Salinity Advice

[RJinPV]

I have been using RO water to calibrate my refractometer for 18 months. Recently I was reading articles recomending calibration of the refractometer with calibration solution instead of RO water. So I bought some cal solution from BRS made by Aqua Craft products and discovered that all this time my salinity was 1.023 and not 1.025. This is surprising because one of my more trusted LFS mixes Red Sea Coral Pro salt to 1.023 and I have measured it with my "high" reading refractometer at 1.021. This implies these "expert" LFS people were extremely low on their mixing.

Is it really possible for my refractometer, using RO water calibration, to be off that much? Or is it more likely my calibration solution is a bad batch?

Second question: Should I now start raising my salinity to the recommended 1.026 level?

 

[nemo1017]

1.023 is fine. That's usually what I run mine at. Also all I've ever used is ro water to calibrate my refractometer. Never had any issues with it being off, so either I've been doing it wrong for years or it's just been working for me somehow. But maybe someone else who has used calibration fluid can chime in.

 

[skim]

I would just follow what the instructions say that came with your Refractometer, especially if it is a D&D or Red Sea or another one that is specific for reading Seawater.

 

[Randy Holmes-Farley Verified Community Expert]

There's no reason I know of to not target natural seawater salinity levels, so a specific gravity of 1.026 seems appropriate to me and I'd raise your salinity.

Many people do not have a good understanding of the many issues around measurement of salinity (temperature effects, the difference between density and specific gravity, how to properly calibrate, etc.), even some manufacturers and resellers do not, and a store selling salt water does have a reason to use lower salinity: sg = 1.022 costs 15% less than making sg = 1.026.

 

[Brew12]

I have been using RO water to calibrate my refractometer for 18 months. Recently I was reading articles recomending calibration of the refractometer with calibration solution instead of RO water. So I bought some cal solution from BRS made by Aqua Craft products and discovered that all this time my salinity was 1.023 and not 1.025. This is surprising because one of my more trusted LFS mixes Red Sea Coral Pro salt to 1.023 and I have measured it with my "high" reading refractometer at 1.021. This implies these "expert" LFS people were extremely low on their mixing.

Is it really possible for my refractometer, using RO water calibration, to be off that much? Or is it more likely my calibration solution is a bad batch?

Second question: Should I now start raising my salinity to the recommended 1.026 level?

Any chance you aren't getting it to the right temperature to read it?

 

[Brian Kim]

I always bring the SW to 78 degrees prior to checking salinity with my refractometer.
Also try calibrating with 53.0ms fluid

 

[Brian Kim]

Most if not all the LFS here in NY keep their salinity at 1.020, also I've checked the water from their new shipments and it was at 1.019
I guess they not only keep them low to save on saltmix cost but also to match most of their distributors salinity for better acclimation.

I would suggest raising your salinity but do it slowly over a few days

 

[skim]

Well I could be wrong but I read, if it is not a sample of Lab grade Seawater standard then it is just Salt and does not contain all the others salts in the makeup of Seawater and units design to read Seawater will be inaccurate.

 

[Randy Holmes-Farley Verified Community Expert]

Well I could be wrong but I read, if it is not a sample of Lab grade Seawater standard then it is just Salt and does not contain all the others salts in the makeup of Seawater and units design to read Seawater will be inaccurate.

Not sure what you mean, but one can make perfect standards for all devices (hydrometer, refractometer and conductivity) using just sodium chloride. You just need to know how much to add.

I show recipes here:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2004-06/rhf/index.htm

It is true that not all 53 mS/cm solutions will be suitable for refractometers at exactly 35 ppt or sg = 1.0264, and in fact, a pure sodium chloride 53 mS/cm solution wouldn't be, but the Pinpoint mentioned above is a seawater mimic and is suitable for refractometers (if made as they claim).

 

[RJinPV]

I always bring the SW to 78 degrees prior to checking salinity with my refractometer.
Also try calibrating with 53.0ms fluid

My cal fluid was a 53.oms solution. Just a different brand. I also let the water and refractometer temperature stabilize before taking a reading. I've small changes if I don't wait a minute or so because the tank water is at 78 deg and the refractometer is at room temp.

 

[Larry L]

Some guy who posts a lot in this forum wrote a really good article explaining refractometer calibration, and why it makes sense to calibrate using a reference solution close to your desired target value: http://reefkeeping.com/issues/2006-12/rhf/

In particular see the section about "Slope Miscalibration".

 

[RJinPV]

Some guy who posts a lot in this forum wrote a really good article explaining refractometer calibration, and why it makes sense to calibrate using a reference solution close to your desired target value: http://reefkeeping.com/issues/2006-12/rhf/

In particular see the section about "Slope Miscalibration".

This is exactly the article that prompted me to start this journey. Since I had been disappointed in the salinity level of the LFS water I have been buying (it was low), I wanted to learn more about the proper levels. I always thought 1.025 was the match for ocean water but this article pointed out ocean water is really 1.026. The slope miscalibration curve convinced me to buy the cal solution. I was surprised at how far off my measurements were.

 

[RJinPV]

There's no reason I know of to not target natural seawater salinity levels, so a specific gravity of 1.026 seems appropriate to me and I'd raise your salinity.

Many people do not have a good understanding of the many issues around measurement of salinity (temperature effects, the difference between density and specific gravity, how to properly calibrate, etc.), even some manufacturers and resellers do not, and a store selling salt water does have a reason to use lower salinity: sg = 1.022 costs 15% less than making sg = 1.026.

I gotta say thank you for the articles you have written. I have learned a lot from them. I went to my LFS and bought some more water. With my calibrated refractometer I measured a 1.022 salinity level. This is higher than I had feared but below the 1.023 minus they claim. I do think it is motivated by money. They have a great store overall.

 

[RJinPV]

Thanks for all your responses. So, I have double checked the calibration of the refractometer by using the cal solution again and letting it stabilize in temperature. That, and my measurements of the fresh batch of LFS water, and my tank water, has helped convince me the cal solution is good. I have always thought closer to natural sea water conditions is the way I want to run my tank. Therefore, I will gradually be increasing it to 1.026. It is at 1.024 now. This has also convinced me to mix my own salt water.