基於離子選擇電極之可攜式水耕巨量營養素感測系統

Abstract

耕作環境中的化學營養成分對於作物生長有密切關係，而農民僅依據酸鹼度、電導度及氮含量作為粗略的判斷指標，因此經常造成過度施肥並引發環境問題。現有的實驗室檢測程序多以耗時之分析儀器為主，尚難滿足即時養分管理之需求。本研究以取樣與樣品前處理較為簡易的水耕養液作為感測對象物，開發具即時性、易攜帶且低成本的現場水耕巨量營養素感測系統，待系統發展完善即可整合土壤採樣與前處理等自動化設備，達到土壤營養素的現場感測。本系統感測器選用鉀、銨根、硝酸根與氫離子選擇電極，經適當的感測訊號調理電路處理，由微處理器控制及運算類比訊號，並以其內建藍芽模組將各離子濃度訊號傳送至手機，進行校正、處理、紀錄與顯示。此外，由干擾修正法計算鉀、銨根與硝酸根離子選擇電極的選擇因子，並確立干擾修正公式，以軟體方式補償離子選擇電極選擇性的缺陷。目前已完成四層堆疊結構 (5×5×8 cm)之電子電路模組，經確效證實訊號處理功能與儀器有極優的一致性 (相關係數0.99)，並具有不同的電氣連接界面膜組可供適合的離子選擇電極接頭組合。嵌入式系統及在智慧型電話 (Smart phone)上所開發軟體之通訊與程序 (含校正)執行，亦經確認可有效地傳送並呈現4通道感測器濃度資料。以本系統量測山崎改良版養液配方，結果顯示鉀與硝酸根離子選擇電極選擇性佳，並不需進行修正；而銨根離子選擇電極經聯立干擾修正後靈敏度由偏移48.1%修正至-5.6%，因其選擇性不足，故此法僅適用於鉀離子不超過銨根離子濃度一個數量級之情況。未來可著重在系統性能驗證與提升系統可攜帶性，並整合自動化設備，實現植物工場養液及土壤養分離子濃度的現場感測。

Precision agriculture for crop management requires the collection of high temporal and spatial resolution soil property data. Crop growth is closely related to nutrition, but farmers currently judge the soil or solution nutrients whether is suitable are based only on electrical conductivity and pH value. Although both parameters can be detected rapidly and use low-cost measurement tools, they do not provide information on individual nutrient concentrations directly. In the laboratory, most of the use of analytical instruments and time-consuming test procedures, and it is difficult for on-site nutrient management. Due to inconvenience of sensing nutrient concentrations, farmers often over-fertilize and cause environmental impact. The aim of this study is to develop a system which is real-time, portable and low-cost for on-site monitoring macronutrients in hydroponics. The sensor sub-system contains 4 ion-selective electrodes (ISE) of nitrate, ammonium, potassium and hydrogen. Then sensing signals are processed by an appropriate signal conditioning circuit and an embedded system. After signal processing, the ion concentration data is transmitted through the Bluetooth module of the embedded system to the mobile phone for correction, processing, calibration, recording and display. Additionally, the selectivity coefficients of ion-selective electrodes are determined by fixed interference method (FIM) and interference compensation by software method. The results showed that this system has a good correlation with the data measured by a commercial pH meter and an electrochemical instrument (correlation coefficient 0.99). Furthermore, an embedded system and an App program developed on Android-based mobile phones have also been verified to effectively transmit and display 4 sensors concentrations data. Both of nitrate and potassium ISEs do not need interference compensation when the system used in a modified nutrient solution developed by Yamazaki due to their well selectivity. However, the ammonium ISE have to be compensated (sensitivity bias was corrected to -5.6% from 48.1%) by a correction equation developed in this work. Due to poor selectivity of the ammonium ISE, this method is only applicable in the condition that the potassium ion concentration does not exceed one order magnitude of the ammonium ion concentration. In the future, the task could be focused on improving feasibility of the system applications in field and developing the process of sample sampling as well as preparation. Furthermore, the system would be tested in different environments, such as plant factory and on-site soil in farm field.