Salinity - Homemade Calibration Standards

BradB

Active Member
View Badges
Joined
Aug 27, 2009
Messages
413
Reaction score
242
Review score
+0 /0 /-0
Location
Hudson
Over 20 years in the hobby, I have never been happy with how I test salinity. I recently switched from using a conductivity probe double checked with my old refractometer to trying to calibrate my probe with a homemade salinity solution. This should work nicely as long as we are close and stay consistent - I don't care if my salinity is really 34 or 36 ppt as long as it stays the same - including when I change brands of table salt.

I read your article http://reefkeeping.com/issues/2004-06/rhf/index.htm and several posts on this forum but had some questions.

You suggest approximating a 3.65 by weight percent "by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified fresh water".

You then suggest a 3.7 by weight percent (or 3.714) solution made from 6.2g Morton's salt in 161g fresh water for specific gravity.

You then suggest a 3.29 by weight percent solution from 6.2g Morton's salt in 182m fresh water to calibrate probes.

Why use over 12% more salt to calibrate by specific gravity than conductivity? Does the same ocean water weigh 1.0264 as much as fresh water and measure 53 mS/cm? Or are the "standards" from different places, such as conductivity being measured in the Atlantic and specific gravity in the Red Sea? If the same ocean water has both properties, than I would assume the ions besides sodium chloride play a significant role, and pure sodium chloride, Morton's and Kosher salts are not interchangeable. Why advise that we can use either? What about the water content of salt made for food?

Is there a reason "Morton's salt" and "fresh water" are appropriate for specific gravity and conductivity but "sodium chloride" and "purified fresh water" are needed for a refractometer? Does a refractometer require lab grade ingredients, or am I being to literal on the order the article was written?
 
Champion Lighting & Supply

Randy Holmes-Farley

Reef Chemist
View Badges
Joined
Sep 5, 2014
Messages
44,831
Reaction score
33,714
Review score
+0 /0 /-0
Location
Arlington, Massachusetts, United States
None of these devices measure salinity directly. They all measure something else.

That something else can be refractive index, density, or electrical conductivity (and there are others).

Seawater has particular values of each of these things at a given temperature as a function of salinity, so knowing one of them (say, refractive index at 25 deg C) can tell you the salinity.

I have provided the amount of pure sodium chloride in pure water that matches each of those values for 35 ppt seawater. 35 ppt NaCl does not exactly match ANY properties of 35 ppt seawater (except perhaps the dried residual mass, but even that is not exactly matching for complex reasons), because it has a different chemical composition.

For example, 3.65% (36.5 ppt) NaCl in water has the same refractive index as 35 ppt seawater. It DOES NOT have the same density or electrical conductivity as 35 ppt seawater. Different concentrations are needed to match those.

The Mortons is just an example I used for folks to be able to use volume measurements without a scale for weighing mass, since different types of sodium chloride or table salt will weigh different amounts for different volumes (that is, they have different powder bulk densities). Any table salt is fine in these recipes if weighed out, as is pure NaCl. The added ingredients in table salt to prevent clumping and such are at such low levels that they will not impact hobby devices.
 
OP
B

BradB

Active Member
View Badges
Joined
Aug 27, 2009
Messages
413
Reaction score
242
Review score
+0 /0 /-0
Location
Hudson
Thanks! Until I saw your answer today, I assumed table salt was closer to evaporated sea water than pure NaCl. So what you wrote made little sense.

The Internet says they actual refine table salt to 97% purity. Which makes no sense because they charge more for gourmet "sea salt" which should be a lot cheaper to make and everyone swears tastes exactly the same as the cheap stuff. Humans are weird.
 

neon_reefer

Active Member
View Badges
Joined
Mar 3, 2021
Messages
133
Reaction score
71
Review score
+0 /0 /-0
Location
Los Angeles covid county
Over 20 years in the hobby, I have never been happy with how I test salinity. I recently switched from using a conductivity probe double checked with my old refractometer to trying to calibrate my probe with a homemade salinity solution. This should work nicely as long as we are close and stay consistent - I don't care if my salinity is really 34 or 36 ppt as long as it stays the same - including when I change brands of table salt.

I read your article http://reefkeeping.com/issues/2004-06/rhf/index.htm and several posts on this forum but had some questions.

You suggest approximating a 3.65 by weight percent "by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified fresh water".

You then suggest a 3.7 by weight percent (or 3.714) solution made from 6.2g Morton's salt in 161g fresh water for specific gravity.

You then suggest a 3.29 by weight percent solution from 6.2g Morton's salt in 182m fresh water to calibrate probes.

Why use over 12% more salt to calibrate by specific gravity than conductivity? Does the same ocean water weigh 1.0264 as much as fresh water and measure 53 mS/cm? Or are the "standards" from different places, such as conductivity being measured in the Atlantic and specific gravity in the Red Sea? If the same ocean water has both properties, than I would assume the ions besides sodium chloride play a significant role, and pure sodium chloride, Morton's and Kosher salts are not interchangeable. Why advise that we can use either? What about the water content of salt made for food?

Is there a reason "Morton's salt" and "fresh water" are appropriate for specific gravity and conductivity but "sodium chloride" and "purified fresh water" are needed for a refractometer? Does a refractometer require lab grade ingredients, or am I being to literal on the order the article was written?
for actual precision salinity readings, I use a tropical marine hydrometer. the best ever!!

I fall back on the refractometers and digital meters as backup, or for different uses..

If Im checking the water Im mixing, Ill use the digital, then verify with the hydrometer, If I want to check the tank salinity, I will just use the hydrometer.
 
OP
B

BradB

Active Member
View Badges
Joined
Aug 27, 2009
Messages
413
Reaction score
242
Review score
+0 /0 /-0
Location
Hudson
for actual precision salinity readings, I use a tropical marine hydrometer. the best ever!!

I fall back on the refractometers and digital meters as backup, or for different uses..

If Im checking the water Im mixing, Ill use the digital, then verify with the hydrometer, If I want to check the tank salinity, I will just use the hydrometer.
This hydrometer is cheap enough and could work well if it is accurate and cannot change. I like the probe since I can tell at a glance where things are - the probe starts to wander over time, and if I don't recalibrate, my salinity wanders too. The Kosher salt calibration works pretty well too, as long as I always remember 3.29%.
 

neon_reefer

Active Member
View Badges
Joined
Mar 3, 2021
Messages
133
Reaction score
71
Review score
+0 /0 /-0
Location
Los Angeles covid county
This hydrometer is cheap enough and could work well if it is accurate and cannot change. I like the probe since I can tell at a glance where things are - the probe starts to wander over time, and if I don't recalibrate, my salinity wanders too. The Kosher salt calibration works pretty well too, as long as I always remember 3.29%.
I have a hanna salinity with temp probe, a refractometer, and the hydrometer, the most accurate is the hydrometer. it's been my go to for precision reading. Ive seen the cheap hydrometers your probably thinking of...

This one is made in Germany. check it out,,

 

What is your most favorite type of shrimp for your reef tank?

  • Peppermint Shrimp

    Votes: 18 4.1%
  • Banded Coral Shrimp

    Votes: 39 8.9%
  • Snapping or Pistol Shrimp

    Votes: 40 9.1%
  • Harlequin Shrimp

    Votes: 40 9.1%
  • Scarlet Skunk Cleaner Shrimp

    Votes: 136 31.1%
  • Blood Red Fire Shrimp

    Votes: 90 20.5%
  • Sexy Anemone Shrimp

    Votes: 30 6.8%
  • Camel Shrimp

    Votes: 1 0.2%
  • Other (please explain)

    Votes: 10 2.3%
  • NO shrimp in my tank

    Votes: 34 7.8%
Reef Kinetics
Top