil Engineering, The University of New Brunswick. 6