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  1. #1
    Senior Member tiffany's Avatar
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    Question some mathematical questions on salinity

    I have to use Aussie measurements here so I'm pre-forwarding an apology!!
    If I have 1 litre of 1.025 and want to reduce it to 1.009, how much fresh water would I need to add?
    If then I wanted to reduce the s.grav. by .005 a day what would be the amount then?? and how would I work it out??
    I'm not very good in the 'ol maths department so a KISS principle would be easy on the brain...ie: 1/2 cup etc...:imwithstu :thanx:
    "The significant problems we face cannot be solved at the same level of thinking we were at when we created them."
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  2. #2
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    You are probably better to work with salinity rather than sg. Salinity is based on weight: the ratio of the weight of 'salt' to the weight of the water. Hence, to dilute water based on salinity is easy: the weight of 'salt' stays constant )if you add 'plain' water) while you add 1kg of 'water' weight for each liter of water you add.

    So, at 78F, an sg of 1.025 corresponds to a salinity of 35.4. An sg of 1.009 corresponds to a sailinity of 13. In order to dilute 1l to the lower salinity, you would need a final volume of about 2.7l (35.4/13). Hence, you would need to add 1.7l of normal water.

    Decreasing by exactly 0.005 sg units per day would be a messy calculation. I suspect that you would be fine to just take the total dilutant you need to add (1.7l) and add it in equal increments each day over the period of dilution.

    Not quite KISS, but......
    Nick

  3. #3
    ijo
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    Nice one nickb!!! karma to you.

    IJO

  4. #4
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    Thanks IJO. I checked the suggestion I made for decreasing by 0.005 daily and the approximation isn't as good as I thought :-(

    If you want to go from 1.025 to 1.020 to 1.015 to 1.010, you would need to add the following amounts of fresh water on each day: 250 ml, 400ml and 800 ml. You would need to add a further 250 ml to get from 1.010 to 1.009.
    Nick

  5. #5
    Senior Member tang_man_montreal's Avatar
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    nickb,

    I may be wrong here, but doesn't a SG of 1.025 correspond to a salinity of 51.5? That's what I've been referring to all this time with my Pinpoint Salinity meter...

    http://www.americanmarineusa.com/sal...onversion.html
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  6. #6
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    I had taken the numbers off my Seatest hydormeter. I have just checked in John Tullock's book. He reports NSW has a salinity of 35 ppt which gives a sg reading of 1.024 at 75F if the device doesn't adjust for temperature. I think the conversions I used were for a lower temperature (70F rather than 75F). At 78F, according to Tullock's conversion table, an observed (non-temperature corrected) sg of 1.025 converts to a salinity of about 36.5. However, if the hydrometer is really doing temperature correction, then we shouldn't need an additional temperature correction factor and the salinity would be 34. This is what I find on the Seatest scale.

    Where did the 51.5 value come from? Values that high aren't showing on any of the tables of salinity that I've been able to track down.
    Nick

  7. #7
    Senior Member tang_man_montreal's Avatar
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    Hi there...

    If you look at the above link that I supplied, you'll see where I got my values.

    The numbers that you've been supplying coincide with the Salt (ppt) levels shown at the link I provided.

    Maybe the numbers that I've been looking at aren't really salinity, but just levels used specifically by the Pinpoint Digital Salinity Monitor.

    I don't use a hydrometer, I use this:


    http://www.americanmarineusa.com/salinityfacts.html
    I am Homer of BORG... Prepare to be..OOOO!! DONUT!!!!!!

  8. #8
    Senior Member ALRHA's Avatar
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    speaking of the two - i have read that different salts mix to a different salinity? how does that make any sense? doesnt the salinity just depend on the amount of salt you add to the water (more salt - higher salinity & less salt - lower salinity?) or is it more complex?
    Albert
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  9. #9
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    I think we are measuring the same thing but using different scales.

    The Pinpoint unit is presenting a direct measure of the electrical conductivity of the salt water. It is usually measured in units of milliSiemens/cm (mS/cm). For NSW, this is around 52-55 mS/cm. This can be converted to salinity and sg (I see that the Pinpoint web page has a conversion chart). When you use that chart, we find that 51 mS/cm converts to a salinity of 33.5 ppt and an sg of 1.0248 (at the reference temperature).
    Nick

  10. #10
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    Vince,
    You're using a Pinpoint meter for salinity??!!! How long have you been doing that?

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