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.
