All three are indirect measures of sailinty, when we are trying to estimate actual amount of salt in the tank, as in ppt (35 being the target usually). And all salt mixes are different from each other. Probably even batches of the same brand have variances in the proportions of each ion etc between batches. Hydrometers measures how bouyant something is in the salt water mix, refracts measure how the light bends,conductance measures the electrical properties of the water mix. But none can tell you exactly the amount of salt overall. Say for example you more iron in the mix, and less Mg, but still the same overall weight, then you'll have different readings on each of the measuring methods above. This is b/c Mg might cause different buoyancy and refraction and electrical conductance than Iron, and vice versa. But both mixes have the same salt content.
You can't really rely on store bought standards because their mix of salt is not what your mix of salt it.
Also I was wondering why people rely more on refracts than swing arm hydrometers. Or where testers like the icecap tester (electrical) fall into how close it is to estimating true density of salt in mine or your specific tank. Which of the three in reality is the best general use to most closely estimate how much salt is actually in the tank? I'm thinking the only way is to have your specfic salt water mix analyzed to find the exact proportions of all the elements etc in the water, figuring out the salinty of your specific tank, and using that as your standard, calibrating whatever you use to measure to that standard.
I used to think salinity was the easiest thing to measure and know for sure if you're off target or not, but thinking about it, it's just not the case I dont think.
You can't really rely on store bought standards because their mix of salt is not what your mix of salt it.
Also I was wondering why people rely more on refracts than swing arm hydrometers. Or where testers like the icecap tester (electrical) fall into how close it is to estimating true density of salt in mine or your specific tank. Which of the three in reality is the best general use to most closely estimate how much salt is actually in the tank? I'm thinking the only way is to have your specfic salt water mix analyzed to find the exact proportions of all the elements etc in the water, figuring out the salinty of your specific tank, and using that as your standard, calibrating whatever you use to measure to that standard.
I used to think salinity was the easiest thing to measure and know for sure if you're off target or not, but thinking about it, it's just not the case I dont think.