Salinity units and conversion

[puks]

Hi guys,

I tried to convert ppt to sg.

Compared table in this (http://forum.neptunesystems.com/showthread.php?800-Salinity-probe-readings) thread and some calculators on Internet. They all provide different values.

And Reef Aquarium Salinity: Homemade Calibration Standards (http://www.reefkeeping.com/issues/2004-06/rhf/index.php) also says

PSU is an acronym for practical salinity units, which is essentially a modern replacement for ppt, since salinity is no longer defined as directly relating to solids in the water, but rather by its conductivity.

[bertoni]

For us:

    35 psu = 1.0264 SG = 53 ms/cm²

The SG and conductivity numbers are for a reference temperature of 25 C, measured at 25 C.

What do you need to convert?

[puks]

I'm trying to match ppt/psu/sg that Milwaukee Digital Refractometer shows and the Salinity probe connected to Neptune PM2 module.

[Barney97]

The conversion using salinity and temp is not simple to do it accurately. It is an involved differential equation that is best left to Matlab and/or Mathematica.

The idea is easy to calculate the SG of saltwater you divide the density of the water you are testing by the density of regular water

SG = (roh)/(roh)H2O

It is just getting an accurate reading on the density of your water is the tough part. I would imagine that this is where most of the online calculators mess up, they probably make some assumptions to calculate the density that puts in a pretty large error

I guess if you take very accurate measurement of the water sample you are testing you could calculate the density easily. I am just not sure how precise it will be over the long run.

[Randy Holmes-Farley]

I'm trying to match ppt/psu/sg that Milwaukee Digital Refractometer shows and the Salinity probe connected to Neptune PM2 module.

This article of mine has some direct comparison tables. For example, table 1:

Table 1. Specific gravity, conductivity, and refractive index as a function of salinity of seawater. The darker blue rows represent the range usually encountered in the open ocean.

Reef Aquarium Salinity: Homemade Calibration Standards
http://reefkeeping.com/issues/2004-06/rhf/index.htm

[puks]

I'm confused with "PSU is an acronym for practical salinity units, which is essentially a modern replacement for ppt" that in my understanding means "PSU is the same as ppt".

[disc1]

I'm confused with "PSU is an acronym for practical salinity units, which is essentially a modern replacement for ppt" that in my understanding means "PSU is the same as ppt".

PSU as I understand it would be the ppt of a KCl solution with the same conductivity. It's really close but not exactly the same thing.

[Randy Holmes-Farley]

Unless you are specifically interested in seawater chemistry details, I'd treat PSU and ppt salinity as the same. :)