Corelation Between SG/Salinity

metweezer

metweezer

Active Member
What's the corelation between SG and Salinity? Does temperature affect both? My refractometer measures SG on one side and Salinity on the other. I have read that temp affects SG. Well then it has to also affect Salinity, right? My Sybon refractometer shows that 1.025 of SG is equal to about 34 ppm Salinity. My mixing bucket is in my garage and it is close to 90 degrees in there most days. If I move my mixing water close to my tank and let the temperature acclimate to my living room temperature, will my SG change? The refractometer that I use supposedly acclimates to any temperature.
 
reefkprz

reefkprz

Active Member
no temperature does not effect salinity. specific gravity is a measure of a liquids density when measured against pure water. water expands and contracts with temperature (water is one of the only fluids that expands both with heat and cold) specific gravity is affected when temperature increases and the water expands so the same actual density takes more room and when cooler the water contracts putting a higher density in the same area. thus skewing the results of a hydrometer.
It is important to understand that salinity and specific gravity are related - but not the same. Salinity can be measured by (a) boiling down a water sample, (b) measuring conductivity by electronic means, and (c) other laboratory methods which are either too complex or too expensive.
Specific gravity indicates density, while salinity refers to the actual weight of the salt.
The hydrometer works on the principle that a solid body displaces its own weight of the liquid in which it floats. The hydrometer is calibrated at 60˚F (15.55˚C) in which distilled water equals 1.000 as the initial point. The readings will rise with increasing density of the water.
As the instrument is calibrated at a temperature of 60˚F (15.55˚C), it requires that the water to be tested will also have a temperature of 60˚F (15.55˚C) in order to get an accurate reading.
Water will expand or contract if temperatures vary; therefore the density fluctuates with temperature as well.
 
xtreeme

xtreeme

Member
Not all hydrometers are calibrated at 60f. Mine is calibrated at 68-85f the instant ocean brand.
Accurate, reliable water test equipment is absolutely vital to creating and sustaining a healthy, successful marine environment. Easy to read and simple to operate, Marineland's Instant Ocean® Hydrometer delivers precise full-range salinity and specific gravity readings - accurate to 0.001 - and does it each time, every time. And thanks to the new improved design, you won't even get your hands wet!
IDEAL FOR USE IN WATER TEMPS OF 68° -- 85° F
 
bang guy

bang guy

Moderator
Refractometers measure Salinity, not Specific Gravity. Just ignore the S.G. scale and use the 0/00 (salinity) scale.
 
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