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Thread: Salinity probes not reading accurately

  1. #1
    Regular Vistor RoyalReef's Avatar
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    Salinity probes not reading accurately

    I bought a PM2 and an extra salinity, Temp probe so I can monitor my salt mixes in my 20 G brute container. During my water change today I put the extra probes in my main tank and notice a huge difference in my salinity. For weeks I've been testing the tank but I never thought to check the salinity with my refractometer because I was relying so heavy on my apex probe to be accurate. I'm glad I finally figured it out but I lost a few corals because of the tanks high salt levels. Has anyone else notice any similar issues with there probes ?






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  2. #2
    Frequent Contributor zombie's Avatar
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    The conductivity reading can drift over time and the probe is very sensitive to electrical interference. Either of those can give you a false reading that you don't want to make adjustments for. Rule of thumb, if the salinity probe seems wrong, it might be. Calibrate it and find out for sure. If you don't have calibration fluid on hand, make some with table salt (see randy holmes farley article) as a quick check or check against a calibrated refractometer.

    You might be an engineer if...You have no life and can prove it mathematically.

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    Bubbles trapped in it will also make it read low. I had mine in the skimmer compartment and it didn't take long for the reading to drop. Once moved to an area with no clouds of fine bubbles it stopped drifting down.

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    Frequent Visitor robbous's Avatar
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    I just noticed my new apex system was reading salt low, I calibrated it twice and still reads 4ppt lower then what I am getting from refractometer, this is a brand new Apex and just set up so I'll give it a few days and re calibrate again and see what happens

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    Frequent Contributor zombie's Avatar
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    Quote Originally Posted by robbous View Post
    I just noticed my new apex system was reading salt low, I calibrated it twice and still reads 4ppt lower then what I am getting from refractometer, this is a brand new Apex and just set up so I'll give it a few days and re calibrate again and see what happens
    A few things to make sure you do on the recalibration.

    1. Make sure there aren't any bubbles in the probe during the 53mS calibration and make sure to rinse in RODI and lightly dab off excess water with a paper towel on the 0 step.

    2. Make sure the packet is the same temperature as your temp probe (float for 15-30 minutes and keep it touching water while the probe is in it.

    3. Verify refractometer reads 35ppt with a couple drops of the calibration fluid.

    4. After calibration is complete, verify a reading of 0 in RODI and 35ppt in the calibration fluid.

    You might be an engineer if...You have no life and can prove it mathematically.

  6. #6
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    I always test and calibrate my salt mix independently, the salinity probe is just in the tank as a double check.

    I would only use it as a clue to do further testing with a digital or optical kit....

    In fact, I never trust a reading from a single device...i always make sure I can repeat the test and correlate....sometimes using the probe, an optical and a digital all on one batch.



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    Frequent Visitor robbous's Avatar
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    Quote Originally Posted by zombie View Post
    A few things to make sure you do on the recalibration.

    1. Make sure there aren't any bubbles in the probe during the 53mS calibration and make sure to rinse in RODI and lightly dab off excess water with a paper towel on the 0 step.

    2. Make sure the packet is the same temperature as your temp probe (float for 15-30 minutes and keep it touching water while the probe is in it.

    3. Verify refractometer reads 35ppt with a couple drops of the calibration fluid.

    4. After calibration is complete, verify a reading of 0 in RODI and 35ppt in the calibration fluid.

    You might be an engineer if...You have no life and can prove it mathematically.
    Did all what you explained here and vola it's reading correctly. Thanks !
    Reefing is my passion, 500 Gallon Display tank with an additional 150 gallon sump room in the basement

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    So, read a bunch of forums on this topic and did the following:

    My salt is 1.025 using refractometer (so I am ok). 2 days ago, I recalibrated the probe, it went from reading 22 to 34 and change, did well for a few hours then dropped immediatly (straight line to 20's again). So I began reading, I moved the probe stand, got bubbles out (or potential bubbles), replaced the probe in the holder all underwater to keep out bubbles and it jumped right into the mid-30's. Felt real good, then it happened again, nothing changed. (see pic).

    Other thoughts, this is a 2 week old system/probe. Could I have gotten a bad probe?

    Image-1.jpg

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    Regular Vistor RoyalReef's Avatar
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    Salinity probes not reading accurately

    Quote Originally Posted by panamagixxer View Post
    So, read a bunch of forums on this topic and did the following:

    My salt is 1.025 using refractometer (so I am ok). 2 days ago, I recalibrated the probe, it went from reading 22 to 34 and change, did well for a few hours then dropped immediatly (straight line to 20's again). So I began reading, I moved the probe stand, got bubbles out (or potential bubbles), replaced the probe in the holder all underwater to keep out bubbles and it jumped right into the mid-30's. Felt real good, then it happened again, nothing changed. (see pic).

    Other thoughts, this is a 2 week old system/probe. Could I have gotten a bad probe?

    Image-1.jpg
    I honestly had something similar happen to me when I first got my apex set up. It’s been 5 months since, I noticed the problem happen every now and then specially during my first two weeks of having the apex.

    I noticed the probe were off by half for awhile and I recalibrated everything which is the biggest pain in the rear at least for me. Never works like it says it does. I’ve tried to clean the probes with DI water and then recalibrate the probes and it’s worked fine. Tiny bubbles can be a pain to deal with.

    I noticed after I moved from MA to CA my probes haven’t had a problem since the first weeks of setting up the apex I was afraid that I would have to recalibrate everything but it’s plug and play now. Crossing my fingers.

    Have you tried the steps listed above to resolve the issues? There was a short time not to long ago that I saw the threads blowing up about the calibration issues. If all else fails try reboot it and then reconnect the probes. I had probe with loose connections and just by reconnecting them fixed the issues.


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    Yes, I did everything listed here. The weird thing is it worked for hours (overnight), then just started reading wacky - not a gradual change from 30's to 20's from 1 read to the next? Not sure why it would work so long, then go nuts? I'll try again tonight, I'll check to make sure its all plugged in good, etc....

  11. #11
    Frequent Contributor zombie's Avatar
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    Quote Originally Posted by panamagixxer View Post
    Yes, I did everything listed here. The weird thing is it worked for hours (overnight), then just started reading wacky - not a gradual change from 30's to 20's from 1 read to the next? Not sure why it would work so long, then go nuts? I'll try again tonight, I'll check to make sure its all plugged in good, etc....
    My first guess would be bubble buildup.

    You might be an engineer if...You have no life and can prove it mathematically.

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    Frequent Visitor Ferrell's Avatar
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    I am having the same problem as well. After 2nd calibration mine read nearly perfect for about a day and has been steadily creeping UP over the last Day. It’s reading 42 now. Calibrated refractometer and checked salinity and it’s at 34. Whew. Going to try rinsing In rodi and then will have to get more solution to try recalibration again. From this thread looks like all done right just a super sensitive hard to manage probe issue. Thanks for all y’all do

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    I have had my Apex for about three weeks. My salinity probe has always be way off. It is showing 46.2 now even though I have checked my salinity multiple times with a Milwaukee refractometer and also taken my water to my LFS (both say the water is at 34).

    I have purchased extra calibration solution and run the calibration routine 3 times and it always says calibration was successful but when I put the probe in the tank I get a bogus reading.

    I will try some of the steps above (pre-cleaning with RODI and changing the location) to see if I can get it working. I was considering putting the probe in the refugium away from any source of bubbles.

  14. #14
    Frequent Visitor Ferrell's Avatar
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    I rinsed mine with RODI, read it at zero and then placed back in the sump where it read 38.5. My refracometer measures it at 34. Will try reocating but not right now. Just knowing that it’s wrong is good enough for tonight is good.
    Last edited by Ferrell; 12-04-2017 at 19:17. Reason: Autocorrect

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    Salinity probes not reading accurately

    Easy way to check calibration. What does it read while in the calibration fluid?

    If you are wondering if there is EMi affecting the reading, take a cup of tank water and place the probe in it. What does it read?

    Be aware that if you are using Fusion to do testing, It can take sometimes up to 30 sec or more for the value to be updated. Use local dashboard instead.

    Bubbles trapped in the probe is always an issue. Align the hole on the side of the probe upwards, tap on the probe to dislodge any bubbles.

    If you calibrate with a bubble lodged in it, it may calibrate fine and read 35 in fluid,but if when you transfer to the sump and the bubble got dislodged, then the reading will jump up even though the water is at 35.

    Reverse will happen if the bubble get lodged when placed in the sump. You will read low.

    If it reads good then slowly drops, you probably have a micro bubble issue.

    If the reading is weird and fluctuates, you probably have EMI.


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    Frequent Visitor Ferrell's Avatar
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    E00D3BE0-525D-4786-A80A-C684C11F83A3.png
    This is what I get. I put the probe in tempered rodi and read 0 then moved to sump then the creep started. Thinking maybe some type of isolation box or putting probe in display tank to try. Is this bubbles or EMi? And what is EMi? Electric interference?

  17. #17
    Frequent Contributor zombie's Avatar
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    Emi is electromagnetic interference. It means that the magnetic or electrical potential waves in the air near the probe cord are inducing a current or voltage onto the probe cord and skewing readings.

    I can't tell from your picture if it's bubbles, EMI, or just the slow change time of the probe (it can take several minutes to an hour or so to reach steady state).

    One quick way to eliminate EMI as a variable is to take a cup of tank water out of the tank and put the probe in that, making sure the probe goes in within a couple minutes (to make sure temp roughly matches tank temp) and also making sure the cord is not nearby any other wires (particularly parallel to other wires).

    You might be an engineer if...You have no life and can prove it mathematically.

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    Frequent Visitor Ferrell's Avatar
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    When I get off work tonight I’m going to try another little experiment.
    Before work today I placed the probe in a cup of rodi in a little spot in the sump that will keep the water at tank temp. It read zero, took it out and placed the probe in the tank where it read 18.5 (3:30 pm) It’s 7:10 pm and the reading has been steadily rising and is up to 27.5. Before I get home tonight or at the latest in the morning it will be up to 40 again.

    Im going to try something tonight: I’m going to take a cup of sump water and place it in that cup and see if that helps. It’ll stay at tank temp but will have no flow. If that does it, it’s placement. If it doesn’t help then it’s EMI. Does that sound about right?

  19. #19
    Frequent Contributor zombie's Avatar
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    That would definately eliminate bubbles and flow as an issue. Doesn't narrow down the k8nd of EMI but would likely be in that realm.

    You might be an engineer if...You have no life and can prove it mathematically.

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