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Thread: Salinity probe calibration.

  1. #1
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    Salinity probe calibration.

    Hi, I calibrated my salinity probe, I floated the calibration packet in my sump for an hour. I followed all the steps with the wizard completing with no issues. Only thing is the probe is reading 36PPT, when I know my salinity is 35PPT. I check it with 3 different refractometers including a digital one. Is there a way to manually set the probe to 35? Thanks in advance for any infomation.....

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    Frequent Contributor zombie's Avatar
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    Nope. Are all three calibrated? If not, although somewhat unlikely the refractometers may all be off. If they are, and your tank temp is a few degrees outside 77F, then you may just need to tune your temp compensation. It is 2.2 by default, but some salt mixes can bring it anywhere from 1.9 to 2.5.

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    Hi, my Tank is at 77.9° Salinity reading 36.4. All of my refractometers are calibrated using 35ppt solution.

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    Quote Originally Posted by Zanclus View Post
    Hi, my Tank is at 77.9° Salinity reading 36.4. All of my refractometers are calibrated using 35ppt solution.
    It's in the advanced tab of the probe page where you did the calibration called TC factor.

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    I found it it was set to 0 I put to 2.2 and now Salinity is reading 35.9....

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    Frequent Contributor zombie's Avatar
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    That would explain it. Since you calibrated with it set to 0 at a temperature other than 77F, the conductivity of the solution was skewed by temp but not corrected. I would let it soak as is for a week to let everything stabilize and see how well the temp compensation is working (you should see no more than 0.5 ppt variation as temp changes otherwise TC needs to be adjusted by 0.1). Then if all is good, repeat calibration with the tuned TC and it should be right on the dot.

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    Thanks I'll do that.....

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    Happened upon this thread as I want to recalibrate my salinity probe as it has never really read correctly for me. I just realized that my TC was also set to 0. Is the TC based off an assumed temp of 77 degrees? My avg temperature ranges from 78 to 80.5 depending on time of day. If that’s the case i believe I’ll need to increase my TC in order to accurately recalibrate the probe. Any advice on doing this properly this time around would be greatly appreciated. Thanks Zombie!

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    Quote Originally Posted by Sanfordnsn View Post
    Happened upon this thread as I want to recalibrate my salinity probe as it has never really read correctly for me. I just realized that my TC was also set to 0. Is the TC based off an assumed temp of 77 degrees? My avg temperature ranges from 78 to 80.5 depending on time of day. If that’s the case i believe I’ll need to increase my TC in order to accurately recalibrate the probe. Any advice on doing this properly this time around would be greatly appreciated. Thanks Zombie!
    Just set it to 2.2 and recalibrate.

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  10. #10
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    Palm to face... LOL.
    I did not know what that setting was compensating for either, My tank temp was reading 77.9 also and now the Conductivity probe is reading 34.6 which is Ok I suppose but now I'm wondering if I should give it another go after a while of running it. Meh, not super important or off I suppose though I do appreciate the information given here and learning a bit more.

  11. #11
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    You're splitting points trying to get it from 34.6 to 35.0 in the tank. Refractometers are only accurate to +- 1 ppt so it's actually much more likely at this point that your tank is actually at 34.6 and there is a teeny bit of error in the refractometer.

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  12. #12
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    I got a lengthy email from Apex reps describing that the reading is not in PPT.

    While it’s a lot to read, it definitely is useful.

    “The Apex probe measures the conductivity of the water, the refractometer measures the change in the speed of light when it goes from air to water, a hydrometer measures how dense the water is. These are 3 different things you are measuring, none of them is a count of the particles of stuff in a million particles of water. All 3 will give different readings if you dissolve different things, 35.0 ppm of sodium chloride will read different than sea water with 35.0 ppm of stuff in a refractometer.

    Our cond probe will not read number to number identically with milwakee's calibration or any other third party tools for checking this measurement.

    If you calibrated the Cond probe properly when placed In the 53mS solution it should read around 34.8~35.2. If it did read that after you calibrated it than you successfully calibrated the probe. That's what's important. That it read 34.8~35.2 in the calibration solution from Neptune systems.

    Can you put the probe back in the 53mS solution and what does it read? If it does not read within that range then calibrate the Cond probe again. If it did, than you're done.

    After that, if you drop it in to a tank or any environment it will read what the conductivity is of that tank. It's not going to be the same as your refractometer, hydrometer, or anything. Our probe reads the conductivity. It's not the same.

    To clarify, the conductivity probe functions by measuring the displacement of NaCl ions in the water. The Conductivity probe is not a refractometer. It's not going to read like your refractometer. This is common. It's a conductive sensing probe. Your refractometer is designed to detect specific gravity/density of all the salts in your water like KCl, MgCl, not just NaCl, but the conductivity probe will give you a closer look at the the true salinity (ppt) reading in your aquarium.

    Typically most refractometers are not calibrated for Seawater, but a brine solution, thus they are reading the SG/salinity of many other salts in your water other than NaCl. So they will read slightly differently than a conductivity probe which only measures your actual NaCl concentration. Here is ant article on Reef Builders illustrating this discrepancy and points to another article discussing these differences. So it is not uncommon to see this difference between most refratometers and salinity probes.

    Here is a great Salinity Calculator the converts ppt to specific gravity and Conductivity
    https://www.hamzasreef.com/Contents/...onversion.php”





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  13. #13
    Master Control Freak RussM's Avatar
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    Quote Originally Posted by Bill_M. View Post
    I got a lengthy email from Apex reps describing that the reading is not in PPT.
    The Apex conductivity probe reading for an Apex 2016/ApexEL is displayed in PPT (part per thousand). The "ppm" in "35.0 ppm" was a typo.
    Please do not send me PMs with technical questions or requesting assistance - use the forums for Apex help. PM me ONLY if the matter is of a private or personal nature. Thanks.

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    Hmm. That’s not what the email from Morgan said. He specially said it does not measure salinity in PPT, rather, it measures the conductivity of the water.


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    Quote Originally Posted by Bill_M. View Post
    Hmm. That’s not what the email from Morgan said. He specially said it does not measure salinity in PPT, rather, it measures the conductivity of the water.


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    Here is a more information that might help

    Conductivity in mS/cm indirectly measures the number of ions in solution. The value can be converted directly to ppt linearly with no error as long as it is compensated correctly for temperature.

    Refractive index measure the bending of light, which is slightly skewed by heavier ions, but not to a large extent. This cant be directly converted to ppt unless you know the composition of the solution (ie CaCl vs NaCl) but as long as one element reigns supreme (NaCl) the error in conversion is small. The potential skewing and measurement error can each introduce about 0.5 ppt error in either direction, which is why I mentioned earlier that refractometers are only accurate within +- 1 ppt.

    Specific gravity measures the weight of dissolved ions and is heavily skewed by heavier ions like magnesium and calcium. The difference in NSW parameters vs SPS dominant parameters can alone make 2ppt difference without factoring in measurement error, which is 1-2 ppt as well.

    The osmotic pressure on marine life is the most important aspect for coral biology with regard to salinity levels. This value is directly related to ppt of dissolved solids or the number of ions in solution and doesn't care what their weights are. Because of this, conductivity is the most reliable measure for what matters most in coral and fish biology, which is the osmotic pressure or number of ions in solution.

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  16. #16
    Master Control Freak RussM's Avatar
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    Quote Originally Posted by Bill_M. View Post
    Hmm. That’s not what the email from Morgan said. He specially said it does not measure salinity in PPT, rather, it measures the conductivity of the water.
    That correct - the probe measures conductivity. But the measurement is then internally converted and displayed as salinity expressed in units of PPT.
    Please do not send me PMs with technical questions or requesting assistance - use the forums for Apex help. PM me ONLY if the matter is of a private or personal nature. Thanks.

  17. #17
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    Quote Originally Posted by RussM View Post
    That correct - the probe measures conductivity. But the measurement is then internally converted and displayed as salinity expressed in units of PPT.
    Mind blown. Lol didn’t know that.


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    I have tried and tried to calibrate my salinity. It won’t get past the dry calibration part. It settles then goes through whole time and acceptable range never checks off. Then after time runs out it says communication error did not calibrate.
    Any ideas?

  19. #19
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    Quote Originally Posted by Guntert View Post
    I have tried and tried to calibrate my salinity. It won’t get past the dry calibration part. It settles then goes through whole time and acceptable range never checks off. Then after time runs out it says communication error did not calibrate.
    Any ideas?
    Reset the probe by running a manual calibration with it unplugged. If your probe hasn't soaked in your tank for a few days, don't bother with calibration or you will just have to do it again in a couple days.

    Then do the calibration. For the dry step, rinse the probe in RODI and pat dry with a paper towel. For the wet step, make sure temp compensation is enabled at 2.2 to start, the packet matches tank temp exactly, and that you shake all bubbles from the probe.

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  20. #20
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    Not accurate

    I did did what you said and now it’s calibrated but doesnt match my Cora life meter. Apex says 27.1 and meter says 31

    how can I fix it.

    Quote Originally Posted by zombie View Post
    Reset the probe by running a manual calibration with it unplugged. If your probe hasn't soaked in your tank for a few days, don't bother with calibration or you will just have to do it again in a couple days.

    Then do the calibration. For the dry step, rinse the probe in RODI and pat dry with a paper towel. For the wet step, make sure temp compensation is enabled at 2.2 to start, the packet matches tank temp exactly, and that you shake all bubbles from the probe.

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  21. #21
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    Is your coralife meter calibrated?

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  22. #22
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    Can any one tell me how to manually calibrate my apex probe? Auto calibration is not cooperating.
    Current Tank Info: Red Sea Reefer XL 425: Born on August 29, 2017

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