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80 PCB007 MAGAZINE I APRIL 2018 to a specific diameter (Nd). By screwing the nuts in or out, the set-point of SG can be set over a 50X range. The calibration curve is seen in Figure 5. The SG operating setpoint is se- lected (horizontal axis) and the span range is selected (vertical axis). This provides the Dd and nut diameter (Nd) for the sensor. To cre- ate your own operating curve, the principle of operation is that according to Archimedes, the fixed weight of the rod and nuts is acted on by the center of gravity (CoG) based on the posi- tion of the nuts. To make the sensor float and close the switch, the volume and center of dis- placed mass does not change regardless of the position of the nuts, only the SG of the liq- uid (buoyancy force) can oppose the CoG. This SG sensor is 25X more sensitive than any that you can purchase and 1/1000 the price, as it has a minimum span of 0.02, with an accura- cy of 0.04% over aqueous solutions of 0.8 to 1.5 units. Two of the most used SG sensors have vari- able read-outs; the hydrostatic head based on the differential pressure of a fixed height of the liquid versus the same height of water. An SG sensor that is useable for organics, solvents as well as aqueous liquids, and at high tempera- tures is the variable-weight-fixed-volume sen- Table 3: Specific gravity sensors, their span sensitivity, the condition of liquids they operate in, and their accuracy. Figure 4: A very sensitive and accurate immersed SG sensor can be made from a purchased GEM level switch that has a threaded hollow rod attached and two threaded nuts. Figure 5 has the Dd and nut diameter of CPVC plastic based on the operating mid-point and total span. On the right, the standard hydrometer in a flask.