Salinity driving me nut's

Discussion in 'Water Parameters and Additives' started by Nemos Janitor, 16 Jan 2012.

  1. Nemos Janitor

    Nemos Janitor

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    Almost every a time reefer gives his/her water parameters it is:

    Salinity 1.025

    This lack of understanding and continuous misquoting is driving me nuts. It highlights how many reefers do not know the difference between Salinity and Specific Gravity.

    Because we are keeping salt water aquariums the first most important parameter to test, measure and understand is salinity and how we measure it. Contra to what most reefers believe it is a little more complex than just taking a refractometer or hydrometer reading or a ms reading if measuring electronically.

    Here is a post by Broomer5 from Saltwaterfish.com that explains the difference.

    Salinity vs Specific Gravity


    "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 ;)"
     
    Last edited: 16 Jan 2012
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  3. rakabos

    rakabos

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    Very good info. BUT people will still say their salinity is 1.025 in posts :p
     
  4. Nemos Janitor

    Nemos Janitor Thread Starter

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    Perhaps i should wear this.

    [​IMG]
     
    Last edited: 16 Jan 2012
  5. rakabos

    rakabos

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  6. rakabos

    rakabos

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    All i know is its important to check salinity or SG :p at consistent temps when ure testing it. So if ure mix is sitting at 1.025 at 24C its gonna be different at 26C. So always test salinity by matching ure DT's temp and the new mix's temp
     
    Last edited: 16 Jan 2012
  7. butcherman

    butcherman Moderator MASA Contributor

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    Perhaps we need to form the SG police :whistling:
     
  8. viper357

    viper357 Admin MASA Contributor

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    I think Keith needs a hug, I nominate you to give him one on the 21st. :biggrin:

    I'll be the first to admit that I haven't got a clue what the difference is between Salinity and SG, I just go by what my refractometer tells me.:blush:

    I can understand your frustrations Keith, and you are 100% correct, but the way I see it is that the majority of hobbyists on the forum are just that, simple hobbyists, not many of us are up to scratch with the scientific terminology and most of the times we'll get it wrong, but it's a term that we all understand and can relate to.

    It's like ammonia (I think :p) There are 2 different types of ammonia we should supposedly be testing for. Or was it Phosphates, or maybe Iodine, I can't remember.

    I reckon we're doing ok from the hobbyist point of view, if anybody wants to really go into the correct scientific terminology or understanding of it then we've always got guys like you, Ivan, Hennie etc. that are there to show us the way. :)

    p.s. I've been pondering on having a section on the forum for advanced topics only, where you big guys can discuss the really advanced and deep aspects of the hobby, but I'm not sure if it would attract much attention. What say you?
     
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  9. butcherman

    butcherman Moderator MASA Contributor

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    We are already getting him a cake with Sarah inside.... :whistling:

    Advanced? But would everyone have access to the info?
    I think it works well like it is, perhaps this thread needs to be made Sticky :whistling:
     
  10. dallasg

    dallasg Moderator MASA Contributor

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    @viper357 an Advanced forum would be cool, would need to be moderated heavily to sift out the junk
     
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  11. dallasg

    dallasg Moderator MASA Contributor

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  12. Clownfish9906

    Clownfish9906

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    Thanks guys for the helpful info...

    But my tank IS really 1.025...measured with a calibrated refractometer from Flamming angel...
     
  13. shaunn

    shaunn

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    but haw accurate are the digital SC pens, is refractometer still the best bet for accurate measurements
     
  14. Clownfish9906

    Clownfish9906

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    i would have thought the digital ones would be better as they should have the technology to auto compensate for temp, etc...
     
  15. Quintus

    Quintus the irish aXeman

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    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.

    well it's not, A Salinity of 35 is a relationship of the molecular Moles influenced by the Convalent bonding of electron's. it is influenced by Temperature(excitation).

    Salinity does Not mean 35 part per thousand
     
  16. Nemos Janitor

    Nemos Janitor Thread Starter

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  17. Broder

    Broder Mudshark

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    It would probably just turn into drawn out arguments between a few members, that nobody ever refers to. I think it would be better if the clever guys just make things accessable and easy to understand for us average hobbyists.
     
  18. Jeann1

    Jeann1

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    On the Reef Angel, it also dispays the Value as 35.0

    The DD range of refractometers show it as 1.025 and 35.
     
  19. Quintus

    Quintus the irish aXeman

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    Correct at STP a SG at 1.027 should show you Salinity of 36.3 and so on

    1.025=33.7
    1.026=35
    1.028=37.3
     
    Last edited: 4 Apr 2012
  20. Jeann1

    Jeann1

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    oops.. sorry. meant 1.026 - 35ppt
     
    Last edited: 4 Apr 2012
  21. Nemos Janitor

    Nemos Janitor Thread Starter

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    Not sure and will check but i think the D-D Refractometer is calibrated to NIST and not STP
     
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