Old and New salinity refractometer difference in readings ?

Question:

Hi there
I bought the new Salinity refractometer i was using the older version in the
blue case
my old ATC reads 35ppt 1.026 the new one is reading 31ppt 1.023 ive
calibrated both instruments with ro-di water at a room temp is about
20celcius
My corals look fine all sps corals
Is there any problems with the new refractometers?

Answer:

Hi Robert

There will be some discrepancy due to the new salinity refractometer
measuring the refractive index of true seawater. The old refract was
designed to measure Brine and when measuring seawater would often measure
1.5-2.0 PPT too high.

Almost all refractometers available to the hobby are designed for brine,
when D-D became aware of the fact that brine refracts gave this error we had
a bespoke refractometer produced specifically designed to measure sea water.
We also advise hobbyists that those using the old D-D refracts or those from
other manufacturers should be taking into account that they will read 1.5PPT
too high, i.e if the refract read 35 PPT the true reading for seawater would
be around 33-33.5PPT.

So with this in mind please expect the new salinity refract to measure
1.5-2.0PPT lower than your older brine refract. So if your old refract is
reading 35PPT your new salinity refract will read around 33.5 PPT

Looking at the results you have mailed you still have a mismatch of 2- 2.5
PPT even when taking into account the offset above. In this instance i would
advise you leave both refracts in a room at 20c for a couple of hours for
both units to equalise in temperature. Clean the optics well on both units
with RO water and flush the pipette well and use a clean piece of kitchen
towel.

Also make sure the plastic flaps on both units are sitting flat onto the
optics when at rest and not sitting up at one edge. If this happens the
water thickness across the optic will be different from one side to the
other and air bubbles will also creep underneath. If this happens light will
refract through the optic incorrectly giving false readings on the scale.
Adjust the flap with the collar that is secured with a screw if the flap
doesn't sit flat.

Now drop a little RO water on each unit and allow the RO water temperature
to equalise with the unit for a minute or two and 0 both units again.

You may also find the following article of interest.

http://www.theaquariumsolution.com/specific-gravity-salinity-and-its-measurement
regards and best wishes Tony