Evaluation of an Interdigitated Array Electrode Device for IoT Water Testing

Current methods of water quality testing require bulky lab equipment or additional chemical reagents added to samples to obtain measurements. It is desired to develop a device capable of evaluating multiple qualities of water samples in a single compact, stand-alone system. With such a device, integrating water testing into the developing Internet of Things would become a feasible system with considerable improvements over current solutions. The interdigitated array electrode (IDA) is a type of circuit that has variable capacitance depending on the environment it is exposed to. With all other properties defined upon the creation of the device, manipulating the permittivity of the surrounding medium results in the capability to measure properties of the medium itself by observing the overall change in the capacitance as well as the detected potential across the device. It is observed that such shifts possess a monotonic relationship to the total dissolved solids (TDS) levels and free chlorine content in aqueous environments similar to that of drinking water. The measurement method to observe these relationships is shown to have a greater sensitivity compared to alternate measurement methods resulting in a more robust system with higher resolution. Long term stability is a concern for electronic devices exposed to water such as IDAs. This paper evaluates two hydrophobic dielectric coatings, HfO2 and OTS-Al2O3, utilized as protective layers for the IDA device in water with considerably greater salinity than the expected operating conditions to prove that such devices are capable of maintaining proper signal integrity over time. Similar experimentation is demonstrated for chlorinated water and the corresponding readings for free chlorine measurement. Further developments that are underway to integrate the IDA into a complete IoT system and the complications or hindrances that must be overcome are also discussed in this paper, and details of alternative applications of the IDA utilizing similar measurement methods are explored.