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How To Read Salt Levels From Hydrometer

Discussion in 'Brackish Forum' started by BobRivers, Apr 18, 2008.

  1. BobRivers

    BobRivers Member

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    Hello.

    I purchased a floating Hydrometer Thermometer from ebay

    link

    http://cgi.ebay.co.uk/ws/eBayISAPI.dll?Vie...N:IT&ih=006

    Without looking stupid :blush: how do i read the salt level indicator, there appears to be no reading.

    The only part i see is the safe level which is coloured green but how do you know when you are at that level?

    thank you.
     
  2. Colin_T

    Colin_T Member

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    The best way to read the floating glass hydrometers is to take a tall glass of water from the tank and float the hydrometer in it. Leave it to settle for a minute and see where the water level is. The water level should be in the green part.
    If the water level is above the green part then there is too much freshwater in the tank. If the water level is below the green then too much salt is in the tank.
    Check the hydrometer in a glass of fresh seawater with a known salinity if you can and check that the salinity is actually in the green. Sometimes the paper inside the hydrometers slips down a bit and they give false readings. The plastic chamber hydrometers are better and the refractometer s better again. Unfortunately they go up in price accordingly.
     
  3. BobRivers

    BobRivers Member

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    Thanks for the advice colin :good:
     
  4. nmonks

    nmonks A stroke of the brush does not guarantee art from

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    While Colin's comment is broadly correct for marine aquaria -- the green section probably corresponding to about 1.022 to 1.026 -- for brackish water aquaria things are different.

    To start with: understand what a hydrometer does. It shows the density of water (usually called the "specific gravity" or "SG"). Freshwater has a density of 1, usually written as 1.000; normal seawater at tropical temperatures has a density of 1.025. In other words, seawater is 2.5% more dense than freshwater.

    With me so far? Good. Now, brackish water contains less salt than seawater. So the density is going to be less than 1.025. If you're keeping "low end" brackish water fish, i.e., species that need just very slightly saline conditions, you're likely going to need around 10-25% the salinity of normal seawater. In density terms, this comes out (at tropical temperatures) to 1.001 to 1.005.

    High salinity brackish water fish need a salinity around 40-50% that of the sea. In density terms, that's around 1.009 to 1.012.

    Now, when you look at your hydrometer you will see that it will be marked off in units. Typically these are one "point" on the density scale. So when you put the hydrometer in plain tap water it will read 1.000. Each mark on the scale is one point upwards, i.e., 1.001, 1.002, 1.003, and so on.

    To explore the relationship between salinity, density (specific gravity) and temperature, download my freebie Mac/Win program Brack Calc. As you'll see, moving the temperature alters the density of the water, which is why you must make the brackish water up with lukewarm water rather than cold water.

    For most practical purposes, you can estimate the amount of salt you need by weight, and then check the final density using the hydrometer, adding salt or water as required. Do always remember that salt absorbs water from the atmosphere, so unless you keep it in an airtight container, after a while any given weight of the stuff will result in lower than expected salinity because part of that weight was water from the air.

    Cheers, Neale
     
  5. BobRivers

    BobRivers Member

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    Neale Thanks for the time and effort will do a bit more reading

    Thanks again.
     

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