Extremely frustrated by the salinty measuring tools available. Does anyone REALLY know what their salinity level are?

IslandLifeReef

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So the salinity being off by 1 or 2 points doesn't matter? This is a surprising answer to be honest. I always figured to have the best chance of success you really need to have a stable/accurate salinity in the tank. When you mention that constant proportions of seawater don't apply in aquariums, why is that?
@Ari Almassi, I do think that we all can get caught up in the weeds a bit and try to control parameters too much. Have you ever snorkeled or dived on the reefs on the Big Island of Hawaii? If you ever do, you will notice that there is an abundance of fresh water coming from the lava rock beneath the reef from rain run-off. The salinity difference is so great, you can see the fresh water vs the salt water. Even in that environment, there are corals that are flourishing and tons of fish. So, I don't think your inhabitants will care too much if your salinity is 34 ppt or 36 ppt. As long as it stays consistent, which it should with an ATO, you will be fine.
 

Koigula

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Funny how this is trying to make a joke about "purists" or something but in actuality I'm talking about how I'm trying to find an accurate tool in the digital age. I'm not an old man trying to saying that we need to go back to the carburetor. I'm saying that the quality leaves a lot to be desired.
I have struggled with this in the past. In this case digital readings are less accurate for hobby grade equipment. I was not thrilled with the rebranded 35 usd refractomers either.

I settled on the VeeGee STX-3 refractometer. It is much clearer and easier to read. I do find value in some Hanna digital meters but definately not touching digital salinity meters. again They last a year and are innaccurate but get glowing reviews.


https://www.veegee.com/products/handheld-salinity-refractometer

A bright incandenscent bulb in a utility room is needed to get a good reading.


https://aquacraft.net/products-page-2/equipment/refractometer-laboratory-standard/

Salinity standard are suspect as well! I have had luck with this one!

Topic of stability only matters for SPS tanks in all likelyhood. I have seen some cray tanks from 90s for soft corals with no clue what they were doing.
 
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ReefRondo

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I started with a fluval plastic swing arm, then got a very cheap and terrible refractometer, better refractometer, better refractometer, digital milwaukee refractometer and just this week the hanna salinity checker. All have their issues but this new hanna one wins for me so far purely for ease of use. Calibrate, stick it in the sample, done. Will of course update if I find issues.
 

snorklr

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with all the talk about salinity lately i decided not to trust my 90's instant ocean swingarm...so i bought the tropic marin hydrometer...when it read 1.0245 my swingarm was 1.0244... paul b took his swingarm diving with him and just put a sharpie mark at the salinity where he was...
 

Laith

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You are not supposed to put them in the tank, you are supposed to take Aquarium water, pour it into some tall vessel like a graduated cylinder, and put the hydrometer in there, take the reading, then pour the water back into the tank

Exactly. I use the hydrometer every once in a while just to double check that my Hanna is reading more or less correctly. Used correctly, a well calibrated hydrometer gives the most reliable results because it uses basic physics to give you a reading.
 

Kris 2020

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I like Refractometers



They are temperature sensitive, so Milwaukee and I assume hanna use more water than the one I linked, so you have to wait for the sample to come to temp equilibrium with the instrument. (the instrument compensates for temp at the instrument temp, not the water sample temp.)
A drop on the vee gee comes to temp immediately.
Use rodi to check zero every month.

I have this same refractometer. I also have 3 different calibration solutions that read the same. I calibrate it once or twice a month, using different solutions to make sure. This refractometer is so much better than my brs one was. I rarely have to adjust it. With the brs one I calibrated every time I used it and often had to make adjustments.
 

Michael E.

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This was my obsession last month. I had an ice cap salinity stick. The first gen. It would read 1.29 on 1.026 water. So, I was running a system with low salt. It was frustrating to find out. Once I did, I went down the rabbit hole of salinity. I Tested multiple standards, refractometers, etc. they were all slightly off. It was hard to find consistency.
I ended up settling on a Hanna conductivity meter. Going off of mS/cm was easier for me. It also satisfied that OCD side. I’ve tested it against an Orion setup we have at work, and it is right there with it . I’ve also been pretty
Lucky to be able to get my hands on some pretty reliable standards that I can reference.
B20273A0-5AF1-4BD8-82AC-65D9A98C22C6.jpeg

Otherwise, I just try to focus on keeping everything stable and consistent.

good luck!
 

Stellar Aquatics

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Whichever you select, take it to a lfs and have them test it more than once. I use the swing arm hydrometer over a year, had them test it 1.025 reads 1.023, I put a mark where I wanted it to be, and It reads the same every time. Doesn't need calibrated, just dip it and rinse for next time.
 

gbroadbridge

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I have tried The Hannah salinity tester (actually I have 2 of them), the brs refractometer, my lfs refractometer. I can't find a tool that makes me feel 100% confident in my salinity. The drift that I have from my Hannah is absurd. After 3 days of use and another calibration my salinity was apparently 4 points higher than what it had originally told me was 35 ppt. I honestly would calibrate the meter every time I used it if it didn't cost $1 per packet of calibration fluid! The refractometers need to be backlit by the sun and the line value changes based on what angle you are looking through it.

Then there is that Milwaukee Instruments meter with amazing reviews, but the issue is that it only has a resolution of 1ppt and an error of +/- 2 ppt! How do you know if your salinity isn't 33 or 37 ppt? How does it have such amazing reviews when your salinity could in actuality be between a range of 33-37 ppt?

Glass hydrometers are a pain because I'd have to completely turn off all my flow every time I want to check salinity.

My desperation is making me look into industrial solutions. I found a $500 industrial salinity meter at Grainger that I might have to suck up and buy. It's so difficult to measure one of the most fundamental parts of this hobby. We have tools for flow, Par, and everything else. No one has been able to make a salinity meter that actually works and you can be completely confident in.

Does anyone have thoughts on this? How do you measure the salinity for your tank? How do public aquariums and food manufacturing facilities measure the salinity of their water? That's what I want. Confidence I can put money on. In this case, its thousands of dollars of quarantined fish and coral.

Grainger meter
A 100ml volumetric flash, an accurate scale, and an accurate thermometer works.

Consistent measurement technique is more important than the absolute value anyway
 

Cory

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You must make your own salinity standard. Heres Randys diy:


Notice added post-publication: the standards described here that use Coke bottles are subject to variation in the volume of a 2-L Coke bottle. It has recently come to my attention that such 2-L bottles can vary in total volume, and that this can lead to at least a 1 ppt error in the salinity of the standards matched to seawater salinity of 35 ppt. Standards made with accurate measurements of salt and water should still accurately match 35 ppt.]]

Refractometer Standard

It is widely believed that only pure water is required to calibrate refractometers. That fact is true of many refractometers, and is certainly appropriate for routine calibration, but it assumes that they were manufactured correctly and have not been damaged since manufacturing. As refractometers used by aquarists become less and less expensive (with some now selling for less than $30), there is every reason to believe that at some point they will no longer be accurate enough.

The only way to be sure that a given refractometer gives useful information is to check its accuracy in a solution similar to aquarium water. I believe that all refractometers should be checked in this fashion when first purchased, and again any time there is a reason to be concerned. For example, an aquarist might be concerned if an aquarium that had been running for years at a salinity of 35 ppt suddenly reads 39 ppt.

In order to provide a standard for refractometers, a solution whose refractive index is similar to normal seawater is required. Seawater with S= 35 has a refractive index of 1.3394.1 Likewise, the refractive index of different sodium chloride solutions can be found in the scientific literature. My CRC Handbook of Chemistry and Physics (57th Edition, Page D-252)4 has such a table. That table has entries for 3.6 and 3.7 weight percent solutions of sodium chloride that span the value for normal seawater. Interpolating between these data points suggests that a solution of 3.65 weight percent sodium chloride has the same refractive index as S=35 seawater, and can be used as an appropriate standard (Table 2).

Table 2.� Refractive Index as a function of the concentration of a sodium chloride solution.1,4 The darker blue row represents the standard.

Sodium Chloride Concentration (weight %)

Refractive Index

Salinity (PSU)

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

This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater.

For a rougher measurement in the absence of an accurate water volume or weight measurement:

1. Measure ¼ cup of Morton's Iodized Salt (about 73.1 g)
2. Add 1 teaspoon of salt (making about 79.3 g total salt)
3. Measure the full volume of a plastic 2-L Coke or Diet Coke bottle filled with purified freshwater (about 2104.4 g)
4. Dissolve the total salt (79.3 g) in the total water volume (2104 g) to make an approximately 3.65 weight percent solution of NaCl. The volume of this solution will be slightly larger than the Coke bottle, so dissolve it in another container.

[[Notice added post-publication: the standards described here that use Coke bottles are subject to variation in the volume of a 2-L Coke bottle. It has recently come to my attention that such 2-L bottles can vary in total volume, and that this can lead to at least a 1 ppt error in the salinity of the standards matched to seawater salinity of 35 ppt. Standards made with accurate measurements of salt and water should still accurately match 35 ppt.]]

How to Use a Refractive Index Standard

One simple way to use this refractive index standard is to measure it with a refractometer, and just remember what setting the standard came to. That setting represents S=35 seawater, with all of the properties shown in Table 1. Hopefully, the reading of the refractometer at that point will be similar to the properties in Table 1 (specific gravity = 1.026 - 1.027, or S=35, depending on the units). Simply using it as the target salinity for the aquarium is a fine way to go.

Alternatively, one can actually calibrate the refractometer using the standard by adjusting it until it reads the appropriate setting indicated in Table 1. Exactly how to adjust it depends on the refractometer, but often it is as simple as turning a screw.

Specific Gravity Standard

Most aquarists recognize that inexpensive hydrometers are often prone to error. In some cases, inaccuracy is due to poor manufacturing, and in other cases it is due to poor usage by aquarists. In a previous article I tested several hydrometers and found variable results, from good to marginal. Beyond the inherent accuracy of the measurement is the confusing problem of how specific gravity relates to the temperature of the measurement, an issue which I detailed in that same article.

The best way to be sure that a given hydrometer is giving accurate information is to check its accuracy in a solution with a density (specific gravity) similar to the aquarium water. In order to provide a standard for hydrometers, a solution of a similar specific gravity to normal seawater is required. Seawater with S= 35 has a specific gravity of about 1.0264 (Tables 1 and 3).

Table 3.� Density and specific gravity as a function of salinity of seawater.3 The darker blue rows represent the range usually encountered in the open ocean.

Salinity (PSU)

Density (25° C)

Specific Gravity (25° C)

0

997.05

1.0000

29

1018.8

1.0218

30

1019.6

1.0226

31

1020.3

1.0233

32

1021.1

1.0241

33

1021.8

1.0249

34

1022.6

1.0256

35

1023.3

1.0264

36

1024.1

1.0271

37

1024.9

1.0279

38

1025.6

1.0286

39

1026.4

1.0294

In order to match this specific gravity to a standard solution made from sodium chloride, look up the density of different sodium chloride solutions in the scientific literature. My CRC Handbook of Chemistry and Physics (57th Edition, Page D-252)4 has such a table (partially reproduced in Table 4), but it has data only for 20°C (68°F). Specific gravity at 20°C is then easily calculated by dividing the density of the solutions by the density of water at the same temperature. This table (4) can then be compared to seawater at 20°C (Table 5). The primary purpose of showing specific gravity at 25°C (77°F; Tables 1 and 3) and 20°C (Table 4) is to show that specific gravity does not change much with temperature (1.0264 vs. 1.0266). Nevertheless, it is only the 20°C data that will be used to devise a standard.

The table in the CRC Handbook has entries for 3.7 and 3.8 weight percent solutions of sodium chloride that span the specific gravity value for normal seawater. Interpolating between these data points suggests that a solution of 3.714 weight percent sodium chloride has the same specific gravity (and density) as S=35 seawater, and can be used as an appropriate specific gravity standard (Table 5). For most purposes, 3.7 weight percent is accurate enough.

Table 4.� Density and specific gravity as a function of salinity of seawater at 20° C.4 The darkened blue rows represent the range usually encountered in the open ocean.

Salinity (PSU)

Density (25° C)

Specific Gravity (25° C)

0

988.2

1.0000

29

1020.2

1.0220

30

1021.0

1.0228

31

1021.7

1.0236

32

1022.5

1.0243

33

1023.2

1.0251

34

1024.0

1.0258

35

1024.8

1.02660

36

1025.5

1.0274

37

1026.3

1.0281

38

1027.1

1.0289

39

1027.8

1.0297


Table 5.� Specific gravity as a function of the concentration of sodium chloride in water. The values in the medium blue boxes are interpolated and the darker blue row represents the standard.3,4

Sodium Chloride Concentration (weight %)

Specific Gravity
at 20° C

Salinity

3.4

1.0243

32.0

3.5

1.0250

32.9

3.6

1.0257

33.8

3.7

1.0265

34.8

3.71

1.02657

34.9

3.714

1.02660

35.0

3.72

1.02664

35.1

3.73

1.02671

35.1

3.74

1.02678

35.2

3.8

1.0272

35.8

To produce a 3.714 weight percent sodium chloride solution, dissolve 1 teaspoon (6.20 grams) of Morton's Iodized Salt in 161 mL (161 g) of freshwater (making a total volume of about 163 mL after dissolution of the salt). This solution can be scaled up as desired.

For a rougher measurement in the absence of an accurate water volume measurement:

1. Measure ¼ cup of Morton's Iodized Salt (about 73.1 g)
2. Add 1½ teaspoon of salt (making about 82.4 g total salt)
3. Measure the full volume of a plastic 2-L Coke or Diet Coke bottle filled with purified freshwater (about 2104.4 g)
4. Add an additional 2 tablespoons of purified freshwater (about 30 g)
5. Dissolve the total salt (82.4 g) in the total water volume (2134.4 g) to make an approximately 3.7 weight percent solution of NaCl. The volume of this solution is larger than the Coke bottle, so dissolve it in another container.

[[Notice added post-publication: the standards described here that use Coke bottles are subject to variation in the volume of a 2-L Coke bottle. It has recently come to my attention that such 2-L bottles can vary in total volume, and that this can lead to at least a 1 ppt error in the salinity of the standards matched to seawater salinity of 35 ppt. Standards made with accurate measurements of salt and water should still accurately match 35 ppt.]]
 

flyfisher2

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Refractometer and Milwaukee Digital Refractometer both give me 1.026 based on calibration with RO water.
swing arm hydrometers are a joke in my experience.
Consistency is key so if I’m consistently off 1 or 2 points Im still good.
Don’t see the need for being exact to a specific part of an ocean?
My corals come from well known vendors and my readings of their water fluctuates from 1.025 up through 1.029.
They are are growing corals successfully and supplying the chain
That’s good enough for me
If someone can show me a swing arm that is consistent and close to my Refractometer I’d consider it for the ease of use.
Haven’t seen one yet.
 

Atrumblood

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Great post, but I prefer a simpler approach to checking if my equipment is calibrated properly.

That is to take a known volume of water and weigh it. You will need a balance sensitive to at least 0.1 grams but those are easy to get.

One thing about all of the salinity checkers out there is they are all temperature sensitive. I use the Milwaukee salinity meter and I've noticed that the temperature the unit is reporting directly affects the reading. I take pains to ensure the meter is at the same temperature as my tank, or at least as close as I can get. Since the unit is water proof, I will dip the end with the optics into my tank for a few minutes to bring the system up to my operating temperature. My reading will be off by as much as 0.002 if I simply wipe the lense and put the sample on and take a reading.
 

Reef.

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The primary purpose of showing specific gravity at 25°C (77°F; Tables 1 and 3) and 20°C (Table 4) is to show that specific gravity does not change much with temperature (1.0264 vs. 1.0266). Nevertheless, it is only the 20°C data that will be used to devise a guy

Didn’t realise a 5c temp difference would only show a 0.002sg difference.

I’ll check that next time I’m mixing salt as I normally try hard to get the temp exactly 25c when testing the salinity with my hydrometer.
 
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flyfisher2

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Great post, but I prefer a simpler approach to checking if my equipment is calibrated properly.

That is to take a known volume of water and weigh it. You will need a balance sensitive to at least 0.1 grams but those are easy to get.

One thing about all of the salinity checkers out there is they are all temperature sensitive. I use the Milwaukee salinity meter and I've noticed that the temperature the unit is reporting directly affects the reading. I take pains to ensure the meter is at the same temperature as my tank, or at least as close as I can get. Since the unit is water proof, I will dip the end with the optics into my tank for a few minutes to bring the system up to my operating temperature. My reading will be off by as much as 0.002 if I simply wipe the lense and put the sample on and take a reading.
I had no idea the unit is waterproof. I’ll try and keep mine closer to the aquarium in a more conventional way in order to achieve equal temps.
 

Craig77

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I have tried The Hannah salinity tester (actually I have 2 of them), the brs refractometer, my lfs refractometer. I can't find a tool that makes me feel 100% confident in my salinity. The drift that I have from my Hannah is absurd. After 3 days of use and another calibration my salinity was apparently 4 points higher than what it had originally told me was 35 ppt. I honestly would calibrate the meter every time I used it if it didn't cost $1 per packet of calibration fluid! The refractometers need to be backlit by the sun and the line value changes based on what angle you are looking through it.

Then there is that Milwaukee Instruments meter with amazing reviews, but the issue is that it only has a resolution of 1ppt and an error of +/- 2 ppt! How do you know if your salinity isn't 33 or 37 ppt? How does it have such amazing reviews when your salinity could in actuality be between a range of 33-37 ppt?

Glass hydrometers are a pain because I'd have to completely turn off all my flow every time I want to check salinity.

My desperation is making me look into industrial solutions. I found a $500 industrial salinity meter at Grainger that I might have to suck up and buy. It's so difficult to measure one of the most fundamental parts of this hobby. We have tools for flow, Par, and everything else. No one has been able to make a salinity meter that actually works and you can be completely confident in.

Does anyone have thoughts on this? How do you measure the salinity for your tank? How do public aquariums and food manufacturing facilities measure the salinity of their water? That's what I want. Confidence I can put money on. In this case, its thousands of dollars of quarantined fish and coral.

Grainger meter
I use a hydrometer and place it inside of an upside down clear gravel cleaner (2” diameter) in my DT. I remove the flexible hose from the gravel cleaner which makes it easy to fill, equilibrate to temp, and empty without any flow interference to the hydrometer. I keep wave makers on with no issue. I struggled quite a bit with reliability of other methods and use this method periodically together with a refractometer more frequently.
 
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