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Thread: KHD works for several days then results drop

  1. #1
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    Frage KHD works for several days then results drop


    Wondering if GHL can help me - I have been struggling for a while to get consistent results.

    Firstly I want to know the exact steps to reset / recalibrate IF I suspect the results are wrong. I have been:
    1 - Empty measurement cell completely
    2 - Recalibrating the pH probe
    3 - Emptying and refilling the Reagent dosing tube
    4 - Vent the reagent tube

    Are these the correct steps and am I doing them in the correct order?

    The problem is that results reads fine and consistent for a few days and then the reading drops suddenly.

    To give you an idea:
    15/06=8.1 16/06 = 7.1 so I increased alk, however, each time I tested the result was below 7 till on the 20th I tested with Salifert (it was over 11). I reset - read 0 then 4.9 then 7.2 then 7.3
    Tried resetting the next day - still 7.3.

    Then turned it off testing manually with Salifert every day.

    On the 25th I set it up again - like calibrated probes and dosing pumps 1 and 2 (tank water and reagent).
    I tested and got 9.5 (Salifert=10.2) - I think this is good. Next day 26/06 ; 7.8! (Salifert 9.6) ; 27/06 = 9.0 (s=9.5) 28/6=9.4

    Then steady till 07/07 = 9.4 (s=9.9) ; 08/07 = 7.7! So I emptied and refilled the reagent dosing line. tRepeated the test = 7.5… (the first test typically runs low after a reset) - fine I waited till the next day… 09/07 8.2 vs s9.9 and 10/07=8.2 (s=9.8); 11/07 9.8 (s=9.7) Therefore I think it is back working.

    Then 13/07 7.4 again! (salifert = 9.0)

    So on the 14th I reset it (like empty measurement cell and vent internal reagent tube) - then I run ATI Standard water (I cannot get GHL Standard here) - The ATI kH should be 7.0) - it’s 0.0.

    Still 14/07, I run test again on tank 7.6 then a few hours later 8.2 and in the night (I schedule it to run at 10:30pm) - 8.4 (vs Salifert 8.7) - this seems right - Salifert is typically 0.1-0.7dkH higher according to my testing in comparison to when I think the GHL is correct.

    So today I empty measurement cell completely - run new reagent into the tube - vent reagent tube - test... I ran it again on the ATI (I just put the tube in the ATI) - I get 8.1 (should be 7.0) - Then I run it on the tank 8.3 - Tested it just now with Salifert - 8.6.

    Note: I did Salifert on the ATI and got 7.7 - which seems right as I have heard Salifert reads higher by about 0.5-1dkH).

    So once again for clarity I run on ATI Standard - get 8.1 (when it should be 7.0) - Then I run it on the tank - Get 8.3 - Then I test the tank water with Salifert - Get 8.6 - then I test ATI with Salifert and get 7.7.

    I think this is a big difference when tests are run so close together. GHL KD says the 2 sources are 0.2 apart - Salifert says 0.9.
    Is it possible the measurement cell is not emptying completely after each test?

    So there are 3 big issues -
    1 - When I reset it I often get a low value and seems to take a couple of tests to get to where it likely should be
    2 - It seems to be producing consistent results and then suddenly it tests way lower
    3 - Those consistent results whilst inline with what I expect but then I test against the standard solution and I get a close result to my last test which I would expect to be 1dkH higher than the standard solution.

    I have had the KHD for a few months. I have an Environmental Science degree (incl oceanography and chemistry). I cannot understand why I cannot get it to work.

    I was told here by GHL to get a standard solution and check for bubbles in the lines. The intake in the tank clears the line each test and when filling it seems to have some small bubbles from the filter. I followed the setup guide - which way up should the filter be?
    Does the filter need to be lying in a particular position - mine is line from tank comes into the small end and filter is angled mostly down so the water flows down (thicker part at the bottom).
    The Reagent line seems free of bubbles.
    The wastewater line has bubbles sometimes (I would expect this doesn't matter as it is just draining the last of the waste).

    My best guess is that the wastewater drain is not working. I am going to re-enter the value for that dosing pump so it is lower than it is - then it should drain longer. But I am running out of ideas.

    Honestly the complexity to try and set this up was way too difficult. But the issue is it's too complex when there is a problem. There should be a troubleshooting guide. There should be a maintenance guide. Like This is how you maintain it every month - step by step. These are the things to check if you suspect the readings are not accurate.

    I bought this so I could get accurate alk test results and could maintain more stable alkalinity when I am away. So far this has failed on both counts and I have lost a lot of acros. I should be getting better results from my tank not worse and I am spending crazy amounts of time constantly trying to work out what is wrong, testing with Salifert 5 times a week etc.

    So I would really appreciate some good answers and some instructions or links to other threads for people that had similar issues so I can work this out.

    Thank you

  2. #2
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    And again - after 5 days of reading 8.7 - 9.0 it suddenly jumps to 10.5.

    I test with Salifert which reads 9.6 - I had tested it 3 days before and it read 9.5, when KHD read 9.0.

    This is VERY frustrating.

  3. #3
    Join Date



    you ask for a trouble shooting guide.

    This and many other helpful resources for the KHD, such as videos, knowledge base, etc., are available on the GHL support page:

    Maintenance guide - not available since there is not much maintenance to do. Calibrate probe from time to time, replace filter when dirty.

    I guess your dosing pump calibration is not correct or something is wrong with the tubing. All explained in above mentioned support resources.
    No support or warranty issues over PM! Please send PMs to the moderators only if you have general problems with using the forum! Thanks for helping us to keep the support efficient.
    Kein Support oder Reklamationsabwicklung über PM! Bitte senden Sie an die Moderatoren nur PMs bei allgemeinen Problemen mit der Verwendung des Forums! Danke dass Sie uns dabei helfen, den Support effektiv zu gestalten.

  4. #4
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    Thank you Matthias - that helps.

    I went through everything. I think the issue is the measurement cell not emptying after the test because when I flush the measurement cell and test again it seems better. Also that 3rd dosing pump makes a lot of noise.

    I am just going to decrease the calibration ml/min from 35 (to 30 I did but didn't work) to 17.5 and see how that works.

    Let's see how this goes

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