"Boomer" wrote in message ...
There is less error involved with a refract. I have yet seen a Deep-Six that is even close
to what it should read. Swing arms units also have a habit of collecting salt deposits on
the swing arms and pin, which can cause very low readings. Thus, they need to be soaked in
vinegar every month to remove such deposits. Floating hydrometers have the issue of temp
compensation, where most in this hobby seem to get lost. Refracts are much more accurate
out of the box. However, they are also in error but we can compensate for that.

Looking at the scale of mine refractometer with divisions for 34 and 35 ppt so close
to each other and comparing wide scale of swing arm with a centimeter between 34 and 36
is it hard to believe refractometers are "much more accurate"... But I will take your word.

Also, would you agree that Coralife statement of +/-0.001 SG accuracy of their
DEEP SIX is not true if you say +/- 1ppt refractometer is "much more accurate"?

For the
last couple of months Randy and I have been colleting data so he could redo the old DIY
calibration solutions from months ago. We knew something was wrong with them and we wanted
something reefers could make easily for a DIY calibration solution. The first ones we had
were to low.

I will wait then... thanks. But now I remember I have a postal scale with 1g acuracy...
It should give me acurate enough result for the calibration.

In short your refract reads 1.5 ppt to low. So if you want a salinity of 35
ppt (3..5 %), on your refract, it needs to read 36.5, which equals 35 ppt NSW. Refracts
from the factory are not calibrated to seawater but NaCl, which does not have the same RI
( Refractive Index) as NSW. You can read all about it here. It just came out the other day
:-)

Thank you.
So it looks like people adjusting their tanks to a refractometer reading 35 ppt
are really using sea water with less than 34 ppt of salt in it... Adding on top
of it +/- 1ppt manufacturer stated measurement error margin we could be off by
more than 2.5 ppt if we do not correct NaCl offset... Sounds very pessimistic :-)