Design and test of high accurately measuring equipment for NaCl water solution utilizing ultrasonic velocity with temperature correction

Abstract: In food, chemicals and pharmaceuticals industries, the concentration is a very important parameter for quality control. Continuous process monitoring is a fundamental requirement for the process control in these industries. Besides process parameters, such as temperature, pressure, liquid level, flow rate, concentration measurement is also of special interest. More qualified information obtained from new or better sensors can significantly enhance the process quality and thereby product properties. Low-intensity ultrasonic sensor system can contribute to this development, and it has been widely used in many fields for its characteristics are non-destructive, non-invasive and rapid. In this study, a method based on ultrasound for determining the concentration of NaCl water solution with high accuracy was presented. The ultrasound velocity in different temperatures and in solutions with different concentrations was measured using an emitter-receiver method. The experimental set-up was composed of two matched piezoelectric transducers with the center frequency of around 5 MHz (one used as emitter and the other as receiver), a thermostatic tank, a CTS-80787PR pulse emitter-receiver (used to excite and receive electric signals) and a data acquisition card. The electric pulse at the emitting voltage of -25 V, the width of 100 ns and the repeated frequency of 100 Hz was sent from the CTS-80787PR to the emitter transducer and then transformed to ultrasound wave. This wave traveled through a liquid sample cell with 5 mm wide to reach the receiving transducer, and was transformed back to electric pulse signal. The signal was acquired by the CTS-80787PR and sent to a computer equipped with the LABVIEW via a 16-bit data acquisition card of PCI (peripheral component interconnect) bus with the sampling rate of 40 MS/s. A Labview interface was developed to collect signals, analyze signals, build models and validate models. Through analysis, the measurement accuracy of ultrasound velocity was 1 m/s for this equipment, which ensured the accuracy of concentration measurement. At 2.5°C gradient in the temperature range of 10-30°C, and at a gradient of 0.5 g/100 g in the concentration range of 0-10 g/100 g, NaCl water solutions were prepared. Some were regarded as calibration samples and others as validation samples. In order to predict the concentrations of NaCl water solution, a calibration model was established using a quadratic polynomial approach based on the experimental results. The coefficients of this polynomial were obtained based on ordinary multiple linear regression (OMLR) method by inserting the measurement points, analyzing the residual to eliminate abnormal sample points and checking out the regression coefficient of the model by t-test. The developed model had high prediction accuracy and stability with the maximum prediction error of 0.25 g/100 g, the determination coefficient of calibration (Rcal2) of 0.9992, the determination coefficient of validation (Rval2) of 0.9988, the root mean square error of calibration (RMSEC) of 0.0894 g/100 g, the root mean square error of prediction (RMSEP) of 0.1015 g/100 g and the ratio performance deviation (RPD) of 28.57, which indicated that the model could be used for practical detection accurately and steadily, and was helpful for on-line measuring.