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APRIL 2018 I PCB007 MAGAZINE 81 sor. It works on the princi- ple of a fixed-volume dis- placer balancing a variable weight, in this illustration (Figure 6), a titanium met- al chain. The height of the displacer is measured with a linear variable differential transformer (LVDT) by plac- ing a metal core in the dis- placer. The differential coils provide the location of the metal core. An example of using SG for analysis is the analysis of the sulfate acid concen- tration in a sulfuric-peroxide microetch. First measuring the temperature gives you the copper sulfate concen- tration from its temp-satura- tion curve. Then the specif- ic gravity is measured. From the saturation curves of sul- furic acid and copper sul- fate, the percent by volume of sulfuric acid can be deter- mined. Or, if you know the percent by volume of sulfu- ric acid, say by conductivi- ty, the concentration of the copper sulfate can be deter- mined. Colorimetric (Spectrometers) A third common indus- trial chemical technique is color indicators. These sim- ple chemical techniques use sensor strips or chemical ad- ditives to the solution to de- velop a color that is com- pared to the standard col- or wheel. Unfortunately, it requires the human eye to judge when the colors are the same. This works OK un- less a person is color blind! Figure 5: The calibration and operating curve for the SG sensor shown 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: Diagrams of two popular industrial specific gravity controllers. On the left, the hydrostatic head SG sensor working off the principle of differential pres- sure of the solution compared to a column of water. On the right, the displacer has a chain with weights attached and will rise or lower until there is equilibrium. An LVDT measures the rise and lower position of a metal rod attached.