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Thread: Turbidity measurement (was "what is NTU?")

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    Junior Member Atredeis's Avatar
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    Re: A Better CYA Test Kit?

    Does anyone know the relationship between CYA and NTU for the TF-100 test kit? If not, I guess a quick and dirty two point calibration could be done using the 50CYA standard and dilution.

    I have use of a Hach-2100Q and would rather measure the turbidity directly.

    Mods, please move this if it should be it's own topic.
    If the thought occurs to you, please put it in a new topic in the first place. Thanks JasonLion

    Thanks!
    -Paul
    10,000 gal plaster pool, sand filter, Hayward swim pure SWG w/T-9 (25,000g) cell, Hayward Ecostar pump, Barracuda G3, TF-100 test kit

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    Re: A Better CYA Test Kit?

    Does anyone know the relationship between CYA and NTU
    What does NTU mean?
    Dave S.
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    Junior Member Atredeis's Avatar
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    Re: A Better CYA Test Kit?

    Thanks Jason, new topic started with thorough background. Please delete this post.
    10,000 gal plaster pool, sand filter, Hayward swim pure SWG w/T-9 (25,000g) cell, Hayward Ecostar pump, Barracuda G3, TF-100 test kit

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    Re: A Better CYA Test Kit?

    Quote Originally Posted by duraleigh
    Does anyone know the relationship between CYA and NTU
    What does NTU mean?
    http://en.wikipedia.org/wiki/Turbidity
    My cement pond is a 36K gunite 20X40 built in mid 1960s, Hayward S244S filter, Aquarite SWCG, Jandy 1.5HP 2sp, TF-Test kit and Nitro Wall Climber

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    Junior Member Atredeis's Avatar
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    Turbidity measurement (was "what is NTU?")

    Does anyone know the relationship between CYA and NTU for the TF-100 test kit? If not, I guess a quick and dirty two point calibration could be done using the 50CYA standard and dilution.

    NTU is a useful measurement of turbidity. Following is quoted from http://en.wikipedia.org/wiki/Nephelo...urbidity_Unit>

    A property of the particles that they will scatter a light beam focused on them is considered a more meaningful measure of turbidity in water. Turbidity measured this way uses an instrument called a nephelometer with the detector setup to the side of the light beam. More light reaches the detector if there are lots of small particles scattering the source beam than if there are few. The units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU). To some extent, how much light reflects for a given amount of particulates is dependent upon properties of the particles like their shape, color, and reflectivity. For this reason (and the reason that heavier particles settle quickly and do not contribute to a turbidity reading), a correlation between turbidity and total suspended solids (TSS) is somewhat unique for each location or situation.

    Another measurement of turbidity is OD or Optical Density. The following is quoted from http://en.wikipedia.org/wiki/Optical_density
    In spectroscopy, the absorbance (also called optical density[2][3]) of a material is a logarithmic ratio of the radiation falling upon a material, to the radiation transmitted through a material.[4] Absorbance measurements are often carried out in analytical chemistry.
    In physics, the term spectral absorbance is used interchangeably with spectral absorptance or absorptivity. In this case it has a slightly different meaning: the fraction of radiation absorbed at a specific wavelengths.

    Various companies manufacture instruments to measure the above, traceable to NIST. NIST is the National Institute of Standards and Technology. http://en.wikipedia.org/wiki/NIST.

    Hach http://www.hach.com/ and Optek http://www.optek.com/ instruments are used all over various industries for turbidity measurements. Typically, Optek are flow through, fixed installation systems, which are used as feedback in control systems. Hach, on the other hand, makes devices to measure samples or portable applications.

    My personal experience is with the Hach 2100Q which we use as a spot check to measure centrifuge and filter performance in Biotech applications. In these purification applications, the molecule of interest (a protein manufactured by and engineered cell line) is in solution and the solids (cells and cell debris) are waste and must be removed.

    So, the point of all of this is that I have a Hach 2100Q at the office and would like use it on samples of my pool water instead of the sight glass provided in the TF-100 test kit. The sample would be prepared according to the TF-100 instructions, just set into the 2100Q instead of put into the sight glass.

    If anyone has done this kind of thing before and has a CYA to NTU conversion, I'd love to hear of it. Otherwise, I have the 50CYA standard (R-7065) and will use that as well as a 2:1 dilution (read 25CYA standard) to make a quick and dirty two point calibration. I say quick and dirty because we don't have purified water (DI or otherwise) at the office. The best available will be distilled water from the grocery store.

    Notes on purified water: DI is DeIonized water and diluting with such would assure the purity of the standard. http://en.wikipedia.org/wiki/Purified_water

    I'll probably do this in the next week or two and will post results.
    10,000 gal plaster pool, sand filter, Hayward swim pure SWG w/T-9 (25,000g) cell, Hayward Ecostar pump, Barracuda G3, TF-100 test kit

  6. #6
    Administrator JasonLion's Avatar
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    Re: Re: A Better CYA Test Kit?

    Nephelometric Turbidity Units, a standard unit for measuring turbidity, which is what the CYA test measures.

    Most of the affordable electronic turbidity meters are going to have problems measuring a CYA sample, though a better quality meter should work.
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    Junior Member Atredeis's Avatar
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    Re: Turbidity measurement (was "what is NTU?")

    Another challenge might be the range of measurement. The 2100Q measures from 0-1000 NTU. If the CYA samples are at the high end of that, a dilution might be required just to get a reading.

    Another thought is that the CYA to NTU relationship may not be strictly proportional. This thought comes from looking at the graduations on the TF-100 cylinder. The distance from 70 to 80 is very different from the 30 to 40. Hence, the quick and dirty two point calibration might be inaccurate. Maybe a three or five point and some curve fitting would be better.

    Ideally, some function of A*NTU^B+C=CYA would result, where A, B and C are determined experimentally.

    As far as generating the points other than the 50CYA standard, is a dilution strategy valid? In other words, if I dilute the 50ppm sample 2:1 does that equal 25ppm? On the other hand, how do I generate a 75ppm sample from the R-7065 standard?

    As to the Hach turbidity meter, it's a very good and repeatable instrument for turbidity. However, the $900 price tag is quite overkill for pool maintenance.
    10,000 gal plaster pool, sand filter, Hayward swim pure SWG w/T-9 (25,000g) cell, Hayward Ecostar pump, Barracuda G3, TF-100 test kit

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    Re: Turbidity measurement (was "what is NTU?")

    Yes, diluting the 50 ppm standard sample 50/50 with distilled or filtered water (which has no CYA in it) will result in a 25 ppm standard. Those two points should be enough for you to reasonably calibrate. Nephelometric Turbidity Units (NTU) specifically calls for a 90 measurement technique as described in this link with more info on modern instrumentation techniques in this link. I couldn't find any table relating CYA ppm (or melamine cyanurate) to NTU, but I did find this link that relates NTU to absorbance which are linearly related (there is also this file that gives a roughly similar formula). Also, this paper referenced in the preceding link has a table of absorbance to cyanuric acid concentration.

    The following table uses this information to relate CYA concentration in the melamine cyanurate turbidimetric test vs. absorbance and NTU. I'm not sure why the absorbance isn't more precisely proportional to the concentration as with Beer-Lambert's Law.

    [EDIT] See the post after this where CYA ppm is around NTU/3.2 rather than NTU/9.7 shown in the following table. I suspect the difference is in the differing wavelengths for absorbance and NTU readings/formulas. [END-EDIT]

    CYA (ppm) ... Absorbance (420 nm) ... NTU (for absorbance at 750 nm)
    ...... 0 ................... 0.001 ....................... 1
    .... 10 ................... 0.109 ................... 101
    .... 20 ................... 0.204 ................... 189
    .... 40 ................... 0.440 ................... 408
    .... 60 ................... 0.637 ................... 590
    .... 80 ................... 0.831 ................... 770
    .. 100 ................... 1.071 ................... 992

    So if you do end up making known references, it would be interesting to see if the above table is accurate so be sure to let us know.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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  9. #9
    Junior Member Atredeis's Avatar
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    Re: Turbidity measurement (was "what is NTU?")

    Got my hands on some glassware and the Hach 2100Q this morning. Bear in mind that this was an industrial environment vs. a carefully controlled laboratory. I'm sure much error can be eliminated with better instrumentation and control.

    Short Answer
    It seems one can take the NTU reading of the CYA test, divide by 3.2 and get an approximate CYA level. I have all of the raw data in a spreadsheet. PM me and I'll be happy to email it to you. Or if there's a suitable hosting site to recommend, I can upload it there. Furthermore, I'll be happy to answer any questions and if there's a methodology error, the experiment can be repeated.

    Experimental data and description
    All volumetric measurements were performed with 100mL graduated cylinders having 1mL divisions.

    The nephelopmeter used is a Hach 2100Q P/N LPG439.01.00002 S/N 09120C000524

    Calibration standards expired in October 2010. Couldn't buy new ones for this project.

    Verification calibrations for the Hach are as follows:
    Std..........Measured
    20NTU......18.9NTU
    100NTU....95.9NTU

    Equal parts sample and R-0013 were used for all samples generated. Typically 10mL each for a total sample of 20mL.

    All samples were mixed for a minimum of 30 seconds, timed with a stopwatch.

    10mL 50ppm standard R-7065 mixed with 10mL R-0013 reagent for 30 seconds.
    3 reading average with mixing between readings: 156NTU
    3 reading average without mixing: 176NTU

    10mL 25ppm standard (R-7065 diluted 1:1 with distilled H2O) mixed with R-0013 reagent for 30 seconds.
    3 reading average with mixing between readings: 61NTU
    3 reading average without mixing: 70.6NTU

    At this point, it seemed that turbidity was rising over the course of a few minuted, as the sample sat in the Hach. Thus the internal averaging was used and the above repeated with fresh samples. RST is the Rapidly Settling Turbidity function, AVG is the Hach averaging function, 3mix is as above mixing between each of 3 readings, 3unmix is as above without mixing between the readings.

    50ppm (same as above)
    RST: 177NTU
    AVG: 145NTU
    3mix: 134NTU

    25ppm (same as above)
    RST: 111NTU
    AVG: 110NTU
    3mix: 92.6NTU

    My Pool (10mL pool water after Tropical Storm Debbie yesterday + 10mL R-0013) Was 70CYA a few days ago via TF-100 viewtube.

    RST: 174NTU
    AVG: 171NTU
    3mix: 168NTU
    3unmix: 193NTU
    10,000 gal plaster pool, sand filter, Hayward swim pure SWG w/T-9 (25,000g) cell, Hayward Ecostar pump, Barracuda G3, TF-100 test kit

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    Re: Turbidity measurement (was "what is NTU?")

    Thanks for doing the experiment. I edited my post so no one will use that table blindly since it's apparently wrong.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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  11. #11
    Junior Member Atredeis's Avatar
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    Re: Turbidity measurement (was "what is NTU?")

    Here some quick statistics of the NTU measurements taken.

    50ppm standard:
    STDEV 18.92280971
    Mean 156.5454545
    Median 159
    MAX 187
    MIN 130
    Error 28.5

    25ppm mixture:
    STDEV 19.65275368
    Mean 81.25454545
    Median 72.1
    MAX 111
    MIN 54.3
    Error 28.35

    It seems the NTU reading gives tighter grouping than the viewtube, however since all the readings are from two samples each, variations in chemistry are unaccounted for. All in all, it is interesting and nice to get a number from the instrument, however I am having trouble finding a reason to use NTU over the standard methods. Heh. I seem to remember reading this somewhere on this board once or twice.
    10,000 gal plaster pool, sand filter, Hayward swim pure SWG w/T-9 (25,000g) cell, Hayward Ecostar pump, Barracuda G3, TF-100 test kit

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