Application of Total Internal Reflection and Heterodyne Interferometry in Electrical Conductivity Measurements

The studies conducted on light emitting diode plant factories have mostly adopted hydroponics for convenient nutrient management and pest and disease prevention. In particular, in nutrient management, liquid electrolytes can be measured to determine the aqueous nutrient concentrations and absorption rates as well as electrical conductivity (EC) and pH variations. Precisely controlling the aqueous nutrient contents is crucial to large-scale healthy plant production. However, in most of the current conductivity measurements, electrodes are adopted for determining the nutrient additive concentrations. This approach can be problematic in some cases, such as low electrode sensitivity, which results in nutrient overdose, specifically in nonconductive additives, and hinders plant growth. Therefore, this paper proposes an optical method for measuring the aqueous nutrient contents. In this method, the high sensitivity of common-path heterodyne interferometry is incorporated into aqueous measurements to prevent errors caused by interfering impurities on the electrodes and improve measurement and analytical accuracy. The sensitivity of the sensor used in EC measurements can reach 2300°/mS · cm^-1. The method has some merits, e.g., a simple optical setup, high stability etc., high measurement accuracy, high resolution, rapid measurement, and easy operation. In addition, its feasibility is demonstrated.