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Thread: :: grr:: conductivity probe calibration is driving me nuts

  1. #1
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    :: grr:: conductivity probe calibration is driving me nuts

    OK, so I posted earlier about trying to dry my probe for calibration, but this is basically a different issue, so I figured I'd start a new thread.

    I got my conductivity calibration fluid a few minutes... pinpoint brand, 53mS. Went through the whole calibration process... probe showed 34.9PPT in the fluid after calibration, so I stuck the probe in the tank... it immediately dropped to 32PPT. I know the tank is at 35PPT because I just calibrated my hydrometer yesterday. So I go through the calibration process a second time with the same results... 34.9PPT in the solution... 32PPT in the tank. I tried to take a cup of tank water and test that in case there was stray voltage in the tank... still 32PPT. Put the probe back in the solution and it goes back up to 34.9

    So as I'm sitting here pulling my hair out I grab the refractometer and figure I'll try to recalibrate that and test again just in case that reading was off and the water really isn't at 35PPT. As I was doing so, I happened to notice that my refractometer calibration solution says "35PPT Refractive Index Standard Solution", but below that it also says that it's 53mS. So I put a bit of that solution in a bag, get it up to temp in the tank and stick my conductivity probe in it and get a reading of 38PPT.

    So at this point it looks like my two calibration fluids differ by about 3PPT and the question now is, which one do I believe? The one I just got today is pinpoint and the other is from Aqua Craft Products and I only got that a few days ago, so unless they had been sitting on the shelf for a long time at Amazon they should both be fresh.

    Should I get a third fluid to try to test with? One of my LFS's sells a lot of Neptune stuff and they may have a packet of the Neptune solution on the shelf. I avoided that one because I figured that the 2oz bottle I got would last longer than a single packet, but at this point I'm thinking that I might need a third source.

  2. #2
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    Well, I found a recipe to make a 53 mS calibration standard from table salt and purified water from reef chemist extraordinaire Randy Holmes-Farley. Available here if anyone else wants it...

    http://reefkeeping.com/issues/2004-06/rhf/index.htm

    I've got my salt and I'm waiting for my two liter diet coke bottle to fill with RO water right now. I'm not sure I'd trust this solution to calibrate my system, but hopefully it will be close enough to let me know which calibration solution is correct.

    I'll post my results shortly after I do my testing.

  3. #3
    Master Control Freak aquamanic's Avatar
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    It will work if you measure precisely. That's the key.
    Al

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    OK, well that just made things worse. Now all three solutions read different values. I have one that reads at about 35PPT... the one I used to calibrate it with. One that reads 38PPT and the one I just made, which reads 32.4PPT. I'm inclined to trust the one I made the least, since I tried to measure as carefully as possible, but using 2 liter bottle and measuring cup are probably not totally accurate.

    I'm guessing that since it was on the lower end that maybe the solution that's currently reading 35PPT is correct, but at this point I just don't know what to believe.

  5. #5
    Master Control Freak aquamanic's Avatar
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    You did read Randy's article where he says that the '2 Liter' bottles aren't really?
    Al

    I do not work for Neptune. Please do not send me PMs with technical questions or requesting assistance - use the forums!
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    Quote Originally Posted by aquamanic View Post
    You did read Randy's article where he says that the '2 Liter' bottles aren't really?
    I didn't notice that, although he did specifically call for a 2 liter coke or diet coke bottle. I used a diet coke one, so hopefully that's what he was expecting. I'm not sure I put too much faith in the solution I made simply because of a lack of accurate measurements.

    I did find a store with some of the Neptune fluid in stock, so I'll probably try to pick some up later this afternoon and see how that goes. I did see a big discussion on reefcentral about the pinpoint calibration fluid and it seems like people seem pretty happy with it. That's what I'm currently calibrated with... I'd just be happier if I could get a second data point to confirm the accuracy.

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    OMG... just when I think this can't get any worse, it does. I finally got out to my LFS and picked up a packet of the Neptune calibration fluid. Figure I'm gonna get home.. use the Neptune fluid, which will hopefully match one of my other calibration fluids and I'll be happy. So I open the Neptune fluid, drop in the probe and... 0.0. It doesn't even register. Just for fun I put a few drops of the Neptune fluid on my refractometer and it also is registering 0.0. I wouldn't expect the Neptune fluid to register a specific value on the refractometer, but I would expect it to register *something*. I'm pretty sure the fluid I have is just distilled water. I'll have to call Neptune tomorrow and see if they had a bad lot or something.

    I'm laughing at this right now... there's nothing else I can do. I now have 4 different calibration fluids and none of them are giving me the same reading. So much for standards

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    Haha I have the same issues.

  9. #9
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    You are mistaken to think that 35ppt always is = to 1.026 on your refractometer due to chemistry of the water. (magnesium, calcium, etc.) will have different affects on a refractometer vs. a conductivity probe. But the 53,000mus Apex calibration fluid should have a similar affect as the other brand of the same equivalent. Seams a bit weird. Some others with more knowledge on the subject could probably help a little more.

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    It is more than that.

    My tank is 1025SG, checked with 3 floaters.

    When I calibrate with 53/ms reference liquid, and I put it back in my tank, my probe tells me my tank is 44 PPM!

    When I recalibrate with 35 PPM reference liquid and I put it back in my tank, my probe tells me my tank is 36 PPM!

    While it should be about 32 / 33 PPM and I checked stray voltage.

  11. #11
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    :: grr:: conductivity probe calibration is driving me nuts

    Quote Originally Posted by TimS View Post
    My tank is 1025SG, checked with 3 floaters.
    Unless the floaters have been calibrated, then you still need to take those measurements with a grain of salt too. Also keep in mind that that you need to compensate for temperature with the floaters. They probably read a bit low without it (although not low enough to be the difference between 35PPM and 44PPM

    When I calibrate with 53/ms reference liquid, and I put it back in my tank, my probe tells me my tank is 44 PPM!
    After you did this calibration and got the bad reading did you try to put the probe back into the calibration fluid and see if it still read 35PPM there? It's possible it was a bad calibration... Or maybe bad fluid like I'm seeing. You could also put your probe in a cup of tank water just to use water isolated from the tank

    When I recalibrate with 35 PPM reference liquid and I put it back in my tank, my probe tells me my tank is 36 PPM!
    The only thing you can use to calibrate your probe is 53ms solution. Solution meant to calibrate other things (Hydrometers or refractometers) may be accurate for those devices, but may not be 53ms. It is possible that a solution may be useful for calibrating different devices, but unless it specifically states that it is 53mS then it can't be used to calibrate a conductivity probe.

  12. #12
    Frequent Contributor Alain B's Avatar
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    Use the 53ms solution , if the calibration is made properly it can be trusted . I only use the conductivity probe for my system , i could clean up the tool kit as i had throwned all others devices in file 13 .
    Alain

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    Quote Originally Posted by Alain B View Post
    Use the 53ms solution , if the calibration is made properly it can be trusted . I only use the conductivity probe for my system , i could clean up the tool kit as i had throwned all others devices in file 13 .
    Yeah, but my problem is that I can't trust my calibration solution I calibrated with one 53ms solution and when I put the probe in that solution it consistently reads about 35PPM. When I put the probe in another 53ms solution it reads 38PPM. I get 22.6 in a third solution and 0.0 in a fourth (although I'm quite certain there is something wrong with that fourth solution)

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    I opened a support ticket with Neptune for the bad solution I got. They didn't seem to have any idea why the solution would be bad, but they are sending two new packets to replace it, so that's good

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    :: sigh:: Well, I got the new solution packets in the mail today. I went ahead and stuck the probe in the solution and got a reading of 33PPT... interestingly enough, that's closest to my homemade solution. I'm still a bit surprised that I can buy three different 53mS calibration standards and have them be that far off. There's a 14% difference between the one that reads the lowest and the one that reads the highest (and that's not counting the homemade one).

  16. #16
    Master Control Freak aquamanic's Avatar
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    I'd trust the packet. Make sure your temp probe is in the packet with the conductivity probe when you calibrate. And use temp compensation.
    Al

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  17. #17
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    Why would you trust the Neptune packet over the pinpoint or aquacraft? They are all supposed to be the same standard and all at about the same price point.

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    I have a fine calibrated refractometer ( calibrated with 35 ppm solution at 25 °C and read 0 with RO water ) , set my tank to 35 ppm with refractometer and 25 °C and calibrated my salinity probe with water tank , so no pb ... work fine

  19. #19
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    Quote Originally Posted by BrettS View Post
    Why would you trust the Neptune packet over the pinpoint or aquacraft? They are all supposed to be the same standard and all at about the same price point.
    Not trying to sound condescending, but you should pick one and go from there, use the neptune pack and calibrate off of that, trying to double guess yourself is going to drive you , beleive me been there, done that, on ORP reading, PH readings, AMP readings, good luck

    sana

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    Quote Originally Posted by Sanababit View Post
    Not trying to sound condescending, but you should pick one and go from there, use the neptune pack and calibrate off of that, trying to double guess yourself is going to drive you , beleive me been there, done that, on ORP reading, PH readings, AMP readings, good luck

    sana
    If they were closer together I probably would... But the difference between 33.6 ppt and 38 ppt is pretty big. Even if I just go in the middle I could be off by as much as 3 ppt, which is still pretty significant.

  21. #21
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    I use mine as a weather indicator, and check with my refractometer if it changes much. Seems the most stable for me when the probe is in my sump with a power head blowing across it to keep out microbubbles. These probes are Way too sensitive to microbubbles.

    Sent from my SM-N915T using Tapatalk

  22. #22
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    If you want them to match, decide which you trust more- refractometer or salinity meter- and calibrate the other off of it. If the salinity meter says the tank is at 32PPT then take some of the tank water and calibrate the refractometer to 32PPT. From then on they should match pretty well unless calibration drifts or tank parameters change.

    I use the refractometer for mixing up salt water and as a periodic double check of my salinity meter and recalibrate if they don't match fairly well. Otherwise I go by the meter.

    Allen

  23. #23
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    I'm reading this all carefully because I'm going crazy with the same problem... I'm REALLY not comfortable with the idea of deciding one calibration to trust...... My tank was reading 33 on the probe before I calibrated. I had 34 on newly calibrated refractometer. Now reading 29 after 2 calibration with 2 different neptune fluid packet!!! I just CAN'T trust the one from neptune because I'm so convinced that my tank is closer to 34 than 29!!!

    I just dont get it... I wonder if BRETTS found out what to do since his last post was from 07-29-2015...


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