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Thread: Use PM2 and Probe for TDS monitoring

  1. #1
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    Use PM2 and Probe for TDS monitoring

    Hi, I'm wondering if it's possible to use the PM2 module along with its conductivity probe for monitoring TDS levels in a freshwater tank. Handheld TDS meters are just conductivity probes with some conversion build in to display the results in TDS. Is it possible to do the same thing on the Apex and have it display TDS using the PM2 module and probe?

    Thanks,
    Harry

  2. #2
    Master Control Freak RussM's Avatar
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    Yes. See the table on page 3 of the PM2 manual for more info. https://www.neptunesystems.com/support/docs/
    Please do not send me PMs with technical questions or requesting assistance - use the forums for Apex help. PM me ONLY if the matter is of a private or personal nature. Thanks.

  3. #3
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    Quote Originally Posted by RussM View Post
    Yes. See the table on page 3 of the PM2 manual for more info. https://www.neptunesystems.com/support/docs/
    Thanks but that table only shows conductivity measurements and salinity, I don't see any mention of displaying PPM for total dissolved solids. If I used the medium range as mentioned in that table I would have to manually convert the conductivity measurements to a TDS measurement in PPM, is that correct?

    Thanks,
    Harry

  4. #4
    Master Control Freak RussM's Avatar
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    You can measure the conductivity of freshwater, or even purified water, but the display will be in microSiemens or milliSiemens depending on which range you choose. You can't actually have it display in ppm TDS. Multiply microSiemens by 700 or milliSiemens by 0.7 to get an approximation of TDS in ppm.
    Please do not send me PMs with technical questions or requesting assistance - use the forums for Apex help. PM me ONLY if the matter is of a private or personal nature. Thanks.

  5. #5
    Hoff
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    Can the conductivity probe on the base Apex unit (2016) be used to measure in the freshwater range? I've read the PM2 manual referenced above, but when I looked at the calibration steps for the base unit conductivity probe, there was no option to select different ranges. The only option is "Salinity (0-45.0 ppt).

    I'd also like to use this with a planted freshwater tank, but not if I have to buy a separate module.

    Thanks for any help!
    Larry

  6. #6
    Master Control Freak RussM's Avatar
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    The Apex 2016’s salinity probe port is fixed at displaying salinity in PPT, which is what the vast majority of Apex owners use.

    To measure conductivity of other than saltwater, you’ll need to use a PM2 which as you noted has 4 available ranges/modes. You can use the same probe that comes with the Apex 2016 with the PM2
    Please do not send me PMs with technical questions or requesting assistance - use the forums for Apex help. PM me ONLY if the matter is of a private or personal nature. Thanks.

  7. #7
    Hoff
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    Thanks Russ - appreciate the clarification.

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    https://www.neptunesystems.com/downl...PM2_manual.pdf
    Page 5 and 6 talk about the measuring RODI and I am in the process of doing so.


    And I am waiting for my 447 uS/cm calibration solution to come in the mail.

    Question is what unit are the numbers on the list?
    53 is 53ms/cm. That I know.
    74, 447, 718 are uS and 3.9, 6.6, 30.1, and 53.0 are ms?

    I have a Hanna HI98308 water tester with range 0 to 99 uS/cm. I think I can create some 74 uS/cm solution myself.

    Oh, one more question. When measuring RODI, I should not use compensation, right?

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    Quote Originally Posted by derekc View Post
    https://www.neptunesystems.com/downl...PM2_manual.pdf
    Page 5 and 6 talk about the measuring RODI and I am in the process of doing so.


    And I am waiting for my 447 uS/cm calibration solution to come in the mail.

    Question is what unit are the numbers on the list?
    53 is 53ms/cm. That I know.
    74, 447, 718 are uS and 3.9, 6.6, 30.1, and 53.0 are ms?

    I have a Hanna HI98308 water tester with range 0 to 99 uS/cm. I think I can create some 74 uS/cm solution myself.

    Oh, one more question. When measuring RODI, I should not use compensation, right?
    uS and mS are the same unit of measurement just different range. mS is 1000th of a Seimen, uS is 1000000th of a Seimen.

    Temp compensation is helpful when it can be achieved, but I don't expect you could accurately measure the temp where you are checking with the probe easily so in this case just leave it off. Temp will still affect the reading, but if you can't reliably measure it, it makes no sense to compensate for it.

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    I (we) know what uS and mS are.
    I gathered you meant:
    74, 447, 718 are uS
    3.9, 6.6, 30.1, and 53.0 are ms
    The numbers are missing unit in the GUI. My question was to clarify the unit.

    I have the temp probe. It is connected to the same PM2 where the cond probe is.
    How would I not be able to "accurately" measure the temp? I just dump the temp probe into the same container. You meant all temp probes can't accurately measure temperature?

    Page 6 says "Seawater typically hasa temperature correction factor of 2.1 to 2.3%/C."
    If I want to use temp compensation for RODI, what value should I use?

    Thanks in advance.

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    Quote Originally Posted by derekc View Post
    I (we) know what uS and mS are.
    I gathered you meant:
    74, 447, 718 are uS
    3.9, 6.6, 30.1, and 53.0 are ms
    The numbers are missing unit in the GUI. My question was to clarify the unit.

    I have the temp probe. It is connected to the same PM2 where the cond probe is.
    How would I not be able to "accurately" measure the temp? I just dump the temp probe into the same container. You meant all temp probes can't accurately measure temperature?

    Page 6 says "Seawater typically hasa temperature correction factor of 2.1 to 2.3%/C."
    If I want to use temp compensation for RODI, what value should I use?

    Thanks in advance.
    If you have a container that spills over into the storage container, and can fit the temp probe in too, use one and enable temp compensation for best accuracy. A lot of people use the inline tee probe holders for this application, which makes measuring the temp impractical.

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    Hmmm. Interesting. I have three PM2's on my apex jr. Each PM2 has a cond probe + temp probe. I am playing with my setup to test 1.026 SG saltwater and zero TDS RODI at the same time. At this moment, 2 probes in 1.026 SG and 1 probe in 74 uS /cm water. One thing I just learned. The compensation is not per probe. Once I change it, all 3 probes will have the change.

    Page 6 says "Seawater typically has temperature correction factor of 2.1 to 2.3%/C."
    If I want to use temp compensation for RODI, what value should I use? I have not gotten that answer.

    But I guess this is a moot point now. Maybe Neptune will look into this deficiency? I believe each probe should have its own temp compensation setting. Otherwise, no way to have a probe for RODI and a probe for SW. That means no way to measure "accurately".


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    Quote Originally Posted by derekc View Post
    Hmmm. Interesting. I have three PM2 on my apex jr. Each PM2 has a cond probe. I am playing with my setup to test 1.026 SG saltwater and zero TDS RODI. At this moment, 2 probes in 1.026 SG and 1 probe in zero TDS. One thing I just learned. The compensation is not per probe. Once I change it, all 3 probes will have the change.

    Page 6 says "Seawater typically hasa temperature correction factor of 2.1 to 2.3%/C."
    If I want to use temp compensation for RODI, what value should I use? I have not gotten that answer.

    But I guess this is a moot point now. Maybe Neptune will look into this deficiency? I believe each probe should have its own temp compensation setting. Otherwise, no way to have a probe for RODI and a probe for SW. That means no way to measure "accurately".

    The data I saw in an ASTM standard indicated that conductivity for "pure" water is about 5%/C near 20C and the effect is non linear. At lower temperatures the effect is less and at higher temps the effect is more. However accurate values for saltwater are more important than accurate RODI TDS, so I would use 2.2 for everything.

    I don't know why Neptune doesn't allow different temp compensation per probe and honestly forgot they did that. You would still be better served with temp compensation enabled since a 2.8% error per degree C is still better than 5% error. The compensation is independent per module, it just cant be fine tuned differently per probe.

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    Zoombie,
    One more question. I know cond probe is measuring conductivity. Ohm then, right? What is the range and resolution it is measuring? How does it corresponding to the settling number? I sometimes see the settling number negative when probe is out of water.

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    Master Control Freak RussM's Avatar
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    Quote Originally Posted by derekc View Post
    Zoombie,
    One more question. I know cond probe is measuring conductivity. Ohm then, right? What is the range and resolution it is measuring?
    Ohms is a measurement of resistance, not of conductivity. The units and ranges are in the second column of the table from the user guide you posted.
    Please do not send me PMs with technical questions or requesting assistance - use the forums for Apex help. PM me ONLY if the matter is of a private or personal nature. Thanks.

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    Quote Originally Posted by derekc View Post
    Zoombie,
    One more question. I know cond probe is measuring conductivity. Ohm then, right? What is the range and resolution it is measuring? How does it corresponding to the settling number? I sometimes see the settling number negative when probe is out of water.
    The way a conductivity probe works is it injects a small current through the water between two probes and another two probes measure the voltage gradient that was induced across that area. That voltage and known current injected is used to calculate the conductivity based on the following equation

    Conductivity (S/cm or 1/(ohm * cm)) = voltage / (current * distance)

    And is then modified one more time by calibration constants which provide an offset and slope to the base calculated values.


    A negative value can be caused either by an offset that doesn't exactly match reality or electrical interference inducing a voltage in the opposite direction of the injected current.


    Apologies is that was too technical. I talk almost exclusively with engineers all day.

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  17. #17
    Master Control Freak RussM's Avatar
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    Quote Originally Posted by derekc View Post
    Question is what unit are the numbers on the list?
    53 is 53ms/cm. That I know.
    74, 447, 718 are uS and 3.9, 6.6, 30.1, and 53.0 are ms?
    Ignore the 74, 718, 3.9, & 30.1 options; they are for specialized uses outside the realm of the hobbyist. Use only the calibration solution listed in the table for the desired range.
    Please do not send me PMs with technical questions or requesting assistance - use the forums for Apex help. PM me ONLY if the matter is of a private or personal nature. Thanks.

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    "It injects a small current through the water..."
    Sounds like I should not have more than one probe in the water (in my case, just a cup) while I am doing calibration.
    Those settling numbers are so unpredictable... jumping up and down even after a stir and changing over time / night.
    Maybe a rid-volt grounding probe will help too.
    Please don't apology. The technical details help.

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    Quote Originally Posted by derekc View Post
    "It injects a small current through the water..."
    Sounds like I should not have more than one probe in the water (in my case, just a cup) while I am doing calibration.
    Those settling numbers are so unpredictable... jumping up and down even with a stir and changing over night.
    Maybe a rid-volt grounding probe will help too.
    Please don't apology. The technical details help.
    You should not have more than one probe in the same cup during calibration and you do want to have them separated in your sump at least a couple inches to avoid cross interference.

    Grounding probes, while very useful for protecting you against electrical shock, are actually counterproductive for conductivity probe interference. The reason for this is that if an electrical device is leaking voltage into the tank and the tank isn't grounded, all of the water in the tank raises to that same voltage and does not induce a current or voltage gradient (same principle as when lineman work on live transmission lines from a helicopter or when birds sit on transmission lines). Once you add a grounding probe, there is now a path for current to flow, which induces a voltage gradient that can effect the probe reading.

    Sent from my SM-G965U using Tapatalk

  20. #20
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    With 2 experts here, I have learned a lot. Thanks.
    I will play with my setup more later.

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