Salinity checking questions

[Arthacker87]

Alrighty I have a hydrometer refractometer and hannah salinity checker. All have different readings. My calibration liquid might be a bit old but stored well and refract reads 1.027-1.028 hannah is 1.025. I'm sure I'm not awesome w a refractometer because they tell you to wait before checking which feels weird to me. I want to check salinity at the temp I'm at right? Any info will help. Corals are fine but my nem isnt as robust as it usually is and another coral is a but off since calibrating w hannah. Maybe a coincidence but kinda annoying

 

[Ron Reefman]

The difference in SG from 1.025 to 1.028 isn't going to make any long term difference in how an anemone looks. As long as you keep the SG level stable over time.

That 1.025 to 1.028 as a sudden change may get a reaction from an anemone or some corals, but over a few hours they would stabilize and should be OK.

If you expect a hydrometer, a refractometer and a hana checker to all read the same without calibration, you'll have a hard time. New calibration fluid and using your refractometer properly is the best solution.

 

[mike550]

Maybe I’m stating the obvious but Hanna has its own calibration packages because it uses conductivity to measure salinity. Since it’s using conductivity it may also be influenced by other matter in the tank. I typically measure salinity by taking a small sample from my sump near the return pump and let it settle for a few min then take reading

Perhaps you can compare against fresh saltwater you‘ve made to see how your refractometer and Hanna compare.

 

[Miller535]

I know some will argue with this, but I am going to say it anyway. If you are totally out of calibration fluid, use RODI to calibrate your refractometer. You will just be calibrating it to zero. Some will argue that RODI will have you calibrating it off because it;s so far away from the range you are measuring. But I have just found this not be true. I DO use calibration fluid to calibrate my refractometer, BUT I have also had a bad bottle of calibration fluid from BRS which they acknowledged by the way. They promptly sent me out a new one, and it was spot on. But to confirm this, I used RODI after it, and it was spot on as well. So RODI can help you verify your calibration fluid.

Side not, some salinity meters and refractometers, they tell you in their instructions not to use for other equipment, because as others said, there's a difference between a refractometer and a conductivity meter (hanna).

 

[Miller535]

I should add, not only if you're out, but if what you have is old (and they usually have an expiration date) then try the RODI

 

[Uncle99]

Stability is the key to success. Pick one and stick with it.

 

[Randy Holmes-Farley]

Maybe I’m stating the obvious but Hanna has its own calibration packages because it uses conductivity to measure salinity. Since it’s using conductivity it may also be influenced by other matter in the tank. I typically measure salinity by taking a small sample from my sump near the return pump and let it settle for a few min then take reading

Perhaps you can compare against fresh saltwater you‘ve made to see how your refractometer and Hanna compare.

Nothing that settles from reef tank water will impact conductivity. So there's no need for that step.

i describe how conductivity works in substantial detail here (one of my first reef articles, from 2004):

AquariumFish.com :: Aquarium Fish International :: The monthly magazine for both novice and expert aquarium hobbyists.

AQUARIUM FISH MAGAZINE is THE monthly magazine for both novice and expert aquarium hobbyists. Aquarium Fish Magazine provides information on choosing, caring for, and breeding freshwater and marine fish.

web.archive.org

 

[Randy Holmes-Farley]

I know some will argue with this, but I am going to say it anyway. If you are totally out of calibration fluid, use RODI to calibrate your refractometer. You will just be calibrating it to zero. Some will argue that RODI will have you calibrating it off because it;s so far away from the range you are measuring. But I have just found this not be true. I DO use calibration fluid to calibrate my refractometer, BUT I have also had a bad bottle of calibration fluid from BRS which they acknowledged by the way. They promptly sent me out a new one, and it was spot on. But to confirm this, I used RODI after it, and it was spot on as well. So RODI can help you verify your calibration fluid.

The reason you give is only part of, and perhaps not the most important reason to use 35 ppt calibration fluid.

Yes, what you state is an issue if the refractometer is not perfect. It is reason enough to do it.

BUT even if the refractometer is PERFECTLY made and functioning, many MUST read incorrectly if they are not true seawater refractometers. Many if not most hobby refractometers are brine refractometers. They are made for measuring NaCl, not seawater. So they will be wrong if functioning correctly and calibrated with RO/DI.

From the article below:

"If you have this type of refractometer, and it was perfectly made and calibrated in freshwater, it will ALWAYS read seawater to be higher in salinity than it actually is (misreporting an actual 33.3 ppt to be 35 ppt)."

Perhaps your refractometer is a true seawater refractometer, or perhaps it is broken in just the right way to read right, or perhaps your requirements for accuracy are not high enough to see the difference.

I address these issues here:

Refractometers and Salinity Measurement by Randy Holmes-Farley - Reefkeeping.com

from it:

Imperfect Refractometer Use: Scale Misunderstanding and Salt Refractometers
Refractometers can lead to incorrect readings in additional ways and, again, these issues abound for reef aquarists. One is that many refractometers are intended to measure sodium chloride solutions, not seawater. 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. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an appropriate salinity in the first place. Figure 23 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in salinity. This figure shows the measured salinity reading for seawater being about 1.7 ppt higher than it really is.

It turns out that this is a slope miscalibration in the sense that a perfectly made sodium chloride refractometer necessarily has a different relationship between refractive index and salinity than does seawater. This type of problem with a refractometer IS NOT at all corrected by calibrating it with pure freshwater. If you have this type of refractometer, and it was perfectly made and calibrated in freshwater, it will ALWAYS read seawater to be higher in salinity than it actually is (misreporting an actual 33.3 ppt to be 35 ppt).
Even more confusing, but perhaps a bit less of a problem in terms of the error's magnitude, salt refractometers sometimes read in specific gravity. But that value is specific gravity of a sodium chloride solution with the measured refractive index, not seawater with that refractive index. A sodium chloride solution with the same refractive index as 35 ppt seawater (which turns out to be 36.5 ppt sodium chloride) has a specific gravity matching 34.3 ppt seawater. So this type of refractometer, when perfectly calibrated, will read the specific gravity of 35 ppt seawater to be a bit low, at 1.0261 instead of about 1.0264. That error (reading 0.0003 or so too low) is, however, probably less than most reef aquarists are concerned with. Figure 24 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in specific gravity. This figure shows the measured salinity reading for seawater being about 0.0003 lower than it really is.

Regardless of a salt refractometer's scale reading (ppt or specific gravity), aquarists can get around this problem by calibrating this type of refractometer in a seawater standard (see below). Because that type of calibration also gets around important manufacturing errors (slope calibration defects due to the scale being the wrong dimensions), it solves both problems at once.

 

[Miller535]

The reason you give is only part of, and perhaps not the most important reason to use 35 ppt calibration fluid.

Yes, what you state is an issue if the refractometer is not perfect. It is reason enough to do it.

BUT even if the refractometer is PERFECTLY made and functioning, many MUST read incorrectly if they are not true seawater refractometers. Many if not most hobby refractometers are brine refractometers. They are made for measuring NaCl, not seawater. So they will be wrong if functioning correctly and calibrated with RO/DI.

From the article below:

"If you have this type of refractometer, and it was perfectly made and calibrated in freshwater, it will ALWAYS read seawater to be higher in salinity than it actually is (misreporting an actual 33.3 ppt to be 35 ppt)."

Perhaps your refractometer is a true seawater refractometer, or perhaps it is broken in just the right way to read right, or perhaps your requirements for accuracy are not high enough to see the difference.

I address these issues here:

Refractometers and Salinity Measurement by Randy Holmes-Farley - Reefkeeping.com

from it:

Imperfect Refractometer Use: Scale Misunderstanding and Salt Refractometers
Refractometers can lead to incorrect readings in additional ways and, again, these issues abound for reef aquarists. One is that many refractometers are intended to measure sodium chloride solutions, not seawater. 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. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an appropriate salinity in the first place. Figure 23 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in salinity. This figure shows the measured salinity reading for seawater being about 1.7 ppt higher than it really is.

It turns out that this is a slope miscalibration in the sense that a perfectly made sodium chloride refractometer necessarily has a different relationship between refractive index and salinity than does seawater. This type of problem with a refractometer IS NOT at all corrected by calibrating it with pure freshwater. If you have this type of refractometer, and it was perfectly made and calibrated in freshwater, it will ALWAYS read seawater to be higher in salinity than it actually is (misreporting an actual 33.3 ppt to be 35 ppt).
Even more confusing, but perhaps a bit less of a problem in terms of the error's magnitude, salt refractometers sometimes read in specific gravity. But that value is specific gravity of a sodium chloride solution with the measured refractive index, not seawater with that refractive index. A sodium chloride solution with the same refractive index as 35 ppt seawater (which turns out to be 36.5 ppt sodium chloride) has a specific gravity matching 34.3 ppt seawater. So this type of refractometer, when perfectly calibrated, will read the specific gravity of 35 ppt seawater to be a bit low, at 1.0261 instead of about 1.0264. That error (reading 0.0003 or so too low) is, however, probably less than most reef aquarists are concerned with. Figure 24 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in specific gravity. This figure shows the measured salinity reading for seawater being about 0.0003 lower than it really is.

Regardless of a salt refractometer's scale reading (ppt or specific gravity), aquarists can get around this problem by calibrating this type of refractometer in a seawater standard (see below). Because that type of calibration also gets around important manufacturing errors (slope calibration defects due to the scale being the wrong dimensions), it solves both problems at once.

You are correct, it did not even occur to me that someone would use a refractometer that was not a seawater refractometer.

As I stated in my other comment, I use 35ppt calibration solution. And I always think it should be used. I just think RODI is a good double check. If I calibrate with my 35 ppt solution as then check rodi water and it is really off, then that's a red flag, TO ME. Like I said, I had a bad bottle of solution from BRS that their support acknowledged was bad. Also as you pointed out it would show that there was something wrong with that refractometer itself. When I had that bad solution, i actually bought a second seawater refractometer thinking my first one was bad.

 

[Randy Holmes-Farley]

You are correct, it did not even occur to me that someone would use a refractometer that was not a seawater refractometer.

As I stated in my other comment, I use 35ppt calibration solution. And I always think it should be used. I just think RODI is a good double check. If I calibrate with my 35 ppt solution as then check rodi water and it is really off, then that's a red flag, TO ME. Like I said, I had a bad bottle of solution from BRS that their support acknowledged was bad. Also as you pointed out it would show that there was something wrong with that refractometer itself. When I had that bad solution, i actually bought a second seawater refractometer thinking my first one was bad.

Unfortunately, most sold to hobbyists are not. If it doesn't say true seawater refractometer, it very likely is not. It's a dark secret that resellers do not want you to know, and some of them likely do not know either.

 

[Arthacker87]

Haha stirred up a can of worms. My solution is under a year old and I know they will all read different (especially the hydrometer imo) just wondering if hannah has worked well keeping at 35ppt (1.026 sg) for anyone using it or if a refract is better

 

[Miller535]

Unfortunately, most sold to hobbyists are not. If it doesn't say true seawater refractometer, it very likely is not. It's a dark secret that resellers do not want you to know, and some of them likely do not know either.

Hmm. I do not remember the brand of the first refractometer I bought, but I do know it said for seawater. The second one I bought was from BRS. Interestingly it does not say on their website for seawater (even though one would assume), but based off of your comment I now wonder about my second refractometer. I actually do not use the BRS one very often. It is the new one they have with the LED on it. I think it's harder to read then my other one.

 

[Arthacker87]

I'll double check mine as well. The reason I ask all this is because I checked the salinity with the hannah and it said I was like .5 off I did some water changes using this hannah and stuff seems off lol. Maybe it's something else I'll check some stuff tonight

 

[Randy Holmes-Farley]

I was reminded in a different thread today how one can tell if you have a true seawater refractometer or a brine refractometer:

Salinity

Is the iodized salt replaceable with table salt? For this purpose, yes, especially if you are weighing it rather than by volume. The biggest issue with volume measurement is how "fluffy" any given brand might be.

www.reef2reef.com

from it:


In fact, this is a way to know if you have a true seawater refractometer or a brine refractometer. For seawater, 35 ppt and sg = 1.0264 will be approximately across from one another. In a brine refractometer, 35 ppt will be across from about 1.025.