Certified Pool Operator (CPO) training -- What is not taught

Retep

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Mar 24, 2011
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Re: Certified Pool Operator (CPO) training -- What is not ta

I am not disputing that this "final" measurement might not be 100% accurate due to those reasons you described and I am also very well aware of that.
However - it still gives you a fairly accurate idea - does not really matter if we are off by a few mV.
Reality is that public pools have to work with the equipment they got - if I want to be 100% sure I'd have to bring a water sample
every time to a lab.
 

JasonLion

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Re: Certified Pool Operator (CPO) training -- What is not ta

In most pools, the reading after 3 minutes is going to be more accurate.
 

Retep

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Mar 24, 2011
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Re: Certified Pool Operator (CPO) training -- What is not ta

JasonLion said:
In most pools, the reading after 3 minutes is going to be more accurate.

It has nothing to do with the pool, but which ORP equipment is being used.
3 min. might be sufficient with certain ORP meters, but not with others.
This is something very very important to pay attention to and ( as I stated in a previous posting) many times public
pool operators are not aware of this .
 

JasonLion

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Re: Certified Pool Operator (CPO) training -- What is not ta

It does have to do with the pool, for all the reasons I outlined in my earlier post. Depending on what your chemical levels are, outgassing may or may not make very significant changes to the ORP reading. If there are going to be significant changes to the ORP reading due to outgassing, then you don't want to wait all that long.
 

chem geek

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Re: Certified Pool Operator (CPO) training -- What is not ta

Retep said:
I've seen it over and over again in the field ( public pools) that pool operators
take a water sample and submerge the portable ORP meter in a cup filled with pool water. After 1-3 min they think they got their final reading.
Wrong ! It takes up to 60 min. to get the "final" reading.
They waited an hour before taking a reading in most cases, though 30 minutes for some where they were more rushed (I communicated extensively with the health director who did the study and they put the sensor with separate probe in first, then did other testing and inspection around the pool, pump room, shower facilities, etc.). They understood the need for waiting for equilibration. Also, I had the data from the in-line sensors and that had no better (or worse) correlation with calculated hypochlorous acid level than the portable device.

The difference in readings between different manufacturers isn't just from that field testing but from manufacturers own test data (charts/tables). If you read the links I gave to you on ORP you would have seen that. There simply is not a consistent "standard" for an absolute ORP reading. The differences aren't just in the absolute amount where a single adjustment would be OK, but there are differences in the mV per doubling of hypochlorous acid concentration which means their SLOPES are different. Chemtrol has 22 mV/doubling, Oakton has 28 mV/doubling, Aquarius has 46 mV/doubling and Sensorex has 84 mV/doubling (seems ridiculous, but that's what the manufacturer tables show). That causes huge divergences in ORP readings at low active chlorine levels.
 

Retep

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Mar 24, 2011
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Re: Certified Pool Operator (CPO) training -- What is not ta

I did read the links and also searched in my documents about this subject.
From what I have seen in public pools I would not be surprised that those different readings what people get also has to do with
the fact that lots of pool operators simply don't clean their ORP probes from time to time.

One of the pools I am consulting keeps their probes in top condition ( Andress&Hausser-Amperometric with an extra probe for ORP as "helping parameter" ) A service Technician comes in from time to time , compared the probe with
his standard ( solution) and everything is well. When I compare it with my portable ORP meter (Hanna)we do get more or less the same readings.
Also from time to time I verify if my portable ORP meter gives me the accurate results by calibrating / comparing it with a solution which has 750mV.

Another Pool I got is a mess. . . Starts with the personal - dirty, filthy and nothing gets cleaned out of laziness. . . :? No surprise everything is off and the water quality is a mess . . .
 

chem geek

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Re: Certified Pool Operator (CPO) training -- What is not ta

As noted here in Figure 7, it takes time for sensors to converge for a common reading, though even after 20 minutes the difference between two of the same manufacturer's sensors (Chemtrol in this case) differs from 867 to 880 or a difference of 13 mV. Even after 16 hours, there is a range of 6-7 mV. So while not huge differences, they would still require a setpoint approach to chlorine levels since 22 mV is a doubling of chlorine concentration. Figure 8 also shows the time effect of ORP measurement (how it rises slowly over time, roughly stabilizing after about 10 minutes).

[EDIT]
I just added Sensorex data to my ORP post to show the absolutely ridiculous 83 mV per doubling of chlorine concentration. So we've got Chemtrol at 22 mV, Oakton from field measurements at 28 mV, Aquarius at 46 mV and Sensorex at 83 mV. How do you explain such HUGE slope differences that cannot be compensated by any sort of single-point calibration technique? Look at the following table of ORP mV vs. FC and tell me how anyone can possibly think to use ORP as any sort of absolute measurement that truly means anything? All values are at 7.5 pH and 80ºF temperature.

FC (no CYA) .... Chemtrol ... Oakton ... Aquarius ... Sensorex
.... 0.01 .............. 619 ........... 559 ......... 428 ........... 144
.... 0.10 .............. 693 ........... 653 ......... 581 ........... 423
.... 1.00 .............. 768 ........... 746 ......... 733 ........... 702
.. 10.00 .............. 842 ........... 840 ......... 886 ........... 980

If one just set 650 mV as their "standard", then their pool would have around 0.03 ppm FC with Chemtrol, 0.1 ppm FC with Oakton, 0.3 ppm with Aquarius and 0.65 ppm with Sensorex. Gee, what a great standard that is where the actual disinfection rate would vary by a factor of over 20 between the Chemtrol and the Sensorex.
[END-EDIT]

The main point is that there is no set standard. Every manufacturer that refers to an FC vs. ORP table has different values. One can't have a "650 mV is good" standard with such variations. Chemtrol looks to have done a reasonable job in being self-consistent to the degree that they can. However, even calibrating to a standard solution will not work when the slope of sensors varies since such calibration is single-point (some calibrations are 2-point, but at 90 mV and 265 mV so not in the normally measured range for pools) and even a "standardized solution" has no real meaning since the "standard" ORP value again depends on the sensor that is used if one is looking at chlorinated water.

So if you believe in absolute ORP so much, then why do you bother testing FC at all and have an amperometric sensor for FC control?
 

Retep

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Mar 24, 2011
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Re: Certified Pool Operator (CPO) training -- What is not ta

chem geek said:
So if you believe in absolute ORP so much, then why do you bother testing FC at all and have an amperometric sensor for FC control?

For the following reasons:
in order to see/verify if I get proper disinfection.
Again - the ORP is a "helping parameter" . I can reasonably assume that by having more than 700 mV there should not be any issues in terms of microbiology. (BTW - once a month "my" pools are being tested by a lab for microbiological parameters).
The amperometric sensor is in order to assure that there is always a residual of FC in the water ( min. 0.50 ppm - might vary from Province to Province).
I "bother" testing for FC and TC in order to see how efficient the filtration works. Obviously the lower the combined chlorine , the better it is.
Furthermore I test for Turbidity. Logically the more precursors we are able to remove beforehand , the less we need to disinfect / oxidise afterwards.

(in my own private hot tub I actually don't bother taking FC and TC readings anymore. By now I "know" my hot tub - I take readings for pH and
ORP only. More or less I can tell exactly how high my FC readings are when I know my ORP and my pH - which I try to keep at 7.1 .)
 

chem geek

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Re: Certified Pool Operator (CPO) training -- What is not ta

Retep said:
I can reasonably assume that by having more than 700 mV there should not be any issues in terms of microbiology.
You would be just as well off looking at the FC level (of FC/CYA ratio when CYA is used) to determine disinfection rates than looking at ORP. That's the whole point. ORP does NOT measure disinfection rate and is only a rough relative proxy for the disinfecting form of chlorine, hypochlorous acid. The hypochlorous acid can be precisely calculated knowing the FC, CYA, pH and temperature. If you are only using ORP to tell you microbiological safety (other than the explicit bacterial testing), then you are wasting your money on the ORP device since your accurate amperometric FC reading (assuming no CYA) is already telling you the amount of disinfectant in the water and THAT is what determines kill times.

So what makes you think that Chemtrol's number is the right one? What about those other manufacturers I listed who also claim to be "right" yet are much different. And again, why use a proxy (ORP) for the active chlorine level when you already are measuring that directly?
 

Retep

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Mar 24, 2011
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Re: Certified Pool Operator (CPO) training -- What is not ta

chem geek said:
If you are only using ORP to tell you microbiological safety (other than the explicit bacterial testing), then you are wasting your money on the ORP device since your accurate amperometric FC reading (assuming no CYA) is already telling you the amount of disinfectant in the water and THAT is what determines kill times.

Not necessarily.
The "kill Time" depends -of course - also of the pH. And that's exactly my point. It's all about the ORP's.
I can have 2 ppm and a pH of 8.2 or 1 ppm with a pH of 7. In which example I got a faster killing time and in which example I got a higher ORP ?
See what I mean ?
 

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Retep

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Mar 24, 2011
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Re: Certified Pool Operator (CPO) training -- What is not ta

chem geek said:
So what makes you think that Chemtrol's number is the right one? What about those other manufacturers I listed who also claim to be "right" yet are much different. And again, why use a proxy (ORP) for the active chlorine level when you already are measuring that directly?

I could argue in saying "what makes you think your test kit for measuring FC is the right one ?"
You might be using a Lamotte or a Taylor or whatever. . . How do you know if "yours" gives you the right reading ?
 

chem geek

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Re: Certified Pool Operator (CPO) training -- What is not ta

Retep said:
Not necessarily.
The "kill Time" depends -of course - also of the pH. And that's exactly my point. It's all about the ORP's.
I can have 2 ppm and a pH of 8.2 or 1 ppm with a pH of 7. In which example I got a faster killing time and in which example I got a higher ORP ?
See what I mean ?
Well, this depends on what kind of sensor you use. If you use a hypochlorous acid selective membrane sensor then the pH doesn't matter because you are measuring hypochlorous acid directly. So you can directly get the disinfecting capability, though really that varies with temperature so you'd want that as well. Of course, that doesn't tell you the FC level (if you wanted to know that for any reason), but you can either calculate that or get a sensor that also measures pH and will calculate the FC for you. With your example, the sensor at 2 ppm FC (with no CYA) with pH of 8.2 would read 0.31 ppm HOCl while at 1 ppm FC with a pH of 7.0 it would read 0.74 ppm HOCl.

For some types of amperometric sensor, you are right that you need to measure the same sorts of things you would measure with a DPD or FAS-DPD test since some amperometric sensors measure something more akin to Free Chlorine (FC) depending on the specifics of the sensor -- titrating amperometric measures all FC (used in laboratory systems) as do "bare-electrode" real-time sensors, but many real-time amperometric sensors are more or exclusively sensitive to hypochlorous acid, though this depends on whether they have a selective membrane. For example, the Hach 9184sc can measure Total Free Chlorine (what we normally call FC) that requires a pH measurement to calculate the hypochlorite ion concentration (and doesn't count any chlorine bound to CYA), but its native mode is to measure hypochlorous acid directly since it uses a selective membrane:

Is offered in models available to measure Hypochlorous Chlorine (HOCl) only (part number LXV430.99.00001), or Total Free Chlorine (TFC), when combined with a pH probe for accurate compensation of pH fluctuations (part number LXV432.99.00001)
However, even these real-time amperometric sensors are calibrated to a setpoint so they are still not an absolute standard. Their main advantage over ORP is that they are not affected by other factors such as pH and hydrogen gas concentration. See this link for a good explanation of typical amperometric sensors and note that they measure hypochlorous acid (HOCl) directly and though they note that this affects accuracy of FC readings requiring you to measure pH, for the purposes of knowing disinfection you don't care about hypochlorite ion (OCl-) anyway and want to know HOCl so don't need any pH adjustment when using this type of sensor.

As for ORP you are assuming that the variation in ORP with pH is due solely to the active chlorine level, but this is not true. pH also affects ORP directly since that is part of the electrochemistry (Nernst equation). Specifically for your Chemtrol sensor, going from a pH of 7.5 to 7.0 at your 0.7 ppm FC with no CYA at 82ºF would go from 754 mV to 793 mV. The actual hypochlorous acid concentration goes from 0.34 ppm to 0.52 ppm. However, this higher ORP is the same as found with 2.35 ppm FC at a pH of 7.5, but would have an HOCl concentration of 1.12 ppm. Basically, pH more than doubles the effect on ORP compared to the effect from HOCl alone. This means that with ORP at lower pH you are over-estimating the disinfecting capability while at higher pH you are under-estimating it. Specifically, if 754 mV were your "target" and the pH dropped to 7.0, then an ORP process control system would lower the FC to 0.18 ppm, but would result in lowering the HOCl from 0.34 ppm to 0.13 ppm while if your pH rose to 8.0, then an ORP process control system would raise the FC to 3.0, but would result in raising the HOCl from 0.34 ppm to 0.67 ppm. Gee, what a great system ORP is, undershooting your FC when the pH goes down and overshooting it when the pH goes up. Of course, one is normally controlling pH as well so in practice this isn't a real problem, but I'm just pointing out to you that ORP is not as great as you think.

Retep said:
I could argue in saying "what makes you think your test kit for measuring FC is the right one ?"
You might be using a Lamotte or a Taylor or whatever. . . How do you know if "yours" gives you the right reading ?
When using a FAS-DPD chlorine test the various vendors that offer such a test all have roughly similar accuracy to 0.2 ppm (when using a 25 ml sample size for the Taylor test). The variations in test readings are more for the DPD tests including those read by a colorimeter where higher FC levels above 5 ppm are not as accurate. Figure 6 in the Chemtrol document is all about the errors in the visual DPD test and doesn't talk at all about FAS-DPD. This is one reason we suggest using a FAS-DPD chlorine test since it is accurate, doesn't bleach out at higher FC (just add more powder if one sees a flash of pink), and is an easy test to do even for color-blind people since one counts the drops until the sample turns from pink/red to clear (see this link for a demo of this test). The Taylor FAS-DPD titrating dropper tips are calibrated to be 24 drops/milliliter +/- 1 drop. So that's an error of less than 5%. The error in the FAS-DPD titrating reagent concentration is likewise less than 5% so the overall test error is conservatively the greater of 1 drop (i.e. 0.2 ppm if using a 25 ml sample size) or 10% of the FC or CC level being measured.
 

chem geek

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Re: Certified Pool Operator (CPO) training -- What is not ta

I have added a section on "Turnover Rate and Law of Dilution" to the first post in this thread since I just discovered that the pool/spa industry is incorrectly using the Gage and Bidwell Law of Dilution to claim that the percentages in that model are the amount of water seen by the filter in one turnover. In fact, the model percentages are clarification percentages for a steady-state level with once a day contaminant introduction. So for one turnover the Gage and Bidwell clarification percentage is 42% meaning that the steady-state level of contamination in the water is 58% of the amount added per day, but it is still true that 1-e-1 = 63% of the water is circulated through the filter in one turnover. The Gage and Bidwell formula is just e-1+e-2+e-3... for a sufficiently large number of terms (it's asymptotic) for 1 turnover which results in 58%. For 2 turnovers, the formula is e-2+e-4+e-6... which results in 16%. For 3 turnovers, 5%; 4 turnovers 2%; 5 turnovers 1%.

The thing is that their model isn't very realistic since the introduction of dirt or contaminants would really be more continuous throughout the day so during the duration of the turnover(s) of circulation. In that scenario, the number of turnovers doesn't clear the water nearly as effectively since the benefit of multiple turnovers is counteracted by the continuous introduction of additional dirt or contaminants. The steady-state for one turnover per day is the amount introduced per day while the steady-state for two turnovers per day is half the amount introduced per day, etc. Basically, each turnover removes the same as the added amount during the turnover when such additions are continuous.

As is typical in this industry, scientific data is misinterpreted or misapplied, though this error is not as egregious as the misapplication of the 10x rule of chlorine oxidation of ammonia incorrectly applied to Combined Chlorine (CC).

[EDIT] It's not just the CPO handbook that incorrectly uses the Gage and Bidwell data, but so does Kent Williams at PPOA when he writes "over half of the pool’s water during the first so-called turnover gets nowhere near the filter". [END-EDIT]
 

mas985

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Re: Certified Pool Operator (CPO) training -- What is not ta

Not only that I would expect that the circulation of pool (aka location of returns and skimmers) would also affect the dilution so every pool should have a slightly different dilution table. For example, suppose someone points a return directly at the skimmer to get good capture of debris. Under that scenario, I would expect high volumes of the same water to be continuously filtered while other parts of the pool don't get filtered at all. I would also expect a different dilution ratio for different speeds of a VS pump.
 

chem geek

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Re: Certified Pool Operator (CPO) training -- What is not ta

Yes, both of these theoretical dilution calculation models assume instantaneous and perfect complete mixing as well as 100% complete filtration (i.e. an ideal filter). Real-world scenarios will be very different and dependent on circulation design and actual filtration percentage. Nevertheless, it still shows why there is a need for a bulk-water disinfectant.
 

loop_pea

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Re: Certified Pool Operator (CPO) training -- What is not ta

I've just passed the exam for the pool plant operator certificate in the UK (still have a written assignment to do).

The course was good, but my understanding of the effects of cyanurates comes from this forum. So thank you folks!
 

waste

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Re: Certified Pool Operator (CPO) training -- What is not ta

CONGRATULATIONS!! :cheers:

Remember to only believe ~ 1/2 (that'd be .5 in metric :lol: ) of what they told you was proper pool maintenance. :p
 

loop_pea

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Re: Certified Pool Operator (CPO) training -- What is not ta

Indeed. I can start my own "what is not taught" post.

Actually 90% of the course was fantastic. But in the 10% that wasn't, we had target levels for FC depend on the source of chlorine. :shock:
 

waste

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Re: Certified Pool Operator (CPO) training -- What is not ta

I hear you!

A few chemical manufacturers are on the board of directors for the folks who certified me, and I can see their influence on the chemical maintenance parts of the training :pukel:

At least you know 'boot polish" from the 'pile of manure' they tell you and can adjust your pool care accordingly :cheers:

CHEERS!
 

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