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Thread: Refractor or probe?

  1. #1
    Regular Vistor mkeller088's Avatar
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    Refractor or probe?

    I understand the cond. probe IS accurate when calibrated properly. I also understand the cond. probe is measuring salt only (not all the other components in aquarium water like magnesium etc.)
    Last, I understand refraction meters measure specific gravity. Two different measurements and two different ppt readings.
    What I donít understand is when (assuming all instruments are properly calibrated) the probe reads 31.3ppt and the refractor reads 1.025.
    If the true salt content (salt only) should be 35ppt then salt should be added. (Using ocean levels as the bar)
    For years the refractor reading is used to adjust.
    What reading should I monitor and adjust? Refractor or probe?
    I know consistency is the key, but I wish to maintain what is the very best for the fish and corals.

  2. #2
    Frequent Contributor zombie's Avatar
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    Cond measures all dissolved solids, not just salt. It converts electrical conductivity in mS/cm with a direct and fully proportional equation that is independent of the makeup of the solution.

    Refractometers measure refractive index, which can be converted into ppt (more accurate) or SG (less accurate). Both conversions have error because they assume a pure solution of NaCl, which a reef tank does not have.

    Your refractometer if calibrated using 35 ppt seawater solution, will have an error of 0.0005 SG due to measurement error and another 0.0005-0.001 SG of error due to variations in the chemistry of your tank from pure NaCl solution. So if your refractometer says 1.025, it actually means it is somewhere between 1.0235 and 1.0265 (or 31.6 ppt to 35.7 ppt). If you calibrated with RODI and not target salinity, then this error grows even further.

    Your cond reading is sitting near the edge of the error bars of your refractometer, so while it's possible both are correct within their respective errors, it's more likely that one or both was not correctly calibrated.


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    Regular Vistor mkeller088's Avatar
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    Thanks for the reply. So best practice is to cal the refractometer using ppt scale and 53 solution? I have pinpoint 53 and is a 35 ppt solution.
    The refractometer docs say to use RODI and scale to zero. I used Hanna lab grade water.
    Iím comfortable with the probe calibration. It settled at 34.8 in pinpoint 53 solution. Cal temp was 77 degrees.
    Probe is at 31.2ppt in the tank. Iíll add salt using the probe as my standard, targeting 35ppt.

  4. #4
    Frequent Contributor zombie's Avatar
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    Quote Originally Posted by mkeller088 View Post
    Thanks for the reply. So best practice is to cal the refractometer using ppt scale and 53 solution? I have pinpoint 53 and is a 35 ppt solution.
    The refractometer docs say to use RODI and scale to zero. I used Hanna lab grade water.
    Iím comfortable with the probe calibration. It settled at 34.8 in pinpoint 53 solution. Cal temp was 77 degrees.
    Probe is at 31.2ppt in the tank. Iíll add salt using the probe as my standard, targeting 35ppt.
    Yes. Any variation in the glass can skew the end result when calibrated with RODI. When calibrated with 35 ppt the error is reduced to essentially measurement error and temp error. Using RODI you also have glass error.

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  5. #5
    Regular Vistor mkeller088's Avatar
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    You are exactly right zombie. The difference was somewhat significant. My probe now matches my refraction meter within .5 PPT
    My standard is now 35 PPT with both measuring devices. Once again I am indebted to you and your brain of course. LOL
    Last edited by mkeller088; 1 Week Ago at 13:58. Reason: Spelling error

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