## 3 Experiment Results and Discussions
### Turbidity Sensing  
Before the turbidity sensing experiment, we performed calibration to clarify the correlation between turbidities and output photovoltages. In accordance with international standard ISO 7027 [11], a formazine solution is recommended as the turbidity standard solution for measuring turbidity by the optical method. However, formazine is difficult to handle because a hydrazine sulfate, which is a carcinogenic agent, is necessary in the mixing process to obtain the formazine solution [12]. On the other hand, kaolin has been prevalently utilized as a turbidity standard solution for a long time. Since kaolin is a clay mineral, it is completely free of anything harmful, is easy to handle, and is a low-cost material. However, a kaolin solution settles easily. It causes variations in turbidity during the measurement. In order to resolve the problem, a styrene-divinylbenzene (St-DVB) copolymer microbeads solution was proposed as the turbidity standard solution. However, a St-DVB copolymer microbeads turbidity standard solution is much more expensive than other turbidity standard solutions. In the calibration, we utilized both a kaolin solution and a St-DVB copolymer microbeads solution. First, we prepared 7 different concentration calibration solutions of kaolin. The prepared kaolin concentrations are 0, 10, 20, 50, 100, 200, and 500 mg/L, as shown in Figure 4. Then, we measured the photovoltages for each kaolin solution with the optical measurement setup shown in Figure 3. In the measurement, the red LED was set to the LED1 position in Figure 1b, and we measured the output photovoltages at three positions: PD1, PD2, and PD3, also shown in Figure 1b. The measurement results are represented in Figure 5. The photovoltages measured at the position of PD2 are not represented in Figure 5 because the position of PD2 is aligned with LED1 and because the photovoltages were out of range. In other words, PD2 measured the light intensity of the light source LED1 rather than that of the scattered light caused from the suspended kaolin particles. However, if the sample is a dense suspension or if the LED and PD are far away from each other, the measurement with LED1 and PD2 may also be valid. As a result, we were able to distinguish the degrees of turbidity according to the measured photovoltages, but we could not distinguish them with the naked eye, especially for low concentration samples shown in Figure 4. Furthermore, we were also able to confirm the linear relationship between the concentration of the calibration solution and the scattered light intensity. The linearity was confirmed for the positions of both PD1 and PD3. We observed more accurate linearity in the measurement results from PD3. This linearity comes from the rapid precipitation effect of kaolin in the measurement, which means that the photovoltage measurement at the bottom of the sample yields a more accurate turbidity. Moreover, especially in low concentration regions of less than 50 mg/L, which is a region that is more meaningful for the sensors during detection, it shows better linearity. In turbidity sensing with environmental water samples, zero calibration is necessary only when the scattered light intensity is considered. Figure 4: The 7 different concentration calibration solutions of kaolin: From left to right, the concentrations are 500, 200, 100, 50, 20, 10, and 0 mg/L. To convert the photovoltage value to the Nephelometric Turbidity Unit (NTU), we measured the same kaolin calibration solutions with a commercial turbidity sensor (Hydrolab DS5X, OTT Hydromet GmbH, Kempten, Germany) calibrated with a formazine calibration solution. The NTU is defined by the formazine calibration solution. The turbidity measurement results are shown in Figure 6. Using the measurement results in Figure 6, the measured photovoltages were converted to NTU turbidities. We also performed turbidity measurements with environmental water samples from Lake Koyamaike located in the Tottori Prefecture of Japan.