Salinity vs Specific Gravity

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#22
Thought I'd post this since it took me an hour to think about and write down for another thread.
The whole salinity vs specific gravity issue is not too tough to figure out. It all depends on the instruments used to do the measurements, what temperature they are calibrated for and which engineering unit you prefer to go by.
Salinity is expressed in parts per thousand ( ppt ).
It's just a measurement of the weight of the salt/minerals compared to the weight of the pure water.
Normal seawater is typically 35 ppt in most areas of a reef.
It could be 35 grams of salt per 1000 grams of water.
It could be 35 pounds of salt per 1000 pounds of water.
It could be 35 tons of salt per 1000 tons of water.
You could even use your own body weight as an engineering unit.
I weigh about 175 pounds.
I could easily say that the salinity of seawater is 35 broomer5's of salt per thousand boomer5's of water.
The weight ratio of salt (solids ) to water ( liquid ) is 35 TO 1000
Using ppt as the units then - the salinity would be considered to be 35 ppt at ANY temperature.
SPECIFIC GRAVITY on the other hand is not a weight to weight comparison. It is a weight to volume comparison.
When we talk weight to volume ~ we are talking DENSITY.
Density is just how much something weighs compared to how much space it takes up.
Picture a little clear acrylic cubic box that measures 1cm x 1cm x 1cm on the inside.
Fill it with pure water.
It now contains 1 cc ( cubic centimeter ) of water.
This little clear box of water contains exactly 1 milliliter of water.
1 cc = 1 ml
Now if you could weigh just the water in this little box ~ How much would the water alone weigh ?
The answer is IT DEPENDS.
The actual weight of this volume of pure water will depend on the temperature and the atmospheric pressure.
The metric standard for weight is grams.
It just so happens that this little clear box of water we have, if it's temperature was 4 degrees C ( 39.2 F ) and it was at sea level where the atmospheric pressure is 1 atmosphere ( 14.7 psi ) then the 1 cc of water ( 1 ml ) would weigh in at a whopping 1 gram.
In other words ~ 1 cc of water is 1 ml of water and weighs 1 gram. Were talking PURE WATER here.
This is how people came up with the term "Specific Gravity"
The DENSITY of water ~ how much it weighs ~ for a given VOLUME is referred to as the SPECIFIC GRAVITY.
In our pure water example above - the specific gravity would be 1.000
1.000 gram of pure water in a 1.000 cubic centimeter container.
Now the tricky part.
Water ( in a LIQUID state ) expands and contracts with temperature changes.
The warmer it is - it expands.
The cooler it is - it contracts.
When water expands or contracts - it either takes up more volume or less volume. But the amount of salt in the sample does not change. Only the volume of the water changes.
The ppt stays the same.
So ......... if we're using specific gravity as our measurement - we need to always look at the temperature of the saltwater too.
Specific gravity is a measurement of the WEIGHT of salt to the VOLUME.
Specific gravity is a measurement of the saltwater's DENSITY as compared to DENSITY of pure water - for the same volume of each.
There are a ton of charts on the internet, and in most any decent marine aquarium book. I can send you a chart if you want a good one. Be careful though - there are some contradicting charts floating around out there.
We'll use 3 different temperatures as examples, and the chart goes something like this.
At 60 degrees F
35 ppt = 1.026 S.G.
At 70 degrees F
35 ppt = 1.025 S.G.
At 80 degrees F
35 ppt = 1.023 S.G.
As you can see - the salinity is the same for each example.
It's still 35 ppt
But as the temperature goes up - the specific gravity goes down.
This is simply because the water expands at a higher temperature - and the density must therefore change as well.
Back to our little clear acrylic box of saltwater now.
The box is full of saltwater with a salinity of 35 ppt.
At 60 degrees F - the saltwater in this little box now weighs 1.026 grams.
At 70 degrees F - the box of same saltwater weighs 1.025 grams.
At 80 degrees F - the box of same saltwater weighs 1.023 grams.
The temperature of the saltwater MUST BE considered when using specific gravity as the measurement.
Okay you say - How does all of this crap mean anything to me and my marine tanks ???
You need to see what TEMPERATURE your hydrometer is CALIBRATED for.
Most refractometers or hydrometers are calibrated for liquids at a given temperature. The swing arm units like FasTests - I don't know what they are calibrated for. I don't like using them for that reason - plus they don't seem to give me good results.
They're okay if that's all you have, I still have one too.
The refractometer I use is calibrated for 68 degrees F.
The floating glass hyrdometer is calibrated for 75 degrees F.
What you need to do then - is measure the mixed up aerated saltwater with your instrument and thermometer, and write both values down on paper.
Then refer to a specific gravity vs. temperature chart - find your temperture you just wrote down and see what the S.G. is for that given temperature.
From a good chart for the marine hobby - you will also be able to determine the PPT salinity as well.
I like to mix my IO and RO/DI water to a salinity of 35 ppt.
The actual specific gravity of that batch of saltwater may be differerent - depending on the actual temperature I read on my thermometer, the instrument I use for S.G./Density and the chart I'm looking at to do the conversion.
Normally - I heat the RO/DI water with a heater - get it up around 78 -80 F ( my tank temperature ) mix in IO saltmix at 1/2 cup per gallon freshwater - toss in a little more salt and let it run overnight.
Then the following day - when I plan to do the water change.
Get out my tools - take the two measurements, thermometer and refractometer - look at the chart and tweek it up or down with a little more salt or a little more fresh RO/DI.
After doing this several times - I get lazy too - repeat the same procedures as I have before, get it as close as I can - and move on from there.
It doesn't have to be exact. It does need to be very close though.
The reasons why density, ppt and specific gravity are what they are IS very exact in every respect ......... but for my tanks - getting it real close is fine for me.
If you made it this far reading - congratulations - you must be interested.
If you have anything to add to this thread - please do so.
We're always interested in trying to figure all this stuff out - and any other views would be appreciated ;)
Plus - if it looks like my chart is not correct - please let me know.
As I said - there are several different charts out there - and I may have the wrong one too :eek:
 
#24
I am running a salt water tank with live coral and fish but i seem to have algea problem being green and red hair algea i run a sump tank with a phosphate, nitrate reactors as well as a skimmer any suggestion beside regualar water changes as i do that thanks
 

trigger40

Well-Known Member
#25
I am running a salt water tank with live coral and fish but i seem to have algea problem being green and red hair algea i run a sump tank with a phosphate, nitrate reactors as well as a skimmer any suggestion beside regualar water changes as i do that thanks
do you have a refugium? how old is your set up?
 
#26
Yes i have a refugium my tank is about 5 years old and refugium about 6 months old it is only just starting to grow sea grass in it i run as well a skimmer, nitrate reactor and phosphate reactor any suggestions would be helpful as just about to quit but don't want to give up as i love my tank thanks
 

flower

Well-Known Member
#27
Yes i have a refugium my tank is about 5 years old and refugium about 6 months old it is only just starting to grow sea grass in it i run as well a skimmer, nitrate reactor and phosphate reactor any suggestions would be helpful as just about to quit but don't want to give up as i love my tank thanks
Hi,

Sea grass? I tried seagrass, because seahorses love it....it took a deep sand bed and nutrients to keep it growing, Those nutrients fed hair algae that was unstoppable because the miracle mud nutrient rich stuff I used, fed it to plague proportions. If you are doing that, I suggest you redo your refugium with regular sand, and grow out some caulerpa prolifera, which looks like seagrass, but does not require all those nutrients to live...but rather feeds on the nasty stuff like phosphates and nitrates that are not good for the display tanks water quality. Eliminating the need for a nitrate or phosphate reactor.
 
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