Factors affecting the accuracy and precision of ion-selective electrodes for hydroponic nutrient supply systems

Accurate and precise sensing of ion-selective electrodes is very crucial for real-time ion-specific nutrient management in hydroponic crop cultivation systems. Sensing performance can be affected by nutrient mixing time, solution's pH, temperature, and motion state. The objective of this study was to investigate the factors affecting the accuracy and precision of K⁺, NO₃^-, and Ca²⁺ ion-selective electrodes (ISE) attached to a hydroponic nutrient management test bench. The test bench consisted of ISE (K⁺, NO₃^-, and Ca²⁺) sensors, liquid tanks, diaphragm pumps, electric valves, and a controller. A small submersible pump was installed inside the nutrient mixing tank for quick nutrient mixing and keeping the solution moving. A water heater was used to control the nutrient solution temperature. A LabVIEW-coded program was used for data acquisition and relevant equipment operation. Three different pH levels (3.5, 6, and 8.5) and eight temperature levels (5-40°C with a 5°C increment) were selected to check the effect on the sensing performance of the ISEs. Low pass filtering was applied to remove noise from the acquired data. The experimental results showed that it took around 10 s for the nutrient mixing pump to make the nutrient ions uniform; the 3.5 pH level showed a high Ca²⁺ ion concentration, and Ca²⁺ ions sediment when pH was 8.5. Accurate and precise data was obtained by K⁺, NO₃^-, and Ca²⁺ ISEs when the solution temperature was 25, 20°C, and 20°C and moving in state, respectively. This study could provide guidelines to improve nutrient management practices for accurate and precise data acquisition by ion-selective electrodes.