Refractometer and hydrometer

Discussion in 'Tips and Techniques' started by iidylii, May 14, 2016.

  1. iidylii

    iidylii Active Member

    so I know this topic has been beaten to death but I just wanted to hear some more lol...

    Thought it also might help some new ppl

    I have the atc refractometer and try to keep my ppt at 35ish...

    My dad has a hydrometer at the moment...I know that I always hear that your specific gravity reading is going to read lower then it actually is...for example 1.023 might very well be 1.025-1.026 and so question the ppt reading on the hydrometer going to read lower as well or is that going to be accurate? I guess I'm also not sure if the atc unit is also reading false in the ppt? I assume on both units the ppt is correct and the specific gravity reads a little low
  2. jay0705

    jay0705 Administrator Staff Member

    If its an accurate hydro it should be the same at the avg tank temp. The problem is alot of them just are not accurate
  3. iidylii

    iidylii Active Member

    Yeah but I thought they were calibrated to read specific gravity at 60 degrees? Like I said a reading of 1.023 at 78 degrees would be more like 1.026 or whatever...maybe i dont know what I'm saying lol
  4. 2quills

    2quills Administrator Staff Member

    Temp and humidity will cause them to read differently.
  5. bang guy

    bang guy Administrator Staff Member

    You are right that a hydrometer calibrated for 60F will read a slightly lower specific gravity (density). The ppt reading on a hydrometer will not be accurate and is a useless scale. In order to determine the ppt (salinity) with a hydrometer you need three things, the S.G. reading, the water temperature, and a conversion chart.

    The reading on an ATC refractometer will give a very accurate ppt (Salinity). ATC = Automatic Temperature Compensation. Placing the water drop and spreading it out on the glass quickly adjusts the water to room temperature.
  6. iidylii

    iidylii Active Member

    Thanks bang...that's what I was looking for...I told my dad to use the ppt reading on his hydrometer but I will be telling him of my new found knowledge :)
  7. bang guy

    bang guy Administrator Staff Member

    In short -

    Hydrometers measure water density (Specific gravity eg. 1.025) which varies quite a bit with temperature. They do NOT measure salinity.

    Refractometers measure Salinity (PPT eg. 35ppt) which does not vary significantly with temperature. They do NOT measure density.
  8. 2quills

    2quills Administrator Staff Member

    Yep, you also should account for atmospheric pressure if we're talking about a device that uses the density of water as a means of measurement.

    Higher pressure means water occupies less space and becomes more dense.

    Someone living in Denver on a nice dry day could see a hydrometer reading lower than that of someone in Florida on a cloudy/rainy day of the same temperature.
  9. iidylii

    iidylii Active Member

    Thanks again guys
  10. Clownfish22

    Clownfish22 New Member

    Hello I just started my saltwater tank I currently gained experiment from the hobby. I am also getting a refractometer and I own a hydrometer so I will be uploading videos on my YouTube channel.
    If you find my channel helpful a like and subscribe will be helpful thanks

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