Why do my Salinity measurements vary by 0.002 sg between refractometer and conductivity testers?

[Tim Olson]

I have a Hanna HI98319 and a no-name refrac.

I use pinpoint calibration fluid for the refrac and the Hanna calibration fluid for the digital meter.

After calibrating both devices, I measured the salinity of TLF AccuraSea. The hanna read 34.9ppt and the refrac was 35 +/- 0.1 (my eyes aren't good enough to see whether the line was on, just above , or just below the 35ppt line)

I also used the refrac to check a sample of Fauna Marin Multi-Reference solution (supposed to be 33.3 ppt) Again, it was +/- 0.1 of the ref solution. I didn't use the Hanna on this solution as the bottle is only 100ml.

That sounds pretty good and encouraging. I'm looking forward to testing my Hanna and refractometer against the AccuraSea.

 

[bluprntguy]

I‘ve switched to using the Acurasea solution to calibrate my hanna salinity checker instead of their solution. I did a couple of tests where I calibrated using their solution and then checked it against the Acurasea. Then did it in reverse. I think I remember their solution was a tiny, tiny, tiny bit lower, but it was close enough for me at the time.

If you are trying to compare the different products, it seems like they should all be calibrated to the same solution. I don’t think there is anything special about the Hanna solution other than its packaged in a convenient way for their checker.

 

[Tim Olson]

I‘ve switched to using the Acurasea solution to calibrate my hanna salinity checker instead of their solution. I did a couple of tests where I calibrated using their solution and then checked it against the Acurasea. Then did it in reverse. I think I remember their solution was a tiny, tiny, tiny bit lower, but it was close enough for me at the time.

If you are trying to compare the different products, it seems like they should all be calibrated to the same solution. I don’t think there is anything special about the Hanna solution other than its packaged in a convenient way for their checker.

Thanks for the help ... My understanding is I can use AccuraSea to calibrate my conductivity meters (Hanna, Pinpoint), refractometers and hydrometer, since it's real seawater. Having one calibration fluid will hopefully make the results more consistent. Up to this point I've been using/making different calibration fluids for each type of device (conductivity, refractometer, hydrometer). We'll see how it goes.

 

[mtfish]

It is not rocket science to make a standard solution, but you do need an accurate scale. I can relate to the persons posting in this thread, but people can overthink things. Using 14 different ways to measure one thing can give you 14 different measurements. Stick with the one you think is the most precise and accurate. Again, most things we keep do not care if salinity is 35 ppt; or 34.6 ppt; or 35.4 ppt (you will have to convert to specific gravity as my science background mind works only with ppt! lol).

 

[Tim Olson]

It is not rocket science to make a standard solution, but you do need an accurate scale. I can relate to the persons posting in this thread, but people can overthink things. Using 14 different ways to measure one thing can give you 14 different measurements. Stick with the one you think is the most precise and accurate. Again, most things we keep do not care if salinity is 35 ppt; or 34.6 ppt; or 35.4 ppt (you will have to convert to specific gravity as my science background mind works only with ppt! lol).

Thanks ... but that's what's driving me nuts. It really should be simple. I made the solutions with an accurate scale, although, not a lab scale and currently have readings that are 0.003-4 sg different from each other, which is significant.

FYI, what got me going on this was my calibrated devices were reading about 1.024, but I brought a sample to my LFS and they got 1.027. So that got me worried over what the tank's salinity really is. My guess is it's around 1.025 sg, but with a low degree of confidence.

Hopefully I'll get this resolved soon. In the meantime, I'm keeping my Tank's salinity stable , since the corals and other inhabitants are doing well. Oh, I also like using ppt, but it seems people relate better to sg.

 

[mtfish]

Thanks ... but that's what's driving me nuts. It really should be simple. I made the solutions with an accurate scale, although, not a lab scale and currently have readings that are 0.003-4 sg different from each other, which is significant.

FYI, what got me going on this was my calibrated devices were reading about 1.024, but I brought a sample to my LFS and they got 1.027. So that got me worried over what the tank's salinity really is. My guess is it's around 1.025 sg, but with a low degree of confidence.

Hopefully I'll get this resolved soon. In the meantime, I'm keeping my Tank's salinity stable , since the corals and other inhabitants are doing well. Oh, I also like using ppt, but it seems people relate better to sg.

OK. See what you mean. Your listed salinites (1.024 = 31.967 ppt) and (1.027 = 35.827 ppt). are significantly different. But this is where you are going down the rabbit hole. You are wanting lab accuracy without a lab scale. Tell you what. PM me your mailing address and I will send you some calibration solution I had made up the the Texas Dept of Health Lab that used Randy's formula for refractometer. Note, this is not a conductivity standard, refractometer only.

And your last sentence says it all to me. Your tank salinity is stable and your stuff is doing well.

 

[Tim Olson]

That's awesome! PM sent.

 

[Pickwun]

Maybe somebody already said this, and I missed it, but none of those devices measures salinity.
They all measure some property of fluids that can be correlated with salinity depending on certain variables. Keep in mind that you can achieve the same index of refraction, the same specific gravity, or the same conductivity with 0% NaCl.
If you calibrate them all with the same standard at the same temperature then you can reasonably expect to get the same reading on that same standard at the same temperature - that's it. As soon as any variable changes then the readings will start to diverge.
Just pick one device and use it properly. Don't expect magic.

 

[Tim Olson]

Maybe somebody already said this, and I missed it, but none of those devices measures salinity.
They all measure some property of fluids that can be correlated with salinity depending on certain variables. Keep in mind that you can achieve the same index of refraction, the same specific gravity, or the same conductivity with 0% NaCl.
If you calibrate them all with the same standard at the same temperature then you can reasonably expect to get the same reading on that same standard at the same temperature - that's it. As soon as any variable changes then the readings will start to diverge.
Just pick one device and use it properly. Don't expect magic.

Good points ... Thanks!

 

[Pntbll687]

I just skimmed through the thread, so I might have missed it. But, why are you chasing salinity through testing?

It seems that all differences in the tests can be attributed to both margin of error in the test itself and testing error by the user.

What are you going to gain by getting your salinity to test exactly 1.0260? or whatever number you're shooting for?

I'm not trying to be a jerk, I'm genuinely curious as to why you want to nail down salinity to such precision. This is literally the opposite approach of my own, I use a swing arm hydrometer. I can say I mix and test my salinity to be consistent, but the swing arm may not be accurate.

 

[Tim Olson]

I just skimmed through the thread, so I might have missed it. But, why are you chasing salinity through testing?

It seems that all differences in the tests can be attributed to both margin of error in the test itself and testing error by the user.

What are you going to gain by getting your salinity to test exactly 1.0260? or whatever number you're shooting for?

I'm not trying to be a jerk, I'm genuinely curious as to why you want to nail down salinity to such precision. This is literally the opposite approach of my own, I use a swing arm hydrometer. I can say I mix and test my salinity to be consistent, but the swing arm may not be accurate.

Thanks for the questions ... Basically what I'm trying to accomplish is to get an accurate value for my tank's salinity and to keep it consistent over time.

Right now, with the different calibrated devices I have, the results vary from reading 1.024 to 1.027, which is a pretty big swing. What I'd love to get to is which device I have is the most accurate. Then all I need to use is that one device, like my refractometer. My goal as well is to keep my salinity consistent and around 1.025 to 1.026 sg.

Also, a big part of why I'm doing this is because I love the testing and analysis part of having a reef tank. I hope that makes sense.

 

[Tim Olson]

Per Cory's suggestion I mixed up some saltwater, based on Tropic Marin Pro Reef's instructions

I had to make 2 batches, though, since they had a low end and high end of expected salinity from 1.025-1.027 sg at 77°F. The batches started with 15 liters of RODI.

• The first batch was at the low end of the directions was 38 grams/liter (570g salt):
- Vee Gee was 1.0250 sg on the Vee Gee refractometer, which seems to be right where it should be
- Hanna was 32.1 ppt or 1.0242 sg or about 0.0009 sg low of Vee Gee
- Pinpoint was 47.8 ms/cm or 1.0230 sg or 0.0020 low of Vee Gee

• The second batch was at the high end of the directions was 40 grams/liter (600g salt):
- Vee Gee was 1.0270 sg on the Vee Gee refractometer, which seems to be right where it should be
- Hanna was 34.1 ppt or 1.0257 or 0.0013 low low of Vee Gee
- Pinpoint was 50.8 ms/cm or 1.0247 or 0.0023 low of Vee Gee

So at this point it would appear the Vee Gee STX-3 refractometer is the most accurate, which was calibrated using Brightwell's Refractometer Calibration Standard. Also, BTW, the tank water measures 1.0250 sg with the refractometer, which is good.

The final calibrations and test will be with AccuraSea, which I should get in a couple of days.

Thank everyone for listening and your feedback.

 

[Tim Olson]

Update - I believe that I now have a fully calibrated refractometer (VEE GEE STX-3) and Hanna Salinity Tester (HI98319).

For the refractometer calibration, I ended up using 2 standards, including "AccuraSea Seawater Reference/Calibration Solution” and a lab-made Randy Holmes-Farley refractometer standard that @mtfish graciously sent me.

The result is my refractometer is calibrated as accurately as possible. Also, the VEE GEE STX-3 is very easy to read compared to my less expensive refractometer, with the view screen being about twice the size. Therefore, I can get reasonably precise readings.

Also, I calibrated the Hanna Salinity Tester (HI98319) with the AccuraSea standard, which worked out great. Before, I calibrated it with Hanna's standard that was provided with the tester. Although, it would always read about 0.0013 sg low, compared to the refractometer. Now, it correlates perfectly with the refractometer.

Overall, I'm excited that I can finally measure salinity accurately and precisely. Of course, within in the device tolerances.

So, thank you all once again for all the help on this. I never could have done it on my own.

 

[Randy Holmes-Farley]

I‘ve switched to using the Acurasea solution to calibrate my hanna salinity checker instead of their solution. I did a couple of tests where I calibrated using their solution and then checked it against the Acurasea. Then did it in reverse. I think I remember their solution was a tiny, tiny, tiny bit lower, but it was close enough for me at the time.

If you are trying to compare the different products, it seems like they should all be calibrated to the same solution. I don’t think there is anything special about the Hanna solution other than its packaged in a convenient way for their checker.

Not necessarily so. It depends on what the solution is made of. My diy solutions are sodium chloride, and a sodium chloride solution that reads 35 ppt Seawater equivalent by conductivity will not read exactly 35 ppt using a refractometer.

 

[mtfish]

This is the formula that was used. Is this not the correct one for refractometers?

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). 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 fresh water.

 

[Randy Holmes-Farley]

This is the formula that was used. Is this not the correct one for refractometers?

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). 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 fresh water.

That is the correct formula for a refractometer, but not other types of devices.

 

[Stigigemla]

The life will be simplier if You discuss the salinity instead of the weight. The salinity in weight percent does not change with temperature.
Refractometers check the refractive index and is most often calibrated with tap water of about 68 degrees as salinity zero or weight 1.000. The refractometers normally have ATC (automatic temperature compensation). That means that a few degrees warmer or colder doesnt matter.

If You use a hydrometer it is usually calibrated for a real density of 1.000 in weight/volume. But at 68 degrees the water is a bit expanded and will actually have a density of about 0.998 (wich we call relative density 1.000 when we use refractometers). The refractometer will compensate for this but a hydrometer not. The hydrometer is often made for measuring cooler water and the intended water temperature is often printed att the bottom of the density scale. It can be 50 degrees or anything else. When you measure warmer water the water will expand and the hydrometer too but much less. The hydrometer shall show about 0.002 less!

If you use an electronic meter there are 2 possibilities. Some show the conductivity in ms/m and some convert the value to salinity in parts per tousend. There are even some capable of converting into weight but there I dont know if it is relative or real weight. I wouldnt use that scale. If You use the build in conversion for salinity it should be correct.
I use a noname called 8371 and I tested it with 68 degrees warm water that i heated in a microwave oven to 86 degrees and I got exaktly the same reading.

Here ( http://reefkeeping.com/issues/2004-06/rhf/index.php ) is a good article on how to make Your own standard to test salinity.

But the most important with the salinity is to keep it constant. It can be 3,45% in the Indopacific area, a bit more in the Carribean and up to 4,1% in the Red Sea. And there are corals in all that places.

 

[Tim Olson]

To set it straight in my mind, I found this simple definition ... “Salinity is the concentration of dissolved salts found in water. It is measured as the total amount of dissolved salts in parts per thousand (sometimes called PSU or Practical Salinity Units by scientists). Ten parts per thousand is equal to one percent. Salinity in seawater averages 34 parts per thousand or 3.4%“

Also, it sounds like we should be able to use either Randy Holmes-Farley's refractometer standard or the AccuraSea standard to calibrate a refractometer for salinity, if the calibration standard and sample temperatures are equal?

My Hanna HI98319 Salinity Tester “measures conductivity (EC) to determine the salinity of natural and artificial seawater. It displays results in parts per thousand (ppt), Practical Salinity Units (PSU), or specific gravity (S.G.).” I’ve set it up to read salinity (ppt), then convert if to specific gravity using hamzasreef calculator.

Does that sound right?

 

[Jeepjeep_0]

Good thread! Moved my question to a new thread.

 

[Garf]

Need some help folks. I have the Hanna HI98319, and this is where I’m at.

I swapped over to the Hannah about 4 months ago. I calibrate it monthly with the Hannah satchets and it always calibrated flawlessly.

I test weekly with The Hanna HI98319. I test my DT water and mix water for a water change to the same specific gravity. I am ALWAYS @ 1.026 per the Hanna.

The other day I brought a sample of my water up to a reputable LFS To compare my measurements of all parameters. Everything was pretty similar but I was at awe to see they were measuring my salinity at 1.030 specific gravity using a refractometer. (LFS said they calibrate their refractometer with RODI every morning.)

I went home and pulled out my two old Refractometers, cleaned them and calibrated them with RODI as I was out of calibration fluid.

Upon testing my water I measured 1.030 on one refractometer and 1.029 on the other.

I hastily started heating fresh RODI and was going to start bringing down my salinity, but after taking a deep breath held off.

I called Hanna the next morning and spoke with a tech. He stated that results of the Hanna “conductivity” tester could not be compared with the refractometer because they were different in how they measured. He insisted Hannas was more accurate (which I would expect) and that as long as I was able to calibrate the Hanna HI98319 Without any issues I should trust the more accurate conductive reading than the other 3 refractometer readings.

Now I don’t know what my best course of action is. After reading through this thread everyone has discrepancy’s but mine seems dangerously bigger than the others. On that note I have a large mixed reef of softies, LPS, & SPS as well as an abundance of fish & Shrimp and everything seems great and all colors and growth are amazing.

Any suggestions on how to proceed? I want to think this one through prior to making any rash decisions.

Your refractometer should be calibrated with 35ppt solution, and so should your LFS, not RODI.