This project came from the need to develop a simple, accurate and robust system to measure fuel level in tanks using a capacitive probe installed directly inside the tank.

The solution was developed for a client that provides technical services to several companies and needed an architecture that could be applied across different installations, with good accuracy, electrical isolation and easy integration with supervisory systems.

The core idea of capacitive fuel measurement was to turn the change in capacitance caused by the fuel level into reliable digital information.

How capacitive fuel measurement works

The sensing element was a capacitive rod installed vertically inside the tank.

As the fuel level changed, the capacitance measured by the probe changed too.

The electronics attached to the rod used an IC dedicated to capacitance measurement. This circuit converted the capacitance value into a frequency, making it possible to carry the information simply and with good noise immunity.

The generated frequency directly represented the capacitive state of the probe.

Optical isolation between sensor and processing

Electrical isolation was one of the key points of the architecture.

The frequency signal generated by the measurement circuit went through an optical isolator before reaching the processor.

This kept the measurement stage installed at the tank electrically separated from the processing and communication electronics.

This separation helped reduce grounding, noise and coupling problems between the sensor area and the rest of the system.

Simultaneous capacitance and temperature reading

The processor received two main pieces of information: the value associated with the probe capacitance and the temperature measured in the system.

Temperature mattered because the electrical properties of the fuel and of the sensor assembly can vary with environmental conditions.

This information was processed and made available to the supervision center through a Modbus interface.

Table-based calibration

After the probe was installed in the tank, a calibration step was carried out.

The procedure consisted of filling the tank to several known levels and recording, for each point, the fuel height or volume, the corresponding capacitance value and the measured temperature.

These points formed a calibration table specific to that tank.

During normal operation, the system compared the measured value with the points stored in this table.

Interpolation between calibration points

Between two known points, the level was calculated by interpolation.

For example, if one capacitance value corresponded to an empty tank and another to a full tank, intermediate values could be estimated between those points.

In practice, the more calibration points spread across the tank’s range, the better the real behavior of the assembly could be represented.

This was particularly useful because tanks do not always have perfectly linear geometry. Using several points made it possible to compensate for some of these geometric differences without requiring a complex mathematical model.

Frequency-based measurement

The chosen architecture had an important advantage: the measurement signal was transmitted as a frequency, not as a small analog voltage prone to degradation along the way.

The dedicated Analog Devices capacitance measurement circuit provided high accuracy when converting capacitance into frequency.

This frequency could then cross the optical isolation and be measured by the processor in a very stable way.

Pulse-based transmission also simplified the interface between the sensing section and digital processing.

Modbus communication with the central system

After processing the capacitance and temperature data, the device made the information available on a Modbus bus.

This made it possible to integrate the sensor with an acquisition center or existing industrial systems.

The architecture clearly separated three functions: capacitive measurement, processing and compensation, and industrial communication.

This organization made maintenance, diagnosis and adaptation of the system to different applications easier.

A simple circuit with good accuracy

One of the most interesting aspects of this project was achieving accurate measurement with a relatively simple architecture.

Using a dedicated capacitive measurement circuit, frequency transmission, optical isolation, digital processing and table-based calibration avoided the need for a complex analog chain.

The result was a capacitive fuel measurement system suited to industrial monitoring, with good repeatability, isolation and integration with existing systems.

Electronic design driven by real measurement

This project shows that a good instrumentation solution does not necessarily depend on a complicated architecture.

By correctly choosing the measurement principle, the way the signal is transmitted and the calibration method, it is possible to obtain a robust and accurate system with few functional blocks.

ECK took part in developing the electronic architecture, the capacitive reading, isolation, processing and the Modbus interface, turning the probe’s capacitance variation into level information the client’s system could use.

See other electronic engineering projects developed by ECK.