A PCB Technology Electrical Conductivity Sensor for the Measurement of Saltwater Contamination

Abstract

Saltwater intrusion is when sea water moves into freshwater aquifers and contaminates them. It can be a serious problem for coastal communities and inland ecosystems. A number of factors, both natural and manmade, contribute to this contamination. Natural factors include events such as storm surges from hurricanes that push seawater into inland freshwater bodies. However, manmade factors contribute far more to saltwater intrusion than natural factors. The biggest human factor is the overuse of freshwater aquifers in coastal areas. Saltwater has a higher density than freshwater, and hence a higher static pressure. So when a freshwater aquifer is partially drained by over usage, the higher pressure saltwater will percolate through the bedrock into the aquifer under the freshwater. Contamination of the freshwater body then occurs through dispersion and diffusion. Manmade canals and channels for shipping, drainage and irrigation can also provide a pathway for seawater to move into coastal bodies of freshwater. Oil and gas production and mining can also contribute to saltwater contamination of freshwater. Saltwater contamination can render freshwater aquifers non-potable, kill freshwater fish and other inhabitants, kill freshwater plants and deleteriously affect inland freshwater ecosystems in coastal areas. Dissolved salt ions in water are excellent charge carries, resulting in a large difference in the electrical conductivity between freshwater and saltwater. Therefore the measurement of the electrical conductivity of a water sample can be used to gauge the level of possible saltwater contamination. Commercial printed circuit board (PCB) technology affords the realization of a low-cost sensor probe for measuring the electrical conductivity of aqueous samples. This sensor makes use of the materials and processes inherent in commercial PCB manufacturing, including the FR4 substrate, patterned Cu cladding and soldermask. The non-conductive E-glass FR4 substrate is used as the rigid backbone of the sensor probe. The patterned Cu cladding is used for electrodes and signal traces. The polymeric soldermask is used as a thin insulating and moisture barrier layer. With these materials, the sensor consists of two planar exposed metal pads on the surface that are used to measure electrical conductivity. Additionally, the PCB probe is directly compatible with the integration of any desired integrated electronic components and/or other sensors. Furthermore, the turnaround time for a new PCB sensor design can be as little as 24 hours at modest cost, making this technology economically superior to traditional sensor technologies, such as silicon based MEMS, where it can take months to realize a new design and be very expensive. A prototype PCB electrical conductivity sensor probe has been designed, fabricated and evaluated with mixtures of freshwater and seawater, demonstrating its usefulness.