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  1. #1
    Senior Member SloHand's Avatar
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    Jan 2004

    Help Converting S/cm to mg/L

    Hi Everyone, I have a DiST WP #4 TDS meter that measures TDS in S/cm. The instructions with the meter say to multiple the results by 100 to get the value in S/cm. Here are the measurements I'm trying to calculate based on the little (and I mean SLIM) understanding of what I thought was the proper conversions**. I should state that this is all in an effort to determine my magnesium levels (Mg2) in mg/L.

    I'm using the Hagen calcium test kit to achieve this calculation. Before you all fire off responses about "oh...that crappy Hagen test kit stuff" know that this is what I have, all I have access to right now, and yes, someday I'll drop $200 on better test kits.

    In the meantime, here is my results using what I think is the right conversions for the TDS meter;

    - Calcium (Ca2+) = 360mg/L (low I know but this is part of understanding why, I think)
    - Calcium Hardness (CaCO3) is (Ca2+ X 2.5) = 900mg/L
    - ** General Hardness is (TDS Meter = 45, multiplied by 100, multipled by 1.7 [this is the conversion facture to get from S/cm to mg/L, right?]) = 7650mg/L
    - Magnesium Hardness is General Hardness (7650) minus Calcium Hardness (900) = 6750
    - Magnesium is Magnesium Hardness divided by 4.1 = 1646.34mg/L (?)

    Provided I have converted properly , this results seems awfully high ("For practical purposes, 1250-1350 ppm is fine [for magnesium]" Quote from Randy Holmes), and if it is correct should I be concerned? But before being concerned about this reading, can anyone, or does anyone know if my conversion from S/cm to mg/L that I'm getting on this TDS meter is correct? Anyone else use this TDS meter, Hanna Instruments (

    Much appreciated if anyone can comment.


  2. #2
    Senior Member reefmutt's Avatar
    Join Date
    Oct 2003
    Sorry, man. None of that really makes any sense to me...

    Old system torn down to make a playroom.. planning a 62x42x28 high

  3. #3
    Senior Member aragorn69's Avatar
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    Aug 2004
    I found to use 0.6 instead of 1.7 for S/cm to mg/L.
    maybe that will help

    A Black Hole Is Were God Divided By Zero!

  4. #4
    Senior Member
    Join Date
    Aug 2003
    Originally posted by aragorn69
    I found to use 0.6 instead of 1.7 for S/cm to mg/L.
    maybe that will help
    I have to agree with aragorn69 that the 0.6 is a better approximation, there is no straight forward conversion from microsieme/cm to ppm. However, a correlation can be arrived at based on your meter and solution used to test. To get your realistic conversion you can do a few tests.

    Mix Saltwater with specific gravity of say 1.0020 to 1.0030 in increments of 0.0001, now you have 11 samples. Measure the EC in microsiemen / cm from your meter (multiplied by 100).

    1.0020 saltwater contains approximately 2000ppm TDS
    1.0021 saltwater contains approximately 2100ppm TDS
    and so on.

    From a plot of these readings you along with EC measured, will give you a more realsitic conversion. And I am sure you'll find that the conversion is between 0.55 and 0.6 for your Hanna Meter DIST WP 4.

    The major components in Natural Sea Water are:


    All these components contribute to your TDS, so the assumption that all the TDS contributes to Total General Hardness does not hold good.

    The hardness in the water is contributed by Magnesium, Calcium, Sulphates, Carbonates and Bi-Carbonates.

    So using this approach to determine the magnesium composition in your tank may be difficult to achieve, but nonetheless, I have to commend you on the thought process.

    If you find anymore info, please feel free to share it with me.



  5. #5
    Senior Member SloHand's Avatar
    Join Date
    Jan 2004
    Hey Aragon, Venki, Thanks for your help here ... wow! with a conversion factor or 0.6 instead of 1.7 my final results for magnesium are drasticly different (new conversion = 439.02mg/L) I have very poor understanding of this conversion and was surprised that there was no straight forward conversion. I think I'll just have to get the Salfert Mg test kit and be done with it.

    Thanks for your help.

  6. #6
    Senior Member papafish's Avatar
    Join Date
    Nov 2004
    Use you your TDS based on manufacturers recommendations. Don't forget, the instrument compensates for temperature. Following a conversion based on 25 degrees C will not help you. I'm not familiar with Hanna's TDS, so I can't help you there. But, I've been using a sump based on Jaubert's findings since 1993. Hands down, my opinion, the best thing since sliced bread! (with a few modifications). But, do not add any minerals or trace elements before testing. Aragonite is stable at pH7.5, below that it starts to dissolve, releasing all the goodies! Million year old goodies!! I've sent my pH probe through the substrate to touch the bottom glass, found readings of 6.8 and 7.1. In 96/97 I had and major hrdrogen sulfide bout. That's when I changed a few things. I haven't had any trouble since. Nitrates have been undetectable for a few years and I feed my fish...believe me!! During the summer when I have to change my light cycle because of heat, my additives are almost nil. In the winter with 7hrs MH and 11hrs actinic. I'm adding elements every 3-4 days, and Kent Turbo Calcium every 2 days.

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