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Parts Per Thousand, used as a measure of how many gms of salt solutes are in 1000 ml of solvent (water). This is the true measure of salinity, the numbers we often see posted as 1.02xx are actually the Specific Gravity of the solution, an indirect measure of the actual salinity, requiring conversion and corrections for many factors in order to determine the actual salinity in PPT.
As temp of the water changes, the density changes as well. This is why warm water masses float on top of cold water masses, as the water expands while still containing the same number/amount of molecules/mass. With more space between the same number of molecules, the density drops (gms weight/unit volume). These charts allow for corrections of salinity based on the changes (error) induced by temperature change. This is important only when measuring the density of water directly to determine SG. Although there are also changes associated with temp when measuring the refraction of light, the error is slight, and more often is related to making sure tht the refractometer is corrected to the water temp (at the same temp as the water) rather than issues with density of the water on a thin plate (the refractometer). For this reason, most refractometers have automatic temp correction , so the error becomes insignificant (this is the short no-jargon answer).
HTH
__________________
Tom <"))))>(
(TDWyatt)
Wise men speak because they have something to say; fools, because they have to say something. -Plato
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