Salinity vs Specific Gravity

35% Off ! All WYSIWYG fish and corals

broomer5

Active Member
#1
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:
 
N

newreefers

Guest
#2
I have a chart also, and it has the same conversion as yours. I always want my test to show 1.025, because of temp that means my tank is at 1.023.:)
 
1

10k

Guest
#3
Once again, another excellent thread Broomer, just one question though. What about the ATC (automatic temperature compensation) refractometers out there? I myself use one, and just assumed that the samples temperature would not effect the readings. Do they, or do they not actually compensate, or is it just a gimick to sell these higher priced refractometers?
Also, this is another good one for all to see, you want it on the net too? I have the room on my site. Right next to your others. Have you ever thought about being a writer? You sound like the perfect candidate for a new online mag. Maybe broomer's corner? If interested, let me know, we can work something out I'm sure.... Great job though, I love 'em..
 

broomer5

Active Member
#4
Thanks 10k !
If you have a refractometer that has automatic temperature compensation ATC - then you would not have to look at the charts.
These refractometers - once calibrated with distilled water AT THE TEMPERATURE that the refractometer is designed for - the unit will give acurate readings of specific gravity or percent, at any reasonable temperature. Reasonable being the temperature range of the liquid that the unit was built to work on.
If you don't have the refractometer calibrated for the room temperature that you do the test - then all bets are off.
The ambient room temperature is the key - and you gotta let the sample of tankwater reach this ambient room temp ( that the refractometer's been calibrated to ) or it will not read right.
Mine does have ATC.
It must be calibrated with distilled water @ 68 F.
This means the room ( refractometer ) has to be at 68F too - or all bets are off.
If you want to try and experiment - take 3 CLEAN shot glasses.
Place one in the microwave for 5 seconds - this will get the tankwater sample up to around 90-100 F
Place the other shot glass in the freezer for a few minutes - until it reads down around 50 F
Take the 3rd shot glass and fill it with tankwater too, and let it sit out on the counter.
Run your tests as normal on all three samples.
By the time you get the 3-4 drops on the prizm - and flip down the little clear protective lid - and run to a window for daylight - the water sample has either cooled or warmed to ambient room temperature - and the readings are all identical.
I've tried this before ( yeah I know - anal retentive )
I may be too slow - but all my readings were the same.
1.025 S.G.
An ATC refractometer is a great tool to have.
 
1

10k

Guest
#5
Thanks for clearing that up! LOL on the temp testing. I would have never gotten up the motivation to attemp such a thing, but you did, and cleared it all up for us. broomer the Mad Scientist huh? lol...I haven't had my refractometer but about....6 mo I think. Any idea what the normal calibration intervals are? 6 mo, a year, more, less. And as for getting the ambient temp down to 68*, I noticed that this was the calibration temp from the factory, and said to myself..."The power company is gonna love this one!" In order for me to keep ambient temperature down to 68*, I would have to run the A/C almost year round, thanks to the two tanks with halides, and three with PCs. Plus, to me, 68* is a little chilly for the house. I'm used to 100* plus at work. Guess I'll have to wear a parka when I test...lol
Do you mind if I post this one on my website, as I did the last? I just think thousands can benifit from your "anal" behavior. I for one love it. Keep it up!
 

broomer5

Active Member
#6
Maybe I wasn't clear.
The ambient room temperature ( and temperature of the refractometer ) should be 68 F only when RE-CALIBRATING the instrument with distilled water.
Otherwise you're good to go.
My refractometer has an "operating range of 50 to 86 F.
Once I recalibrate the unit to distilled water at 68 F ambient - I can use it in a room that has an ambient room temperature between 50 to 86 degrees F.
Providing I let the tankwater sample drops sit on the prizm for a minute or so before reading it - all readings should be compensated.
Yes you may use this stuff on your website - if you think it's helpful. Please do not take this stuff as gospel though - I'm learning just as others learn. Little each day.
 
1

10k

Guest
#7
OK, I gotcha, only 68* when calibrating. I thought that was correct. Considering it wouldn't be ATC if you had to read at 68* also. Thanks for clarifying.
The reason I want to post these threads is because you make it easier for the people "new" to reefing out there. You lay these things down in simple terms (without all of the mumdo jumbo) that the newbies can understand. I like giving out information to the surfurs out there looking for answers to the simple questions, that have complex answers, that they can't quite understand. I realize that you are doing this for the inexperienced here. But I want to broaden the realm, and pass it along to the ones who haven't found SWF yet, and maybe even point them in this direction. There are too many misguided souls out there, looking for the right path...you know?
Any way, I'll put this one to rest, and post it for the world to see. If you ever have any other brainstorms, let me know...I love 'em!:D
 

broomer5

Active Member
#8
Thanks again 10k !
Sure I like to help other people in this hobby, but I must admit that I have my own selfish reasons to write stuff like this too.
If I think someone may read something I write down - I find that it makes me think about it a lot harder.
I read a lot too, like most on here - then I try and figure out what's what - opinions and stuff ya know.
Then if I can figure it out from my own tank at home - it makes sense to me then.
Repeating it in written words - just makes it more clear for me and drills it down deeper into my gray matter. Not writing it down - I forget real fast.
Unfortunately - many times I'm learning as I go - like us all.
See ya
 

josh

Active Member
#9
Nice article there broomer. Now for a real challenge for ya.
Relate salinity to conductivity! Try to wrap the brain around that one. There have been a few articles written about it, it sounds interesting at least. I use lab grade conductivity meters at work and I am just waiting for the right time to "borrow" it and do some tests at home :)
On the hydrometer note, I am also lucky enough to have a densitometer at work ( yes I do work in a lab ) so I decided to take some water in one day for a test. :eek: Since then I take my water in almost weekly and skip the hydrometer all together.
 

broomer5

Active Member
#10
josh,
If I go any farther today - my head may pop off.
I'll tell ya what I know about conductivity as it relates to seawater salinity.
Conductivity is just a measurement of how well a liquid passes an electrical current from one "plate" sensor to another "plate" sensor. It's value is given in micro-mhos or micro-Siemans ~~ depending on what scale you use. Their the same anyways.
The electrons/electricity passes through the solution by jumping from one ion to another. That's how it gets from one plate to the other. The plates are wired up to an electrical device that can measure this amount of currect - and displays in the engineering unit ( mhos or Siemans )
To one plate - a small voltage potential is applied. The electrons then jump from one ion in solution - to the next. Billions of ions may be in the sample. The electronics "watch" and report back on the local display how much ( or less ) electrons are passing through the liquid.
If a solution does not pass very much electricty it is considered more resistive. We measure resistivity the same way - except it's value is reported in ohms of resistance.
Ohm spelled backwards is Mho - so the term mho was used for conductivity.
In the old days - they actually used metal plates for the conductivity cells. A 1 cm x 1cm metal plate that was located 1 cm away from another 1 cm x 1cm plate. This was termed a cell constant of 1.00
Nowadays - most decent conductivity sensors incoporate these 2 cells into one probe like sensor. The cell constants available are
0.01
0.10
1.00
10.0
20.0
These cell constants cover about all ranges of conductivity that you'd ever expect to encounter.
From DI deionized water ( very low conductivity - not may ions - pretty good insulators - very high resistivity ) to heavy brine or acid/bases that are jam packed with ions ( very good conductors - lots of ions in solution - very poor insulators - very low resisitivity )
Each cell constant is good for measuring a certain "range" of a solutions conductivity.
Saltwater would require a cell constant of either 10.0 or 20.0
The 10.0 would handle a range of 100 micromhos ( bottled water ) to around 200,000 micromho's ( most acids and bases )
This would be an ideal cell for measuring the conductivity of saltwater.
Saltwater falls within the 10,000 micromho's to 50,000 micromho's range ...... depending on the saltwater's salinity !
Ah-ha ! Something meaningful - finally !!
You can get the instrument calibrated to display in dual scale units. A good salinity unit will display the raw value in micromhos ( microsiemans ) AND ppt salinity.
The kicker is this ...
Each solution has it's own "curve" so to speak.
The actual conductivity of a particular solution is not a true "linear" relationship.
Seawater has a curve just as beer, or freshwater would have it's own curve on an X & Y axis.
In other words - it's not a straighline relationship.
This curve is "characterized" into the microprocessor of a good electrical instrument.
When the conductivity cell/probe is place into a sample of say seawater - and the probe has been calibrated to be used with the display instrument that has the "seawater" curve in it's microprocessor - then as the electronics "sees" the micromho reading - the microprocessor goes out to it's onboard "look-up-table" and finds the curve for saltwater.
It mearly coverts the micromhos against this look up table - and corrects the digital display to read out in ppt salinity.
Naturally - these electronic devices are temperature compensated as well. Many times there's a small thermistor down in the body of the cell/probe, and the temperature is sent back to the microprocessor as well.
The microprocessor alters the readout, again by going to it's temperature curve - and offsets the "error" that would normally be introduced by the changes in sample temperature.
A really good electronic conductivity unit is expensive, as I'm sure you know josh .... from working around these devices everyday.
That's all I know about conductivity and seawater.
 

josh

Active Member
#11
I think the trick is to figure out what temp. to take the readings at. We use 0 deg. F to measure diesel conductivity. So I am unsure at what temp to measure the seawater. You know a lot about it, I am going to run this past another chemist at work this week to see what he can add to it.
I love thinking of new angles to approach this hobby at. It seems you also like to tinker around with new things as well :)
And you are right anything laboratory grade is not cheap!!!
 

broomer5

Active Member
#12
Yeah - the trick is to measure the sample at a known temperature.
Most of the curves I have for various liquids are drawn for a temp measurement of 25 C.
The curve shows micromhos on the Y axis and % on the X axis - or the other way around ( can't keep my X's and Y's straight )
You need the chart ( or microprocessor characterized ) for a given solution, at a give temperature in order to determine anything other than the liquids raw conductivity.
Like salinity of saltwater in parts per thousand.
 

josh

Active Member
#13
Hmmm,
I will see what I can dig up, I might even call the company that makes it and have a little informative chat with them :)
The % would be on the X axis....known values are always on the X . . such as time increments.
:cool:
 
#16
Ok so I have a float hydrometer that I have used for the making of wine. It gives me accurate S.G. readings of my wine. Will it work for my saltwater as well?
 

geridoc

Well-Known Member
#17
Ok so I have a float hydrometer that I have used for the making of wine. It gives me accurate S.G. readings of my wine. Will it work for my saltwater as well?
I think that wine hydrometers are calibrated in Brix units, not SG. If yours is calibrated in SG or %, so long as the range covers marine concentrations and temperatures, it should be fine.
 

flower

Well-Known Member
#18
Ok so I have a float hydrometer that I have used for the making of wine. It gives me accurate S.G. readings of my wine. Will it work for my saltwater as well?
GeriDoc answered your question... a floating hydrometer won't work, not because of the readings, but because it floats. You would have to turn off everything in the tank to get it to hold still long enough to read it. I laughed at my silly self when I tried to use mine. Your wine vat sits nice and quiet, but your SW fish tank has (should have) current, it will take your floating hydrometer for a nice crazy ride, and you won't be able to read it.
 
#19
T
I think that wine hydrometers are calibrated in Brix units, not SG. If yours is calibrated in SG or %, so long as the range covers marine concentrations and temperatures, it should be fine.
thanks it does have S.G. On the scale and I have the calculations chart to account for marine differences in temp.
 
Top