644 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 46, NO. 2, APRIL 1997 A Planar Capacitive Precision Gauge for Liquid-Level and Leakage Detection Ferry N. Toth, Gerard C. M. Meijer, Member, IEEE, and Matthijs van der Lee Abstract— A new capacitive precision liquid-level gauge has been developed. The sensor consists of a low-cost planar electrode structure, a capacitance-controlled oscillator and a microcon- troller. The device is able to measure absolute levels of conducting and nonconducting liquids with a 1 mm uncertainty over a 4 m range. The system has a high resolution of 0.1 mm at short measuring times of only 0.2 s. Slow level movements of 0.02 mm/h can be detected within 18 min. I. INTRODUCTION M ANY DIFFERENT types of liquid-level sensors are used throughout industry. The sensor presented here attempts to improve a type of sensor used in storage tanks at fuel service stations [1]. These tanks usually contain noncon- ductive liquids like leaded or unleaded gasoline or diesel fuel. However sometimes conductive liquids like water need to be measured, for instance for calibration purposes. Liquid-level gauges can also be used to detect a leakage in the tank. For this application a high resolution is required. In this paper a capacitive sensor with a high resolution is presented, that uses a new low-cost electrode structure. This sensor is able to measure simultaneously the levels of both conducting and nonconducting liquids with an equal accuracy. II. PHYSICAL ASPECTS The electrode structure spans the whole measuring range and has a maximum length of nearly 4 m. It consist of a long electrode and one that is divided into insulated segments . The relative position of the electrodes is fixed by the mechanical construction, while the absolute position requires a one-time calibration. All capacitances are connected to either the low-impedance voltage source, the low-impedance measurement-system input or to ground (Fig. 1). The level of nonconducting liquids can be calculated by finding the interface segment , which has a value between , the capacitance in the liquid and , the capacitance in air. Then the capacitance of the interface segment can Manuscript received June 20, 1996; revised October 1, 1996. This work was supported by the Dutch Foundation for Technical Sciences (STW) and ENRAF BV. F. N. Toth and M. van der Lee are with ENRAF BV, Delft Instruments Group, 2624 BD Delft, The Netherlands. G. C. M. Meijer is with the Faculty of Electrical Engineering, Delft University of Technology, 2628 CD Delft, The Netherlands. Publisher Item Identifier S 0018-9456(97)01583-0. Fig. 1. Conductive liquid connected to ground. be interpolated to find the interface position accurately (1) where is the length of the electrode segments. To measure conducting liquids such as water, a special provision has to be made: To prevent short-circuiting of the input of the measurement system the electrodes need to be covered with an insulating sleeve. This material also protects the electrodes against the possibly aggressive environment. In the proposed setup (Fig. 1) some electric-field bending around the interface will occur. Assuming a parallel plate electrode structure, covered with an infinitely thin insulator, the conducting liquid can be regarded as a shield (Fig. 1) that is connected to ground. The capacitance between a single electrode segment and the opposite electrode can be calculated as a function of the interface level. The nonlinearity in the characteristic is caused by field-bending effects. The parallel plate electrode structure has the advantage of a simple physical structure that can easily be modeled. However, the costs of constructing such a structure with a length up to 4 m are considerable. To reduce costs, the transmitting electrode segments and the receiving electrode can be integrated on the same substrate. Shi et al. [2] presented a structure where electrode is removed and the capacitances are measured between the successive electrodes and . This method leads to a rather complex model to retrieve the level from the measured capacitances. An alternative approach is to integrate electrode also 0018–9456/97$10.00  1997 IEEE