Accurate Salinity Reading

[Servillius]

Randy,

What’s my best bet for getting an accurate and precise salinity reading? I have a Milwaukee digital refractometer and a DA salinity probe. I checked the error on the refractometer and it’s +-2 ppt. My probe varies by 1ppt per day. Between them I get readings from 33.5 to 37. I’m considering doing a weight/weight by taking 1L of water and boiling it off. Is there a better way?

 

[Martin Kuhn]

I recommend an aerometer(spindle) with a Long scale.
Take your tank’s water to a separate cylindric jar and measure water temperature together with the density measurement.
Than Calculate both values into salinity (online calculator, AquaCalculator, etc)

 

[Randy Holmes-Farley]

There are many good ways, and most ways are good enough for reefers.

My preferred method is conductivity. I really don't understand why the the controller folks seems to have so many people with issues using the built in conductivity measurements, but it might relate to temperature compensation (which is important) and how (or even if) it is implemented.

I have used and like the simple Pinpoint conductivity meter, but I somewhat prefer the old Orion model 128 (get them used on ebay) for several reasons, not least of which is that it responds faster to temperature changes and has a wider range of uses.

As Martin mentions, a good glass floating hydrometer also works well, although is more cumbersome to actually use. I found my Tropic Marin glass hydrometer to be spot on, but you do need to understand how to manually correct for temperature issues if using it at a temp other than the calibration temp.

As an aside, boiling off the water is an inherently flawed method because it won't easily drive off all the water, and if you heat it hot enough to do so, you begin driving off "chemicals".

While I doubt hobbyists implement any of these methods to perfection, a renowned chemical oceanographer, Millero, lists the best that can be obtained (at least when he wrote it in 1996), in salinity measurement had these levels of uncertainty:

Salinity of average seawater = 35 pp +/- ??

composition of major ions: +/- 0.01
evaporation: +/- 0.01
measurement of chloride: +/- 0.002
density: +/- 0.004
conductivity: +/- 0.0001
speed of sound: +/- 0.03
refractive index: +/- 0.05