Can i depend on clinical floating (urinometer)hydrometer .. ??

Can i depend on clinical floating (urinometer) hydrometer .. cause i found refrac. expensive ??

 

It's going to be calibrated for a different temperature. In other words, you can't use the standard reef tank conversion charts for floating hydrometers. If you're good at algebra, you can use the formulas and create your own chart.

In all honesty, if money is an object, why don't you just buy a temperature adjusting swing arm hydrometer. Then you don't need any conversion charts whatsoever.

 

To use a floating hydrometer correctly, you use a columnar cylinder and put the water into the cylinder and then slowly lower the hydrometer into the water until it doesn't sink any more. Let it stop motion and then read it at eye level.
Whether or not you need a calibration chart for conversion depends on the temperature the hydrometer was calibrated at and it should be inside somewhere on the scale in the tube of the hydrometer. Many hobby floating hydrometers are calibrated at an average temperature for salt tanks so if your water is close to the calibration temp, you won't need to adjust.
For instance, my certified calibrated hydrometer is done at 60°F and if my tank water is at 80°F then I have to add .0024 to my reading to be correct, or in other words I would add roughly .00012 for each degree temperature the tank water is over the calibration temp of the hydrometer. You can see if you have a hobby floating hydrometer at say 77° and your water is at 80° then the addition will only be .00036.
If you have one that has a big difference like most quality ones, you can cool the water sample in the refrigerator to the calibration temp and then check the s.g. and it will be correct, assuming you have read it correctly.
The cheaper easiest way to to buy a SeaTest swing arm, (Randy Holmes-Farley found they were the most accurate) condition it in water for a few minutes, and then take a reading. Compare the same water with the stores refractometer, assuming they have used calibration fluid to calibrate it with, and mare down any difference between the two readings so you can adjust the reading you take from then on.
Whenever you use the swing arm, rinse it out with fresh water afterwards and then once a week soak it overnight in white vinegar to remove any deposits that affect the swing arm. (You can't see then so just go ahead and clean it regular)
OR, you can just say to heck with it and buy a refractometer and calibration fluid.
ALL METHODS CAN BE USED.
When I started, nobody around here had refractometers and the swing arms were in use the most.
I had my set of certified calibrated hydrometers for when I worked in a lab and I used the correct one to check calibration on my swing arms that I used for everyday use. 18+ years later, I'm still using them.
Best recommendation is to get the one you think you will be most comfortable using.