Capacitor and virtual instrument used for on line measurement of water film

The thickness of water film is an important parameter in the field of chemical industry. According to the thickness of water film, many other chemical parameters can be determined. Water film is formed by the flowing water, which flows through a narrow channel. So it is difficult to use contact gauges to measure the water film because it's flowing so fast that if it is touched, the thickness will be changed. A low-cost and high precision non-contact measurement method-capacitive sensor is used as the sensor of the thickness of water film, virtual instrument is used to analyze the measurement state. As a relative measurement instrument, capacitive micrometer converts the change of distance to the change of voltage which range is from -10Vsimilar to+10V. And this voltage output signal can be connected to a PC through an analog to digital (A/D) interface card, such as ZTIC8310, ZTIC6319, and so on. After the calibration of capacitive micrometer, a much better linear relationship between the change of voltage and the change of the thickness can be found. The absolute thickness of water film can be got by the computation of voltages and calibration coefficient. In contrast to the conventional stand-alone instruments, a PC based virtual instrument for the measurement of water film is proposed in this paper. The virtual instrument with all controls, indicators and analysis modules required for the measurement has been developed using the software package LabVIEW, which is a well-known commercial product for developing control systems and engineering applications. It offers an easy programming method to solve complex measurement problem, and can interactively control inputs, outputs, and system variables. The virtual instrument analysis software system used, which is a comparatively good method among control systems based on measurement, has front panel, display panel, oscilloscope and filtering analysis modular. This system provides a graphical programming environment that integrates data acquisition, analysis and presentation in one system for effective analysis. There are many virtual instruments, which can be easily added or removed to the measurement program. Based on on-line measurement theory, real time voltages should be collected to PC. These voltages will be computed by formulas in Labview and the real time data of thickness will be plotted on the screen. These data of thickness can be stored in computer with any format for future research. If bad signal got, it will give error alarm. System operation consists of user interfaced on-screen front-end panels designed to acquire and control data acquisition card, and external interface hardware. The method of the measurement of water film can save a lot of fees of research and applications, and be easy to apply in other measurement and control fields. The basic principle and working process of capacitive micrometer, denoise method, the method of capacitive calibration, virtual instrument data acquisition system and on-line measurement of water film, will be further discussed in this paper.