Does 1.026 SP = 35 PPT?

[mook1178]

Specific Gravity is unitless

Yes, but density is not and specific gravity is a ratio of water density. The ratio is the density of freshwater at 4C which is 0.999 g/l. You are essentially dividing the density by 1. Therefore SG is basically the density of water when using SG for water.

 

[mook1178]

Actual salinity is not affected by temp, but all the ways to measure it are affected by temperature.

This is correct, but a good salinometer will take temp into account and 35 ppt at 75f will still be 35 ppt +/- 1ppt at 40f

 

[BeanAnimal]

This thread is rather amusing to me in the sense that that these numbers are thrown around constantly, yet so few people understand how they are related or what the really mean.

Specific Gravity
Salinity
Density
Conductivity
their associated (or lack of) units and the "conversions" between each.

Luckily "close enough" is pretty effective for our purposes!

The calculator that @EeyoreIsMySpiritAnimal linked to is fairly well made and uses an accepted conversion equation. I have a similar calc on my site (though I think it is unpublished currently). Hamza's Reef also has a fairly well vetted calculators that he put a good bit of time into writing. Each of our calculators will vary slightly by ~.1 PPT/PSU simply because we all three likely use a bit different coefficients in our Equation of State calcs - as well the different scripts may handle floating point and rounding a bit differently in the rather lengthy equations and variables

If there is interest: What you need to take into consideration as that there is no "single" equation, they are all approximations based on historical testing and definitions and the (somewhat) ever evolving "Equation of State" for Seawater. It is a very interesting subject if you are into that kind of thing... if not "close enough" is absolutely good enough for our purposes.

 

[KStatefan]

Yes, but density is not and specific gravity is a ratio of water density. The ratio is the density of freshwater at 4C which is 0.999 g/l. You are essentially dividing the density by 1. Therefore SG is basically the density of water when using SG for water.

But that is not correct. Specific Gravity is a ratio of densities and has no units.

 

[mook1178]

But that is not correct. Specific Gravity is a ratio of densities and has no units.

Yes the ratio of densities. So the density of the water in our tanks / the density of water at 4C

So just for ***** and giggles lets use the average density of open ocean water ~1.027 g/l and divide that by 0.999 g/l. So the Average SG of the ocean is 1.028.

Do you really think that 0.001 makes a difference? Do you see how the density and SG is essentially the same number?

we could even draw out the density at 4C to 0.9997 g/l. In the example above the average SG is now 1.027...

So my point stands, when we look at SG of a tank we are essentially just measuring the density and removing the units. If SG is looked at in this light, it is easier to understand how temp has much more effect on SG than measuring salinity.

I personally do not like measuring SG as it does not give good info without a temp given. In the reef world temp range is small, so it is understood. However, salinity and alkalinity have a very strong relationship.

 

[Randy Holmes-Farley Verified Community Expert]

I think this answered my question. When people say they target 35 ppt or 1.026 SG it is assumed they mean at 78 degrees as both measures are affected by temperature.... correct?

No. They are not. Measuring devices such as glass hydrometers need temp corrections, but those values themselves are not temperature dependent.

 

[Randy Holmes-Farley Verified Community Expert]

Yes the ratio of densities. So the density of the water in our tanks / the density of water at 4C

So just for ***** and giggles lets use the average density of open ocean water ~1.027 g/l and divide that by 0.999 g/l. So the Average SG of the ocean is 1.028.

Do you really think that 0.001 makes a difference? Do you see how the density and SG is essentially the same number?

we could even draw out the density at 4C to 0.9997 g/l. In the example above the average SG is now 1.027...

So my point stands, when we look at SG of a tank we are essentially just measuring the density and removing the units. If SG is looked at in this light, it is easier to understand how temp has much more effect on SG than measuring salinity.

I personally do not like measuring SG as it does not give good info without a temp given. In the reef world temp range is small, so it is understood. However, salinity and alkalinity have a very strong relationship.

That actually is not always correct. Specific gravity is most often the density of the fluid divided by the density of pure water at the same temp, and so it changes very little with temperature.

I describe these issues in detail here:

Chemistry and the Aquarium: Specific Gravity: Oh How Complicated!

This month, Randy explains and reviews specific gravity.

reefs.com

 

[mook1178]

That actually is not always correct. Specific gravity is most often the density of the fluid divided by the density of pure water at the same temp, and so it changes very little with temperature.

I describe these issues in detail here:

Chemistry and the Aquarium: Specific Gravity: Oh How Complicated!

This month, Randy explains and reviews specific gravity.

reefs.com

Gotcha. I am interested to read some of your articles on here. I am a chemical oceanographer with a concentration in carbonate chemistry. I know some of that chemistry has slight variations due to being a closed system in aquaria.

I was using the NPL version of SG, which is compared to pure water at 4C. The article you posted will be interesting to read.

 

[KStatefan]

Yes the ratio of densities. So the density of the water in our tanks / the density of water at 4C

So just for ***** and giggles lets use the average density of open ocean water ~1.027 g/l and divide that by 0.999 g/l. So the Average SG of the ocean is 1.028.

Do you really think that 0.001 makes a difference? Do you see how the density and SG is essentially the same number?

we could even draw out the density at 4C to 0.9997 g/l. In the example above the average SG is now 1.027...

So my point stands, when we look at SG of a tank we are essentially just measuring the density and removing the units. If SG is looked at in this light, it is easier to understand how temp has much more effect on SG than measuring salinity.

I personally do not like measuring SG as it does not give good info without a temp given. In the reef world temp range is small, so it is understood. However, salinity and alkalinity have a very strong relationship.

Lets just use the calculator posted earlier in the thread. I think units are important and to call them the same is not good. There have been a few threads here over the years with misunderstanding S.G. vs density. One was someone who bought a Tropic Marin hydrometer while he was in Europe and did not know that it had a density scale instead of S.G. and there have been a couple with GHL controllers that did not know it was displaying density and not S.G.

 

[mook1178]

Lets just use the calculator posted earlier in the thread. I think units are important and to call them the same is not good. There have been a few threads here over the years with misunderstanding S.G. vs density. One was someone who bought a Tropic Marin hydrometer while he was in Europe and did not know that it had a density scale instead of S.G. and there have been a couple with GHL controllers that did not know it was displaying density and not S.G.

The only thing I got wrong was the temp of pure water in my explanation...
Which was pointed out by Randy.

Either way, temp is more important to SG than salinity and was just trying to show why.

 

[Randy Holmes-Farley Verified Community Expert]

Gotcha. I am interested to read some of your articles on here. I am a chemical oceanographer with a concentration in carbonate chemistry. I know some of that chemistry has slight variations due to being a closed system in aquaria.

I was using the NPL version of SG, which is compared to pure water at 4C. The article you posted will be interesting to read.

Welcome.

It would be great to have more discussions with you!

 

[Randy Holmes-Farley Verified Community Expert]

This Reef Chemistry Question of the day may interest readers of this thread:

Reef Chemistry Question of the Day #282: Temperature Effects on Parameters

Reef Chemistry Question of the Day #282 Suppose your coral reef aquarium is chugging along nicely at 80 degrees F. You withdraw a sample to run some tests, and the temperature drops to 72 degree F before you are ready. Which of the following parameters will NOT have changed as the temperature...

www.reef2reef.com

Note in particular choice 2 bears on this thread, and whether salinity in ppt varies with temperature.