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Thread: Conductivity Probe takes more than 24 hours to settle after calibration

  1. #1
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    Conductivity Probe takes more than 24 hours to settle after calibration

    I have made a 3 probe in-line holder from 1" tees with a 1/2 reducer in each to house a BRS probe holder. It's mounted to a board in the basement just off the basement ceiling about 2' below the sump in the room above. intentionally the PVC is mounted with the feed at the bottom with each probe horizontal from the 3 TEEs the output come out of the top and returns back to the tank. This uses gravity to insure the probes are all submerged and any air is at the top before the 1/2 OD tube returns water to the tank 6-7' above.

    ORP and pH work well, conductivity is another story. I test my tank with a Milwaukee MA887 digital refractometer, it doesn't read partial PPT like Neptune, its either 34, 35 or 36, I try for 35, but, I understand this could be 34.6 to 35.4, but honestly IMHO it's not important 34,35 or 36 just that there is consistency.

    I have tried with multiple probes over the years to use tank temp calibration solution (using Apex temp compensation of 2.2), but, I get an erratic reading that often is 2 or 3 points away from my target 35 and it still takes 24+ hours to show that final number. It always reads low and goes up .1 - .3 over the 1st hour... so ... say 32.2 to 32.4 in an hour or so, then say .1 every couple of hours until reaching something other than 35, maybe 33 or maybe 37.

    So... I stopped calibrating with solution and calibrate with tank water that has my (lets say semi-unique) combination of conductive salts, which sets Apex at 53 / 35PPT in the end, but, it does the same thing as calibration solution, it starts low and takes 24-48 hours to hit 35. As I said above consistency is the key for me, IDK if actual salinity is 34, 35 or 36, just that today it's X and a month from now it's still X and not Y.

    The lag is perplexing, I thought if getting clear PVC to confirm no bubbles, but, IMHO I don't think all bubbles would take 24+ hours to go away, and I don't think all bubbles would equal 2-3PPT low. This is easy to re-produce. I can turn off all equipment except the Apex and Salinity probe and get the same results. My EB832 is grounded both to conduit and water main and I have tested for neutral isolation as well as a voltage meter to neutral and ground for any stay voltage and nothing. I just took down my old Apex in July, which had this result for 3+ years and put in a new Apex with the same results. Again, new probes, etc.

    If anyone has any idea I'd be interested. If Neptune systems had a way to set calibration this would be great for my use, right now I have to shutdown the flow to all probes to remove the conductivity probe to dry it for the "0" calibration before putting it back in for 35.0 calibration. It'd be nice to check with a refractometer and when it says 35 hit a button to say Apex is now 35.

    It's not the end of the world if there is no fix to this problem because it's just a reference for me, but, it's annoying and I figured I'd post it to see if anyone had thoughts.

    I have the following points to keep in mind before commenting:
    1.) Apex Classic and New Apex - same problem
    2.) More than 5 probes on the two different systems - same problem
    3.) No stray current in the water tested
    4.) All EBs are grounded.
    5.) All devices unplugged only Apex and CDX probe active - same problem
    6.) Always starts 2-3PPT low and works its way back to reference in 24-48 hours
    7.) Once back to reference it runs for over 6mos without calibration and even then only reason for calibration is I want to remove, check and clean the probes, this maintenance is not because of erratic conductivity readings.
    8.) The is a BRS media reactor in-line before the probe TEES with a sponge, to trap debris so the probes stay clean longer - sponge is cleaned at least once a month.
    9.) Apex FMM shows ~25gph through the BRS/Probes, when it slows sponge gets cleaned.

    Thanks,
    JW

  2. #2
    Frequent Contributor zombie's Avatar
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    Repeat the test where you were getting 32 to start and climbing up to 35. Except this time invert the probe inside your sump to get all bubbles cleared and see if it shows you 35. If it does, than bubbles are your culprit and it is just taking 24 hours to clear all of them the way the probe is oriented.

    Sent from my SM-G965U using Tapatalk

  3. #3
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    No need to turn it upside down

    Quote Originally Posted by zombie View Post
    Repeat the test where you were getting 32 to start and climbing up to 35. Except this time invert the probe inside your sump to get all bubbles cleared and see if it shows you 35. If it does, than bubbles are your culprit and it is just taking 24 hours to clear all of them the way the probe is oriented.

    Sent from my SM-G965U using Tapatalk
    I should have added it calibrates to about 34 using calibration solution in 35PPT water, in the sump. Issue I have with the sump is algae grows on the probes quickly in the sump as I have about 20G refugium section with a MARS 150W growlight. Even though the probe are at the opposite end of the sump the growlight, it's so powerful I get thick growth everywhere in the sump that I clean weekly. So... I prefer to keep the probes in-line, in the dark where they stay clean for months. I'm familiar with light refraction vs conductivity, so not suggesting that one is better than the other, just that no matter what the CDX probe seems to show lower that all other tests.

    So.... no delay in the sump, I have not tried the overflow, but, I would assume faster moving water (no bubbles) is causing the delay anomaly.

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