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Using conductivity meter

Discussion in 'Aquarium Hardware' started by ButtNekkid, Oct 6, 2016.

  1. ButtNekkid

    ButtNekkid Active Member

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    Hi,

    What is the correct way of using conductivity meter?

    I always have to calibrate it before measurements. Because it seems to lose its calibration by, say 200µS.
    The manual says you only have to calibrate it after 10 measurements or 2 weeks.

    Have I "polluted" my calibration fluid when the first 2 times I calibrated, I only rinsed the meter with tap water.
    I don´t pour the calibration fluid out of the bottle that is in the package. I stick the meter into the fluid in the original bottle.

    I have water that is used for batteries. It says that it´s maximum conductivity is 5µS. I use that to rinse the meter nowadays.

    Should conductivity rise over time? Every time i take measurements, it seems to stay in same values. About 140µS.
    Should fertilizers rise it much? I use the "duckweed index" and only dose when the plants start to lose their green.
  2. dw1305

    dw1305 Well-Known Member 5 Year Member

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    Hi all,
    You shouldn't have to calibrate it every time you use it, they aren't like pH meters.
    Possibly, we have some combined pH/conductivity meters where we've had problems with students not rinsing the 4M KCl storage solution off the probe before putting it into the conductivity calibration fluid.
    Rinsing it in DI water is a good idea. What does the meter say when you put it in the DI water?
    I don't tend to get much difference in conductivity values with time. In planted tanks the values can go up and down depending on plant uptake (will lower values) and evaporation (will give higher values).

    cheers Darrel
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  3. ButtNekkid

    ButtNekkid Active Member

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    Between 4-5µS

    Should I get a new calibration fluid?
  4. dw1305

    dw1305 Well-Known Member 5 Year Member

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    Hi all,
    Good, that sounds like the meter is OK, and your 140microS is about the right value for the tank.
    Might be an idea.

    You can make your own "1411 microS" calibration fluid. The usual calibration solution is 0.01M KCl (which has a conductivity of 1411 microS at 25oC). Conductivity is approximately linear so you can go where you like from there.

    I don't like trying to weigh very small amounts of salts, so I use 7.46g of KCl in 1000cm3 H2O = 0.1 Molar sol.
    and then:

    100cm3 of 0.1M KCl in 1000cm3 = 0.01M and 1411microS.

    cheers Darrel
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  5. ButtNekkid

    ButtNekkid Active Member

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    Hi,

    I bought new calibration fluids. Should I rinse the meter with DI water before calibration with the new fluid?
    Should the meter be rinsed between measurements of different tanks and my peat bucket?
  6. dw1305

    dw1305 Well-Known Member 5 Year Member

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    Hi all,
    Yes, you need to rinse it in DI water and then dry any drops of water off (you can just dab it on some kitchen paper).

    The calibration fluid isn't like a pH buffer, any DI water that is added will reduce its conductivity value and any salts that get in increase it. Because I make up my own calibration fluid, I use large volumes and tip it away and start again fairly regularly, if I was buying it I would be a lot more careful.
    You should, but in this case you don't need too because the large volume of tank water will wash away any residual salts. We are only interested in ions in solution so they are, by definition, soluble.

    Also conductivity meters aren't like pH meters, you don't need to calibrate them before every use. I only do ours about every three months, and they are usually pretty close to the calibrated value.

    cheers Darrel
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