How to read a salinity refractometer compared to your true Seawater version | D-D The Aquarium Solution
Question: 

hi just read this on facebook, we have the old type (salinity) dd 
refractometer, we use the left side of the meter and run at 1.025 on the 
gauge, am i running at a lower level than i think?

Just wanted to point out to anyone using a D-D refractometer to test salt 
levels.

The old type that displays the word 'salinity' in the view finder, actually 
reads 1.7ppt over the actual reading! These were designed for brine use a 
long time ago, until D-D released the new type of refractometer that displays 
the word 'seawater' in the view finder, which reads as per the guage when 
calibrated correctly. So in other words, if you're using the salinity type, 
and are aiming for 35.5ppt as recommended by DD you need to read it on your 
refractometer as 37.3ppt when correctly calibrated at 20c

many thanks David

Answer: 

Dear Dave

Yes this is true, our old Brine refractometer was based on Sodium chloride rather than full strength seawater
so it will read 1.5PPT too high compared to our true Seawater refractometers. Indeed other manufacturers
Refractometers will also be brine unless they specifically state otherwise.

We suggest our H2Ocean reef salt is mixed to 35.5PPT (1.0273 s.g at 25c) using our seawater Refractometer.
If using an old style Refractometer please aim for approx 37 PPT.

With regards to your current mix please use the PPT scale, 1.025 at 20c will run at around 33PPT which when you
take the brine offset into account you could be running as low as 31.5PPT.

Aim for 37PPT after calibrating your refractometer with RO water (unit temperature at 20C).

You may also find the following link interesting.

http://www.theaquariumsolution.com/specific-gravity-salinity-and-its-mea...

Best regards Tony