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68 PCB007 MAGAZINE I NOVEMBER 2020 (Dd), and two CPVC plastic nuts are tuned to a specific diameter (Nd). By screwing the nuts in or out, the setpoint of SG can be set over a 50X range. The calibration curve is seen in Figure 5. The SG operating setpoint is selected (horizontal axis), and the span range is select- ed (vertical axis). This provides the drill depth (Dd) and nut di- ameter (Nd) for the sensor. To create your own operat- ing curve, the principle of oper- ation is that according to Archi- medes, the fixed weight of the rod and nuts is acted on by the center-of-gravity (CoG) based on the position of the nuts. To make the sensor float and close the switch, the volume and center of displaced mass do not change regardless of the posi- tion of the nuts; only the SG of the liquid (buoyancy force) can oppose the CoG. This SG sen- sor is 25X more sensitive than any that you can purchase and 1/1,000 the price, as it has a minimum span of 0.02, with an accuracy of 0.04% over aque- ous solutions of 0.8–1.5 units. An example of the analysis of the copper-sulfate concentra- tion in a sulfuric-peroxide mi- croetch with specific gravity is done by first measuring the temperature and then the SG. The intersection of these two points is the sulfuric acid con- centration and copper sulfate concentration. 3. Colorimetric (Spectrometers) A third common industrial chemical technique is color in- dicators. These simple chemi- cal techniques use sensor strips or chemical additive to the so- lution to develop a color that is compared to the standard col- or wheel. Unfortunately, it re- quires the human eye to judge when the colors are the same. This works okay unless a person is color blind (Figure 6). Regarding spectrometric sensors—or more appropriately named, electromagnetic radi- ant energy sensors—the visible light colorime- Figure 5: The calibration and operating curve for the SG sensor in Figure 4. Pick the operating point and span of the sensor. This will provide the outside nut diameter and the drill depth for the rod. Figure 6: Simple color comparisons can be especially useful and accurate; take, for instance, the pH and chlorine content of a home swimming pool.