? for Randy about Salinity Reference Solutions, NaCl

[ReefEnabler]

Hi Randy,

I was just going over your article on Refractometers and Salinity measurement.

I got confused by the whole salt vs seawater refractometer reading, and some numbers reported in a data table for % NaCl to reach various refractive index and PPT values.

... These are often called salt or brine refractometers. Despite the scale reading in ppt (‰) or specific gravity, they are not intended to be used for seawater. Unfortunately, many refractometers used by aquarists fall into this category. In fact, very few refractometers used by hobbyists are true seawater refractometers.

Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large. A 35 ppt sodium chloride solution (3.5 weight percent sodium chloride in water) has the same refractive index as a 33.3 ppt seawater solution, so the error in using a perfectly calibrated salt refractometer is about 1.7 ppt, or 5% of the total salinity.

Are you saying that a SALT refracometer would actually measure 35ppt using the 3.5% NaCl solution, and would be 1.7ppt off when testing a NSW sample, and vice versa (ie, SEAwater refractometer is accurate with NSW, and 1.7ppt off when testing 3.5% NaCl solution)????????????????? Or would any refrac




Basically, whats the different between a salt and seawater refractometer that would lead to different readings? Wouldn't they both suffer innacuracy testing 3.5% NaCl since they can only read refraction, not ppt (even if the results are on a ppt scale).



Would anybody happen to know if the Sybon refractometers are true seawater refractometers? I'm guessing no....


Then in this table slightly different information is provided:

NaCl (weight %)---------Refractive Index---------------(ppt)
3.3-------------------------1.3388-------------------------31.65
3.4-------------------------1.3390-------------------------32.8
3.5-------------------------1.3391-------------------------33.3
3.6-------------------------1.3393-------------------------34.4
3.65------------------------1.3394-------------------------35.0
3.7-------------------------1.3395-------------------------35.6
3.8-------------------------1.3397-------------------------36.7

In this table the 35ppt solution is 3.65% NaCl.

Why is the information different? Are the PPT values simply corrected to match NSW at the given Refractive Indices?

I am able to find some 35ppt NaCl solutions, like this one from DIY reef: https://diyreef.authsecure.com/shop/product_info.php?products_id=36

But how do I know if that is a solution thats 3.5% or 3.65% NaCl??


Thanks for clearing up any confusion!

Ryan

 

[bertoni]

Any refractometer made and calibrated for NaCl solution testing will be off by 1.7 ppt when reading a NSW solution. Likewise, a NSW refractomer would be off by about 1.7 ppt when reading a 3.5% NaCl solution.

The product at your link is supposed to be about 3.65% NaCl, and, if so, is fine for calibrating your refractometer for your tank. The Sybon refractometers likely are for NaCl testing, but I don't know for sure. They're all fine, if calibrated with an appropriate solution.

 

[Boomer]

There is no such thing as a Seawater refract despite what they may say. They are all saline refract's which is not Seawater. You can buy very expensive Digital refracts that can do may solution tests to include Seawater in one meter. ~$600

Basically, whats the different between a salt and seawater refractometer that would lead to different readings?

Salt is on dealing with two ions Na+ and Cl - and Seawater has many many ions all which cause a shift in the RI to be different for NaCl. And the scale you read in the refract window would be different.

Your

Saline Standard, 35ppt (1.0264 specific gravity) 2 oz bottle

Would not tell you much with a cheap chinese refract, accept it can be cal to 35 ppt NaCl. A good lab grade one that ref would be fine as you know it will be off 1.7. This cheap units are often way off and need to be cal with PinPoint 53 mS to read 35 ppt or 1.0264.


The table you posted is for NaCl and the table (below) in the article you linked is for Seawater.

Note that at 35 ppt NaCl the RI = 1.33910

Note that at 35 ppt NSW the RI = 1.33940


<tbody><tr bgcolor="#006699">
<td colspan="3">
<div align="center"><font color="#ffffff" size="-1" face="Georgia, Times New Roman, Times, serif"><b>Table
2. Specific gravity and refractive index as a
function of seawater’s salinity of seawater. The
darker blue rows represent the range usually encountered
in the open ocean.</b></font></div>
</td>
</tr>
<tr bgcolor="#ccecff">
<td width="114" height="26">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif"><a set="yes" linkindex="26" href="http://web.archive.org/web/20030104132911/http:/ioc.unesco.org/oceanteacher/resourcekit/M3/Converters/SeaWaterEquationOfState/Sea%2BWater%2BEquation%2Bof%2BState%2BCalculator.htm" target="_blank">Salinity
(ppt)</a></font></div>
</td>
<td width="202" height="26">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif"><a set="yes" linkindex="27" href="http://web.archive.org/web/20030104132911/http:/ioc.unesco.org/oceanteacher/resourcekit/M3/Converters/SeaWaterEquationOfState/Sea%2BWater%2BEquation%2Bof%2BState%2BCalculator.htm" target="_blank">Specific
Gravity at 25° C</a></font></div>

</td>
<td width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif"><a set="yes" linkindex="28" href="/issues/2004-06/rhf/index.php" target="_blank">Refractive
Index (20° C)</a></font></div>
</td>
</tr>
<tr>
<td bgcolor="#ccecff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">0</font></div>
</td>

<td bgcolor="#ccecff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0000</font></div>
</td>
<td bgcolor="#ccecff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.33300</font></div>
</td>
</tr>
<tr>
<td bgcolor="#ccecff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">30</font></div>

</td>
<td bgcolor="#ccecff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0226</font></div>
</td>
<td bgcolor="#ccecff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.33851</font></div>
</td>
</tr>
<tr>
<td bgcolor="#ccecff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">31</font></div>

</td>
<td bgcolor="#ccecff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0233</font></div>
</td>
<td bgcolor="#ccecff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.33869</font></div>
</td>
</tr>
<tr>
<td bgcolor="#ccecff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">32</font></div>

</td>
<td bgcolor="#ccecff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0241</font></div>
</td>
<td bgcolor="#ccecff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.33886</font></div>
</td>
</tr>
<tr>
<td bgcolor="#ccecff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">33</font></div>

</td>
<td bgcolor="#ccecff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0249</font></div>
</td>
<td bgcolor="#ccecff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.33904</font></div>
</td>
</tr>
<tr>
<td bgcolor="#95d8ff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">34</font></div>

</td>
<td bgcolor="#95d8ff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0256</font></div>
</td>
<td bgcolor="#95d8ff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.33922</font></div>
</td>
</tr>
<tr>
<td bgcolor="#95d8ff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">35</font></div>

</td>
<td bgcolor="#95d8ff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0264</font></div>
</td>
<td bgcolor="#95d8ff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.33940</font></div>
</td>
</tr>
<tr>
<td bgcolor="#95d8ff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">36</font></div>

</td>
<td bgcolor="#95d8ff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0271</font></div>
</td>
<td bgcolor="#95d8ff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.33958</font></div>
</td>
</tr>
<tr>
<td bgcolor="#ccecff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">37</font></div>

</td>
<td bgcolor="#ccecff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0279</font></div>
</td>
<td bgcolor="#ccecff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.33976</font></div>
</td>
</tr>
<tr>
<td bgcolor="#ccecff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">38</font></div>

</td>
<td bgcolor="#ccecff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0286</font></div>
</td>
<td bgcolor="#ccecff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.33994</font></div>
</td>
</tr>
<tr>
<td bgcolor="#ccecff" width="114">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">39</font></div>

</td>
<td bgcolor="#ccecff" width="202">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.0294</font></div>
</td>
<td bgcolor="#ccecff" width="197">
<div align="center"><font size="-1" face="Georgia, Times New Roman, Times, serif">1.34012</font></div>
</td>
</tr>

</tbody>

 

[Flipper62]

<a href=showthread.php?s=&postid=14337173#post14337173 target=_blank>Originally posted</a> by bertoni
Any refractometer made and calibrated for NaCl solution testing will be off by 1.7 ppt when reading a NSW solution. Likewise, a NSW refractomer would be off by about 1.7 ppt when reading a 3.5% NaCl solution.

The product at your link is supposed to be about 3.65% NaCl, and, if so, is fine for calibrating your refractometer for your tank. The Sybon refractometers likely are for NaCl testing, but I don't know for sure. They're all fine, if calibrated with an appropriate solution.

Bertoni.....My findings over the past few years are, the refractometers made for testing salinity...seem to all be accurate if using the same means for the calibration.

If using the PinPoint 53.0mS Calibration Fluid......The salinity will read 35 / 1.0265 on the refractometer

NOW...almost every LFS here in the Chicago land area say that they calibrate there refractometer with RO water.......

So, when They say that my salinty is 1.025.....I see 1.023.

Bottom line............calibrate the refractometer with the PinPoint Fluid.........OR....just RO water.....What is Correct ??

 

[Boomer]

So, when They say that my salinty is 1.025.....I see 1.023.

I would not do that as we have found some of those cheap refracts to be off .000 - .004 or 0 -4 ppt in RO

 

[Randy Holmes-Farley]

Bottom line............calibrate the refractometer with the PinPoint Fluid.........OR....just RO water.....What is Correct ??

If you can make my DIY with accurate measurements (like a balance), that is best. The Pinpoint, assuming it is made correctly and has not been tampered with, will also be perfect. RO water is likely not perfect.