Weird Results from Refractometer/Conductivity Meter

Discussion in 'Reef Chemistry by Randy Holmes-Farley' started by AwesomeusPrime, Mar 8, 2018.

  1. AwesomeusPrime

    AwesomeusPrime Member

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    Hi Guys,

    I was calibrating my Apex Conductivity meter last night, using the Neptune 53,000 microsiemens/cm solution. When I finished the calibration, the probe read 35.6ppt in the solution. I placed the probe in the tank and it read 33.2ppt, lower than I thought because my refractometer was reading 35ppt.

    I then Calibrated my refractometer to 35ppt using the 53,000 microsiemens/cm solution and tested the reef water, which then read 43ppt.

    I repeated the process 3 times with 3 different solutions of the same brand of 53,000 microsiemens/cm solution and got the same results every time.

    In the past I've calibrated my refractometer using the conductivity solution and never had an issue. Why would this solution not calibrate correctly on my refractometer?

    EDIT* This solution is a solution of KCl and RODI water
    Second EDIT* Solution was at 78.3 F
     
    Last edited: Mar 8, 2018
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  2. chipmunkofdoom2

    chipmunkofdoom2 Always Making Something R2R Supporter Reef Squad R2R Excellence Award Article Contributor

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    Was the solution specifically designed to be measured using refractive index? It doesn't sound like it if it claims to be KCl solution with a known conductivity.

    I can't speak as to why this may have worked in the past, but I wouldn't expect to get proper calibration on a refractometer using a conductivity standard. In Randy's article on DIY salinity standards, he lists different recipes for refractive index and conductivity because of the way these measurement techniques differ.
     
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  3. HOOPDEEZ

    HOOPDEEZ Active Member Build Thread Contributor

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    +1 conductivity is not the same as refraction, and Neptune says the solution will not work for calibrating refractometers on their website. Just grab a solution specific to refract. They last a long time if you keep the cap on.
     
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  4. AwesomeusPrime

    AwesomeusPrime Member

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    Ah, well that solves that. This is why I didn't become a chemist I suppose.
     
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  5. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

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    Correct.

    A 53 mS/cm solution of KCl, which is perfect for calibrating a conductivity meter (assuming it is made correctly) is not going to match the refractive index of 35 ppt seawater. It's refractive index matches seawater at much lower salinity than 35 ppt, so it gets calibrated to read too high, and that explains the 43 ppt reading.
     
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  6. daelie

    daelie Active Member Partner Member 2019 Build Thread Contributor

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    Just out of curiosity, what’s the recipe for a 53 mS KCl solution?
     
  7. AwesomeusPrime

    AwesomeusPrime Member

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    32.44 grams of KCl in 1 kg of RODI water @ 59F if you aren't using a temp probe to compensate for temperature.
     
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  8. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

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    I have published a recipe for 53 mS/cm sodium chloride (as well as 35 ppt for refractometers and hydrometers, all are different):

    Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com
    http://reefkeeping.com/issues/2004-06/rhf/index.htm

    In a quick look online, I could not find KCl to match 35 ppt seawater at 25 deg C (only 15 deg C). I expect the error is minimal, however, and that value is about 32.4358 grams of KCl added to 967.5642 grams of pure water (to make 1 kg total).

    http://ieeexplore.ieee.org/document/1145440/
     
  9. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

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    I think that is 1 kg total, not 1 kg RO/DI. :)
     
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  10. AwesomeusPrime

    AwesomeusPrime Member

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    Yeah that's correct, 967.56 grams of RODI not 1kg of RODI
     
  11. Stigigemla

    Stigigemla Active Member

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  12. Randy Holmes-Farley

    Randy Holmes-Farley Reef Chemist Staff Member Team R2R R2R Supporter R2R Excellence Award Article Contributor Expert Contributor

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    Yes, all salt solutions increase in conductivity as the temp rises, and nearly all conductivity meters automatically correct for this effect. There are small differences in the temp effect on conductivity of different salt mixtures, but I think that effect is not significant enough to be a huge impact for reefers. Some fancier meters allow for different types of corrections, or even to program your own preferred correction.

    If you make either my NaCl standard or the KCl standard, it should be adequate at most any temp and the conductivity meter will make appropriate corrections. For folks interested, you can easily measure the conductivity of your standard or tank water at different temps to see how well it is working. If you think it is not working well enough, just make sure that the temp that you calibrate/check the conductivity standard and the tank water you measure are at the same temperature. :)
     
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