## 5 Discussion
For instance, in the field of hydroponic farming (hydroponics is an advanced technique for plant and vegetable production that utilizes water instead of soil as growing matrix [42], implementable in a _Vertical Farming_ setting), the pH level must be maintained in the 5.5 to 6.8 range [42]. Furthermore, the detection of minor changes in the pH level significantly impacts water quality, and thus, the main nutrients of these systems. The instrument presented in this work enables accurate detection of pH variations of 0.1 pH units, with a 68% confidence interval, for hydroponic farming. Hereafter, a brief discussion of the sources of uncertainty, within our instrument, is presented for those applications requiring more stringent pH control. First, with respect to the actual sensor, the efficiency of the electrode can degrade due to the manufacturing process (better vs worse quality), glass membrane, reference cables (e.g., electrical interference or RF coupling), and especially maintenance. A decrease in performance has been validated, by our group, if the electrode is poorly maintained (i.e., cleaned). To counteract this problem, the calibration, as described in this work, was implemented. More concretely, in order to ensure accurate readings, the calculation of the _sensor \"compliance\"_ metric, is advisable. For the case reported in this work, such a metric was demonstrated to be 97.3%. Next, the impact of _Signal Conditioning_ and _Acquisition_ stages, which are closely interrelated, is addressed. In the current version of the pH instrument, refer to Section 4.1; the offset voltage is fixed to its minimum value of 840 mV. This places a restriction on the usable detection range of the ADC to 48%; namely, from 1.32 to 3.73 V (corresponding to pH values from 14 to 0 units). Figure 8 depicts a two-step approach to significantly improve the signal conditioning circuitry. First, the feedback resistor, R7, can be substituted (e.g., 10 k\(\Omega\)) to set the amplification factor to a different value (e.g., GAIN = 2), and the analog reference voltage can be set to 2.56 V. By implementing this first step, the usable detection range of the ADC becomes 65%; that is 0.83-2.51 V. Second, the minimum offset voltage of the divider network can be reduced to a different value (e.g., 420 mV). This is achieved by replacing the fixed resistor, R1, with a 2 k\(\Omega\) component. By selecting the amplification factor correctly, the usable detection range can be optimized to 98% (i.e., in the 0-2.52 V range), which is approximately a two-fold improvement. As noted throughout this work, the expected magnitude of the detected signal will approximate the Nernstian response, with a maximum of 59.16 mV per pH. For our instrument, the measured sensitivity was demonstrated to be 57.56 [mV/pH]. Thus, a difference of 10 mV in the sensor signal implies a variation of \(\sim\)0.175 pH units. Various sources can induce voltage variations of this magnitude, including the communications and ADC stages. The _Network Layer_, or wireless communications stage, is not only necessary to enable transport of pH information, but rather indispensable to minimize coupling and/or layout effects in the instrumentation. In fact, pH information could also be sent to a personal computer (or smartphone) by means of wired (e.g., serial) communications. However, voltage variations on the 10 mV range due to coupling errors are commonly encounteredin wired communications. As noted before, such variations would induce an error that is almost twice the value of the instrument accuracy. Moreover, the evaluation of the dynamic range of the ADC is important because this establishes a minimum detectable voltage (i.e., pH). Thus, the measurement of potentials below a certain threshold is not plausible, and the system is said to be ADC-limited. Furthermore, various deviations inhibit ideal performance of ADCs. The absolute accuracy of an ADC serves to quantify: gain error, offset deviations, differential error, non-linearity, and quantization error.