5.2 Conductive Sensor Design
Conductive liquid detection sensors operate based on the principle that conductive liquids can complete an electrical circuit. The sensor typically consists of two or more electrodes placed where liquid detection is required. The electrodes are typically made of conductive materials such as steel, copper, or other corrosion-resistant metals.
When no conductive liquid is present, the resistance between the electrodes is very high because air or any non-conductive medium between the electrodes provides no path for electrical current to flow. When a conductive liquid (such as water with dissolved salts or other conductive substances) comes into contact with the electrodes, it creates a conductive path between them, significantly lowering the resistance between the electrodes. Then the system will measure this electrical conductivity change. Connect two pull-up resistors to the electrodes, with one resistor for each. The resistors help create a clear distinction between the presence and absence of liquid by ensuring a significant voltage drop when liquid is detected. They also help reduce the noise and interference that might affect the detection circuit.
Conductive liquid detection sensors can come in various shapes and forms, each designed to suit specific applications and requirements. The design can range from simple configurations like two conductive wires to more complex structures like interleaved electrodes.
The simplest form of a conductive liquid sensor consists of two parallel wires or rods placed at the desired detection point. This type of sensor is frequently used to monitor conductive liquid levels in tanks. The advantages of such sensors include simplicity, cost-effectiveness, and versatility.
The conductive electrodes can also be arranged in an interleaved pattern. Due to its sensitivity and reliability, typically, this sensor type is used to detect liquid leaks in different applications across different industries. Such sensors can also be designed on a printed circuit board. The exposed copper traces will act as electrodes and can be shaped in various patterns.
The PCB geometry and the dimensions of the electrodes play a crucial role in the performance of the interleaved sensor. The overall size and shape of the electrodes determine the coverage area. When widespread detection is required, a larger sensor will have an increased detection surface area, making it suitable for applications. The spacing between the electrodes and their width influences the sensor sensitivity. Closer electrode spacing and reduced electrode width can provide higher resolution, allowing the sensor to detect smaller amounts of liquid. However, if the electrodes are too narrow, they may become fragile and more prone to damage, and if the spacing is too small, it might lead to short-circuiting or bridging by contaminants.
A disadvantage of the conductive liquid detection method is that the sensor’s effectiveness depends on the conductivity of the liquid. Nonconductive liquids such as oils cannot be detected using this method. Another drawback is the potential corrosion of the sensor’s electrodes as they come in direct contact with the liquid. The electrodes can also become fouled or coated with residues from the liquid, which can affect their performance and require regular cleaning.
To reduce the overall corrosion rate, the MTCH9010 implements electrode polarity reversal when operating in Conductive mode. In one acquisition cycle, the system is configured so that the current flows from Electrode 1 to Electrode 2. In the next acquisition cycle, the system reverses the current flow. Periodically reversing the current flow helps ensure that both electrodes experience similar wear and tear, extending the lifespan of the sensor.