There are a lot of sets of three hydrometers on the market at the moment. These apparently measure alcohol potential not specific gravity. I believe that these are designed for wine making and therefore assume that the final gravity is around 1.000. In order to measure the SG, I assume that you convert the alcohol reading to SG, assuming a final gravity of 1.000. Am I right? The advantage of these is that they split the alcohol (or SG) range into three, and therefore the measuring scales are much larger than the standard single hydrometer.