Ideal ORP readings

[Retep]

ORP readings are nearly useless unless you are also regulating the PH so that PH does not change.

:?:
ORP readings don't take into account the pH.
Naturally the lower pH the higher your ORP will be, but the measurement of the ORP is just that. . a measurement in mV
regardless how low/high your pH is and regardless how low/high your FC is.

To put it simple ( without using chemical formulas and so on ) :
The higher the ORP the faster your "germ killing time" and the more effective your sanitizer will be working.

Example : pH = 7.2 FC = 0.80 ppm ORP = 780 mV
pH = 7.6 FC = 1.50 ppm ORP = 780 mV

Both examples give you the same "germ/bacteria" killing capacity of 780 mV.
Personally I run rather at the 7.2 pH and low FC than the other way around.

You also may want to read this whole article as well as the references at the very end.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646482/

 

[JasonLion]

Re: ORP readings vs. Free Chlorine readings

You also may want to read this whole article as well as the references at the very end.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646482/

That article is very misleading. It starts from the assumption that an ORP level of 650 is the definition of a safe pool, which is simply not true, and then wanders off from there.

While there are entertaining math games you can play with ORP readings, if you want to do anything practical with them, like automate a chlorine delivery system, you need to be regulating the PH to a constant number. Otherwise you will be unable to create a stable automation system and will simply drive your parameters outside the useful range.

 

[Retep]

Re: ORP readings vs. Free Chlorine readings

First, what is the ideal ORP range for swimming pools?

Generally speaking the higher , the better. :-D

Swiss regulations require a minimum of 650 mV in public Pools, Germany, Austria and FINA set a minimum
of 700 mV.

The aim for proper water treatment is to run with as low as possible Chlorine residuals ( both free and combined)
and still obtaining high ORP readings.

I know some members of this forum may not agree with me though

 

[Retep]

Re: ORP readings vs. Free Chlorine readings

That article is very misleading. It starts from the assumption that an ORP level of 650 is the definition of a safe pool, which is simply not true, and then wanders off from there.

Actually it is not really an article, but rather a research from Minnesota.
As to having 650 mV considering a safe pool I would agree based on what I have been taught , seen , tested over the years .

It has been demonstrated by taking a "Plate Count" on bacteria/ Viruses that it's the ORP's which determine the time it takes to "kill" the microorganism.
3 ppm of FC with an ORP of 550mV is less effective than 0.5ppm with 750 mV.

I know you may not agree with this, but ( like I mentioned in another post) let's agree to disagree

 

[JasonLion]

Re: ORP readings vs. Free Chlorine readings

It has been demonstrated by taking a "Plate Count" on bacteria/ Viruses that it's the ORP's which determine the time it takes to "kill" the microorganism.
3 ppm of FC with an ORP of 550mV is less effective than 0.5ppm with 750 mV.

You should go back and read those studies again. They don't show that at all. What they show is that if you vary the ORP reading by varying the HOCl level, while holding everything else constant, that higher ORP/HOCl levels kill bacteria and viruses and more effectively than lower ORP/HOCl levels. What they haven't looked into is what happens when other things that affect the ORP reading vary while HOCl levels are maintained constant. Various things, such as changing brands of ORP sensors, or changing level of dissolved metals, or changing levels of dissolved hydrogen gas, can cause dramatic changes in the ORP reading without changing the rate at which bacteria are killed at all.

The aim for proper water treatment is to run with as low as possible Chlorine residuals ( both free and combined) and still obtaining high ORP readings.

Not at all. The aim of proper water treatment is to provide conditions that are as healthy and pleasant for the bathers as possible while being respectful of government regulations, practical realities, and budgets. In many cases all of those things are compatible with the your statement, but that is not consistently true. It is important to be aware of the times when the actual goals conflict with your over simplified statement.

 

[Retep]

Re: ORP readings vs. Free Chlorine readings

The aim for proper water treatment is to run with as low as possible Chlorine residuals ( both free and combined) and still obtaining high ORP readings.

Not at all. The aim of proper water treatment is to provide conditions that are as healthy and pleasant for the bathers as possible while being respectful of government regulations, practical realities, and budgets. In many cases all of those things are compatible with the your statement, but that is not consistently true. It is important to be aware of the times when the actual goals conflict with your over simplified statement.

Well - that's exactly what I've been saying all along.
Bather comfort, microbiological safe and healthy water with low chlorine levels , as little by-products of chlorination as possible.
Just like the Germans have in their public pools. I guess that's why they have the toughest laws in the world and I don't think they are wrong.
BTW - I recently had a ( now retired) swimming pool expert here from Germany ( 40 years experiance in the industry) and we visited quite a few public pools in the USA and Canada.
One of his remarks was " this is what we had in the late 70's. . . "
( which reminds me to crack open a cold Loewenbrau )

 

[chem geek]

You are missing the point here. ORP has a rough correlation with active chlorine (hypochlorous acid) level in a RELATIVE sense, but whose ABSOLUTE number varies by manufacturer's sensor and by conditions in the water that have nothing to do with disinfection rates such as hydrogen gas concentration or use of oxidizers that are not fast-acting sanitizers. We are not saying ORP has no value whatsoever, but rather that it's primary use is for process control using a setpoint measuring actual FC, but that the relevant values to measure for disinfection rates are the FC, CYA, pH and temperature. An amperometric sensor or a hypochlorous acid selective membrane sensor would be better, but those are more expensive.

Higher ORP is not always "better" because higher active chlorine levels create more disinfection by-products and oxidize swimsuits, skin, hair faster and corrode metals faster, etc. One wants to use the lowest active chlorine level that still kills pathogens quickly enough and if chlorine is used for algae control then it needs to be high enough for that as well.

Your pools with 0.7 ppm with no CYA have over 3 times more active chlorine than a pool with an FC that is 20% of the CYA level. Is it necessary for a 6-log (99.9999%) kill of bacteria in < 30 seconds or is it sufficient for a 2-log (99%) kill in < 1 minute (so a 6-log kill in < 3 minutes)? That is where the debate should be since the lower active chlorine level would produce disinfection by-products more slowly. The low end of the German standard of 0.3 is close to 0.2 anyway, but it is hard to maintain consistently throughout the pool without getting wiped out locally by chlorine demand.

 

[Retep]

Higher ORP is not always "better" because higher active chlorine levels create more disinfection by-products and oxidize swimsuits, skin, hair faster and corrode metals faster, etc. One wants to use the lowest active chlorine level that still kills pathogens quickly enough and if chlorine is used for algae control then it needs to be high enough for that as well.

As I was saying before : The aim is to run with as low as possible chlorine levels by still obtaining high ORP's for the exact reason you just wrote above.

 

[Retep]

Your pools with 0.7 ppm with no CYA have over 3 times more active chlorine than a pool with an FC that is 20% of the CYA level.

I guess this might be one of the reason CYA is not allowed for public indoor pools.

 

[chem geek]

I guess this might be one of the reason CYA is not allowed for public indoor pools.

But the German standard goes down to 0.3 ppm (0.2 ppm if ozone is used) and other standards are completely inconsistent since CYA is used in outdoor pools yet you don't find pathogenic outbreaks in those pools except for Cryptosporidium that is very chlorine-resistant. The reason CYA isn't used in indoor pools is because the industry only talks about CYA protecting chlorine from the UV in sunlight and doesn't talk about it reducing chlorine's effectiveness, or at least not by how much. It's thought to not be needed, but that results in most indoor pools being over-chlorinated and producing too many disinfection by-products and oxidizing bathing suits, skin and hair more quickly. Most importantly, it is inconsistent with outdoor pools with an order-of-magnitude difference in disinfection and oxidation levels.

Lowering the active chlorine level is a good thing to reduce disinfection by-products and having even 0.1 ppm FC with no CYA equivalent is still a fast kill for most pathogens -- 99% kill in under 1 minute is still likely to be fast enough to prevent person-to-person transmission and is still much, much higher than needed to prevent uncontrolled growth (this post lists kill times for the equivalent of 0.1 ppm FC with no CYA).

 

[Retep]

Lowering the active chlorine level is a good thing to reduce disinfection by-products and having even 0.1 ppm FC with no CYA equivalent is still a fast kill for most pathogens -- 99% kill in under 1 minute is still likely to be fast enough to prevent person-to-person transmission and is still much, much higher than needed to prevent uncontrolled growth (this post lists kill times for the equivalent of 0.1 ppm FC with no CYA).

Exactly. That's one of the reasons I am an advocate of promoting low FC levels .
Thanks for the link posted above.

 

[chem geek]

The thing is that getting to 0.1 ppm FC with no CYA is very hard and most experiments testing for kill times get whacky at such low chlorine concentrations since the chlorine itself tends to get used up. This would happen in pools as well. That's why I suggest using CYA as a chlorine buffer since 4 ppm FC with 20 ppm CYA is roughly the same active chlorine as with 0.2 ppm FC and no CYA; 2 ppm FC with 20 ppm CYA would be roughly similar to 0.1 ppm FC with no CYA. So you can have plenty of chlorine "in reserve" bound to CYA ready to be released quickly as needed to meet localized demand, but in a relatively inactive state that will effectively not oxidize swimsuits, skin, hair, create disinfection by-products or kill pathogens while still having enough active chlorine for disinfection. It's like having your cake and eating it too.

To make this work in high bather load pools, however, you really need to remove organic precursors including urea because the lower active chlorine level will tend to have those build up over time since oxidation rates are slower (especially indoors with no UV from sunlight to produce hydroxyl radicals from chlorine breakdown). That's where coagulation/filtration comes in.

 

[learthur]

Dudes! You may have a sanitized pool with low chlorine levels (which are more difficult to maintain by the way due to smaller margin of error) but you will most likely have an algae issue.

 

[chem geek]

Algae is killed mostly by hypochlorous acid (i.e. the active chlorine level), so with no CYA in the water even low levels of 0.08 ppm FC would be sufficient to control algae growth IF one were able to maintain that level consistently everywhere in the pool. Of course, that's a big "if" which is why I think it's better to use CYA to buffer the active chlorine level.

 

[Retep]

Re: ORP readings vs. Free Chlorine readings

You should go back and read those studies again.

I know quite a few studies which are written in German and French about the importance of high ORP's.
Also found something you may or may not find interesting - IMHO this article summes it up quite nicely ->
http://ppoa.org/?p=572

 

[JasonLion]

That article is written by Kent Williams, who has been pushing ORP since he worked for Stranco, a major manufacturer of ORP controllers. It doesn't present any research results. If you go back to the original papers, you will find that they don't consistently support his summarizations, and that he extrapolates well beyond the conclusions that are supported by the actual research.

 

[Retep]

That article is written by Kent Williams, who has been pushing ORP since he worked for Stranco, a major manufacturer of ORP controllers. It doesn't present any research results. If you go back to the original papers, you will find that they don't consistently support his summarizations, and that he extrapolates well beyond the conclusions that are supported by the actual research.

I suppose since he worked for a company which manufactures ORP controllers his article is not accurate then. . . Mmhh. . .
OK - what about this research ->
http://www.ncbi.nlm.nih.gov/pmc/article ... 6-0108.pdf

. . . or this. . . (from the World Health Organisation) ->

5.12.4 Oxidation–reduction potential
Continuous control of the oxidation–reduction potential (ORP) is considered useful by many
experts. Together with pH and free chlorine, the ORP value gives an indication of the
disinfection efficiency (Carlson et al., 1968; Jentsch, 1973). In chlorinated pools, ORPs of 750
mV (for pH 6.5–7.3) and 770 mV (for pH 7.3–7.8), measured against a silver/silver chloride
reference electrode with potassium chloride electrolyte (Denecke and Althaus, 1986), are
required to guarantee the safe inactivation of microorganisms (Deutsches Institut für Normung,
1997a).

 

[chem geek]

Retep,

You're running around in circles here. ORP roughly correlates with the hypochorous acid level -- it's just that it doesn't correlate consistently in an absolute sense and that it is affected by other factors, but in a relative sense there is a correlation. So obviously ANY experiment showing kill times or oxidation rates is going to show a correlation with ORP. However, NONE of these studies on ORP bothered to look at the hypochlorous acid concentration directly, even calculating it from the FC and CYA levels (along with pH and temperature). If they had done that, they would have found that calculated HOCl correlates at least as well if not better with disinfection and oxidation rates. This link (which is in the CPO post I keep referring you to) actually measured hypochlorous acid concentration and found the correlation to it directly. And of course, I've already shown you this post that shows that ORP roughly correlates to HOCl though not perfectly and there is this paper that showed ORP correlating to FC (with no CYA), again all links that are in the CPO post.

Again, no one said that ORP wasn't useful for process control since it does roughly correlate with hypochlorous acid concentration and one can create a setpoint against actual FC (and CYA) measurements. It's that it is not useful in an absolute sense the same way that the actual hypochlorous acid concentration would be. Obviously, if you specify a "high ORP" then odds are it will be more sanitized water, but it would be better to know the actual hypochlorous acid concentration, even calculated from knowing FC, CYA, pH and temperature, than it would be to know the ORP level. One manufacturer's "650 mV" is not the same as another manufacturer's and even with one manufacturer 650 mV varies too much by pH and hydrogen gas concentration so again, it's not as good as an absolute standard as actual hypochorous acid concentration. Why would one continue to push a proxy that is only approximate and is affected by other parameters when one can calculate the REAL THING that oxidizes and disinfects? If one is going to use a sensor, then use an HOCl sensor since you are at least measuring the relevant disinfectant directly. This would at least handle situations where there may be CYA-like molecules that bind to chlorine but measure as FC and not as CC (just as the chlorinated cyanurates do). ORP will also catch this situation, but since the absolute ORP level isn't definitive, one can't really know the true HOCl level -- again, one really needs a direct HOCl measurement as is done with the selective-membrane HOCl sensors.

I'm really missing the point you are trying to make with these references. What are you saying? We never said ORP didn't correlate at least in a relative sense with HOCl and its associated disinfection and oxidation rates and we never said that ORP wasn't useful for process control if you have a setpoint and control for external factors (pH, dissolved hydrogen gas). I've only said that ORP is useless as an absolute reading (except at extremes) since it varies so much by sensor and external influences not related proportionately to disinfection or oxidation. The 770 mV "standard" you quoted is arbitrary and will be inconsistent depending on the manufacturer's sensor and the pool being tested where a saltwater chlorine generator pool can readily fool the ORP sensor into thinking the ORP is too low and then overshoot the FC level to be too high -- something that has been reported to happen in real pools.

770 mV for Chemtrol at pH 7.5 would be 0.6 ppm FC with no CYA, but for Oakton it would be 1.1 ppm FC with no CYA while for Aquarius it would be 1.3 ppm FC and for Sensorex it would be 1.5 ppm FC. So what about the German DIN 19643 standard that allows down to 0.3 ppm FC? That's only 730 mV for Chemtrol, 697 mV for Oakton, 654 mV for Aquarius, 556 mV for Sensorex (if one believes their tables). You can see how ridiculous it is to just say that a certain mV ORP level is THE STANDARD. Of course, that doesn't stop people from saying it and even 19643 quotes 700 to 770 mV as a range though it is the ppm FC level of 0.3 to 0.6 (0.2 to 0.5 if ozone is used) is the definitive measurement.

Richard

 

[JasonLion]

In http://www.ncbi.nlm.nih.gov/pmc/article ... 6-0108.pdf, their conclusion was that

In short, it can not be claimed that a high redox potential could function as an absolutely sure yard stick of the disinfecting power of swimming-poolwater.

 

[Retep]

Retep,

You're running around in circles here.
Again, no one said that ORP wasn't useful for process control since it does roughly correlate with hypochlorous acid concentration. It's that it is not useful in an absolute sense the same way that the actual hypochlorous acid concentration would be. Obviously, if you specify a "high ORP" then odds are it will be more sanitized water, but it would be better to know the actual hypochlorous acid concentration, even calculated from knowing FC, CYA, pH and temperature, than it would be to know the ORP level. One manufacturer's "650 mV" is not the same as another manufacturer's and even with one manufacturer 650 mV varies too much by pH and hydrogen gas concentration so again, it's not as good as an absolute standard as actual hypochorous acid concentration. Why would one continue to push a proxy that is only approximate and is affected by other parameters when one can calculate the REAL THING that oxidizes and disinfects?

Richard

Richard,
I agree what you are saying. What I am trying to say is that ORP's is a useful parameter in order to see how efficient the water treatment
is. If I see a ( public pool) which has 650mV (assuming that the ORP probe is calibrated and really gives me the correct reading) with a pH of 7.2 and 5 ppm of FC then I could tell that something is wrong.
I am in no way affiliated with any manufacturer of ORP controllers or Amperometric controllers.