The Difference Between Salinity And Specific Gravity

[idlefingers]

I was reading someone's post on here last night where they were referring to specific gravity as salinity and it got me thinking about what the difference is between the two.

According to wikipedia, salinity is the amount of dissolved salt in water which I can understand. But it's definition of specific gravity has me a little confused because I'm not scientifically minded enough:

Relative density is a dimensionless ratio of the densities of two materials. The term specific gravity is similar, except that the reference material is water. A relative density can help quantify the buoyancy between two materials, or determine the density of one "unknown" material using the "known" density of another material.

So, can someone explain what the difference is in layman's terms for simple folk like me? I thought they'd be quite similar, but from what I've read so far, they seem quite different and so (it seems) that someone saying "I checked the salinity and it was 1.025" is quite wrong?

 

[andywg]

Salinity (as you say) is the measurement of how much salt is dissolved in the water. In marine conditions it tends to be about 35 grams per litre, which also tends to be roughly 35ppt.

Specific Gravity relates the weight of something. We refer to the specific gravity of sea water as to how it is different from pure water at sea level. Pure water woul dhave a weight of 1 (identical to itself). Sea water tends to have a Specific Gravity of about 1.025, that is it weighs 1.025 times as much as the similar volume of pure water would. SG is temperature dependant, and a hydrometer will red slightly differently as teh temperature increases (how much depends on the salinity of the water).

Also, when using a refractometer you are actual measuring the refractive index of the water, which is converted using the scales on it to give you the salinity or SG.

 

[idlefingers]

Ah okay.. that makes more sense. Thanks for that andy!

 

[Danno]

Could spacific gravity change by where you live? i.e: 1000 feet above see level vs. 20 feet above see level