Newbie question regarding salinity versus SG

[Bradley Keck]

Hey everyone!
Exciting times here as I have finally reached the point of making my first batch of saltwater and filling my 150g for the first time! I'm using HW marine reefer and standard refractometer. I calibrated it using RODI water(per instructions). I had in my head 1.026 as my target, but noticed after making 145g, I have used almost the entire box. I had read on BRS to go about 1/2cup/gallon RODI. This ratio did not get me close to 1.026. I then decided to research this some and found where HW recommended 1.022 for a salinity of 34 and that going to 1.026 will make for too high salinity of 39. I can fix this by diluting with RODI, but I wanted to get everyone's opinion and confirm that I should shoot for 1.022. My refractometer displays both. Should I just go by the salinity or go by the SG, or does it matter? My plan is a mixed reef of one or two softies, LPS, and some of the easier to keep SPS's. Thank You!

 

[HotRocks]

Hey everyone!
Exciting times here as I have finally reached the point of making my first batch of saltwater and filling my 150g for the first time! I'm using HW marine reefer and standard refractometer. I calibrated it using RODI water(per instructions). I had in my head 1.026 as my target, but noticed after making 145g, I have used almost the entire box. I had read on BRS to go about 1/2cup/gallon RODI. This ratio did not get me close to 1.026. I then decided to research this some and found where HW recommended 1.022 for a salinity of 34 and that going to 1.026 will make for too high salinity of 39. I can fix this by diluting with RODI, but I wanted to get everyone's opinion and confirm that I should shoot for 1.022. My refractometer displays both. Should I just go by the salinity or go by the SG, or does it matter? My plan is a mixed reef of one or two softies, LPS, and some of the easier to keep SPS's. Thank You!

Most people (myself included) keep DT around 1.025-1.026 or 34-35ppt

 

[Bradley Keck]

Most people (myself included) keep DT around 1.025-1.026 or 34-35ppt

Well, that's what I thought, but then when I went through almost the whole box making 145g and it took a lot more than the general suggestion of 1/2cup per gallon of RODI (as suggested on BRS's site) I thought I should look into it. That's when I found an old thread where someone contacted HW marine and quoted them as saying 1.026 would equal too high a salinity(supposedly 39). Is there any chance that the HW marine reefer is different or was this just false info? Thank you for your reply. I was thinking good grief, I just made my first batch of saltwater and already screwed up.

 

[Larry L]

1.022 for a salinity of 34 and that going to 1.026 will make for too high salinity of 39

Those numbers are off - salinity of 1.022 is 29.2 ppt, and salinity of 1.026 is 34.5. There's a good calculator at: https://www.hamzasreef.com/Contents/Calculators/SalinityConversion.php

Most people (myself included) keep DT around 1.025-1.026 or 34-35ppt

Agree.

I'm not sure about HW but I know with most commercial salts the number of gallons marked on the packaging assumes mixing up to a salinity of around 1.023, not the 1.025 or 1.026 most people like to run in reef tanks. E.g. for Instant Ocean the 1/2 cup per gallon rule will only get you to somewhere between 1.021 and 1.023. So you pretty much always end up using more salt to mix up the number of gallons marked on the package (or put another way, if the package says 150 gallons you will get less than 150 gallons if you mix it up to 1.026).

 

[NY_Caveman]

Well, that's what I thought, but then when I went through almost the whole box making 145g and it took a lot more than the general suggestion of 1/2cup per gallon of RODI (as suggested on BRS's site) I thought I should look into it. That's when I found an old thread where someone contacted HW marine and quoted them as saying 1.026 would equal too high a salinity(supposedly 39). Is there any chance that the HW marine reefer is different or was this just false info? Thank you for your reply. I was thinking good grief, I just made my first batch of saltwater and already screwed up.

I will just note that for my salt (LiveAquaria brand) I need to add considerably more than the recommended half cup to get to 1.026. About 15% more.

 

[HotRocks]

Im terrible. I don't pay any attention to how much I use. I just use the same scoop, and mix in the same brute cans.

I know how many scoops I need for a DT change and a QT change. I do regularly calibrate my refractometer with calibration fluid, as I got myself in trouble with my DT before with low salinity prior to having the fluid.

 

[Bradley Keck]

Thanks guys. I have mixed the first 3 50g batches to 1.025 and will stick to that. You guys make sense and everything you are saying is what I thought to be true. If I mix to a SG of 1.025 then that should equal a salinity of 34. I feel better about it now, but I am going to go back and read that thread again. Either I read it wrong or something was not right. Thanks guys! Hey, on another note, I did get my tank filled and have no leaks, the pumps are all working, and the Bean is silent. Took like 2 minutes to adjust it. ;Happy

 

[Bradley Keck]

OK. I found the thread. So a reefer contacted the company to find out estimated values of Mag, ALK, and Ca. Their response was this:
"Weestimate that the value of 1.026 is a density and not a gravimetric based value. If you mix to this density you will reach a very high salinity of 39 psu which is the upper limit value of natural marine environment (only possible to find in some limited areas of the red sea). The values for HW marine mix reefer at a salinity of 34 psu ( density of 1022.6 @ 25 deg C) is Ca= 420-440, Mg 1310-1330, dKH 8,4 - 8,7. "So this is a little over my head. Does this sound accurate?

 

[Bradley Keck]

This thread is old and on anther site. It is from 2009. But, it has me wondering. I want to make sure I have the basics right before I add one living thing to this tank.

 

[NY_Caveman]

This is a good article on Salinity and creating calibration solutions for the different forms of measurement.

http://reefkeeping.com/issues/2004-06/rhf/index.htm

 

[Bradley Keck]

Thanks! Good article! So, using a refractometer to measure specific gravity is OK? Does it measure density as well as a hydrometer? The way I understand it, if the reading of 1.026 is accurate, then it has to be a salinity of 34-35? It couldn't possibly be at 39 like the manufacturer was quoted as saying. correct?

 

[NY_Caveman]

Thanks! Good article! So, using a refractometer to measure specific gravity is OK? Does it measure density as well as a hydrometer? The way I understand it, if the reading of 1.026 is accurate, then it has to be a salinity of 34-35? It couldn't possibly be at 39 like the manufacturer was quoted as saying. correct?

Correct. FYI, I calibrate my refractometer with RO/DI every time I use it. For mine I did test with a salt solution and it is dead on after calibrating with RO/DI.

 

[Forsaken77]

Thanks! Good article! So, using a refractometer to measure specific gravity is OK? Does it measure density as well as a hydrometer? The way I understand it, if the reading of 1.026 is accurate, then it has to be a salinity of 34-35? It couldn't possibly be at 39 like the manufacturer was quoted as saying. correct?

Just an FYI... The manufacturer was saying PSU, which is "Practical Salinity Unit, not PPT (parts per thousand). PSU is a third term of measuring salt content. For our purposes, PSU is equivalent to PPT because the differences in the range we use it is insignificant. PPT is not temperature dependent while Specific Gravity (S.G.) (1.025) is temperature dependent. So make sure your refractometer has "Auto Temp Compensation" (ATC). A hydrometer does not compensate for temperature.

 

[Bradley Keck]

Just an FYI... The manufacturer was saying PSU, which is "Practical Salinity Unit, not PPT (parts per thousand). PSU is a third term of measuring salt content. For our purposes, PSU is equivalent to PPT because the differences in the range we use it is insignificant. PPT is not temperature dependent while Specific Gravity (S.G.) (1.025) is temperature dependent. So make sure your refractometer has "Auto Temp Compensation" (ATC). A hydrometer does not compensate for temperature.

Good to know. Thank you both for your information. It is very helpful. I am going to order some calibration fluid for my refractometer and will stick with 1.026 sg. My refractometer does not have auto temp correction, the instructions say the sample should be the same temp as the ambient air temp the refractometer is in. That was not a problem initially when I measured because the water was not heated, but now when I check the DT salinity, it's at 78 degrees and the refractometer at room temp of 71 deg. So, that may not be an accurate reading. I will research a reliable one with temp compensation. Thanks!

 

[Martin Kuhn]

Water temperature in my eyes is not relevant at all using a refractometer. Just imagine: you have a refrac of about 300 grams and add a drop of water with about 1 Gramm or less. The water adjusts to the refrac’s temperature just immediately (entropy....)

What eventually influences your results, if your refrac is not an ATC one, is the temperature of the refrac itself ..... adjusting the temperature of the water measured

Kind Rgds
Martin

 

[Bradley Keck]

Water temperature in my eyes is not relevant at all using a refractometer. Just imagine: you have a refrac of about 300 grams and add a drop of water with about 1 Gramm or less. The water adjusts to the refrac’s temperature just immediately (entropy....)

What eventually influences your results, if your refrac is not an ATC one, is the temperature of the refrac itself ..... adjusting the temperature of the water measured

Kind Rgds
Martin

Ah yes, Entropy. You are taking me back to the early 90's when I was chemistry major at Purdue. That makes sense. You have a mass at one temp that is roughly 300x greater than the mass of the sample at another temp. The small mass will quickly loose energy to the larger mass (in the case of the refract being cooler) thus the temps will be equal in no time. I am ashamed I could not make sense of this myself, but I did switch major's to Physical Therapy in 1992 after a year and a half of Chemistry and Physics at Purdue. Now I know why Ironically enough, my first major when I started college was Marine biology... Now, if my salinity is lets say 35ppm at a DT temp of 78 deg. I measure it with my refractometer and the temp of the sample reduces to lets say 71 deg. Will it still read it as 35ppm or is refraction altered by energy level in a solution?

 

[Randy Holmes-Farley Verified Community Expert]

OK. I found the thread. So a reefer contacted the company to find out estimated values of Mag, ALK, and Ca. Their response was this:
"Weestimate that the value of 1.026 is a density and not a gravimetric based value. If you mix to this density you will reach a very high salinity of 39 psu which is the upper limit value of natural marine environment (only possible to find in some limited areas of the red sea). The values for HW marine mix reefer at a salinity of 34 psu ( density of 1022.6 @ 25 deg C) is Ca= 420-440, Mg 1310-1330, dKH 8,4 - 8,7. "So this is a little over my head. Does this sound accurate?

They are wrong, sadly. [emoji30]

 

[Randy Holmes-Farley Verified Community Expert]

The specific gravity (not density) of 35 ppt seawater (a typical worldwide average) is about 1.0264.

 

[Bradley Keck]

They are wrong, sadly. [emoji30]

Thank You! I can now have the piece of mind that I'm not failing with one of the first and most basic measurements to get right! It is disappointing, and they sure made it sound like they knew what they were talking about, and had me questioning everything I had learned about salinity measurement for the past two years!