Using An Orp Meter During Soaks

[Dietitian]

I'm wondering if anyone uses an ORP meter during soaks. I'm thinking I could keep it next to the spa and test periodically during hot tub parties in order to make sure the chlorine doesn't drop to zero (or below 650 mv).

Does anyone do this? Is it effective?

[chem geek]

It is far better for you to use a Free Chlorine (FC) test. If you are only concerned about whether the chlorine gets towards zero, then even test strips would be OK for this, though they are not nearly as accurate as a FAS-DPD chlorine test.

ORP only has a rough correlation with the hypochlorous acid concentration and will vary by manufacturer even measuring the same identical water. So while it's OK for automation once you set a setpoint, it isn't very good for the purpose you describe.

During a hot tub party, a LOT of chlorine will get used. In a 350 gallon tub of hot (104F) water, the sweat and urine from every person-hour of soaking can consume up to 7 ppm FC.

[Dietitian]

It is far better for you to use a Free Chlorine (FC) test. If you are only concerned about whether the chlorine gets towards zero, then even test strips would be OK for this, though they are not nearly as accurate as a FAS-DPD chlorine test.

ORP only has a rough correlation with the hypochlorous acid concentration and will vary by manufacturer even measuring the same identical water. So while it's OK for automation once you set a setpoint, it isn't very good for the purpose you describe.

During a hot tub party, a LOT of chlorine will get used. In a 350 gallon tub of hot (104F) water, the sweat and urine from every person-hour of soaking can consume up to 7 ppm FC.

Wow, that is a lot of chlorine. If I have five people in there for an hour i need to add half a liter of bleach!

How should one go about adding that much bleach? Should I dose with MPS instead? I don't see how five people in a 280 gallon tub can keep any chlorine in the tub at all. After about 10 minutes it will all be gone.

Is it safe to let the chlorine go to zero during soaks and dose after getting out?

[chem geek]

The actual amount of bleach will depend on how much people actually sweat (or urinate), but it will probably need at least half that calculated amount and most likely more than half. The rough rule-of-thumb independent of spa volume is that every person-hour of soaking takes around 5 fluid ounces of bleach or 3-1/2 teaspoons of Dichlor or 7 teaspoons of non-chlorine shock (43% MPS). 5 people for an hour is around 3 cups of 6% bleach, but as I said it may take less than this amount and realize that the consumption rate will not all be during the soak itself.

Most people just start out with 1-2 ppm FC when they start to soak and then add chlorine afterwards. You could do that as well, but you are right that there will be no chlorine during most of the soak and that means there is the risk for person-to-person transmission of pathogens, mostly via the fecal-to-oral route. If you wanted to minimize that, then you would need to start out with 4 ppm FC (assuming around 30 ppm CYA) and then add more chlorine fairly frequently during the soak -- a few fluid ounces of 6% bleach every 10 minutes, for example, but I don't know how you would add that while people are in the tub. In commercial/public spas, there is continual dosing that is incrementally rather small so it's not noticeable. Also, though the ammonia in sweat/urine will react almost immediately with the chlorine to form monochloramine, it will take more minutes to an hour to oxidize completely while urea (which is the largest component in sweat/urine) will take longer to oxidize (probably at least an hour to several hours). So you may not find the FC dropping quite as fast, though it will still be noticeable.

Note that if you have an ozonator, that the amount of chlorine needed is far lower since the ozonator should oxidize a lot of the bather waste so that chlorine doesn't have to. The downside is that the ozonator can oxidize some of the chlorine itself (to chlorate).

[Retep]

It is far better for you to use a Free Chlorine (FC) test. If you are only concerned about whether the chlorine gets towards zero, then even test strips would be OK for this, though they are not nearly as accurate as a FAS-DPD chlorine test.

ORP only has a rough correlation with the hypochlorous acid concentration and will vary by manufacturer even measuring the same identical water. So while it's OK for automation once you set a setpoint, it isn't very good for the purpose you describe.

During a hot tub party, a LOT of chlorine will get used. In a 350 gallon tub of hot (104F) water, the sweat and urine from every person-hour of soaking can consume up to 7 ppm FC.

chem geek- I do disagree with you on that one

I still believe that ORP readings are an excellent indicator how "germ - free" your tub water is, regardless what your Chlorine Level is in your spa.

Of course you are right when you say it will vary by manufacturer, however it still serves as a "helping parameter".

As we all know the FAC effectiviness is also very much dependable of the pH.

If I want to achieve an ORP reading ( for example) of 750 mV only 0.6ppm at a pH of 7.2 would be sufficient.

If the ph is 7.8 we need around 2 ppm in order to have the same effectiviness from our Chlorine .

(Those figures are based on a public pool, where the water temperature is much lower - I assume we need a higher chlorine levels in spas due to higher water temperatures).

Public Pools / Public Spas have usually an automatic controller which inject Chlorine / and/or Acid when there is a demand for it.

Those controllers work either through ORP readings or so-called Amperometric units.

ORP controlled units takes a very long time to react , for this reason it is preferable to use automatic controllers which work Amperometric.

On those the operator sets the Chlorine level at ( for example 1.5ppm) and if it drops below,Chlorine will be injected till we reach the desired 1.5ppm.

Even those units have ORP readings, but these are meant as an additional "helping parameter"

So - and now the Original Poster makes me go into my tub with my little hand held ORP meter to make my own little experiments.

Thanks a lot - the Olympics are on and I have to do tests in order to satisfy your ( and mine) curiosity

I'll report later with the results

Cheers

[chem geek]

Take a look at this post with a lot of links and info to information that isn't taught currently in the industry or that is taught incorrectly. In particular, this link shows the traditional industry graph of hypochlorous acid level vs. pH compared to the true graph when CYA is present. While the active chlorine level drops by 50% going from 7.5 to 8.0 if there is no CYA, it only drops by 15% when CYA is present because CYA is a hypochlorous acid buffer. You can use this spreadsheet I wrote to calculate hypochlorous acid concentration though will need to turn on temperature dependence by setting line 225 to TRUE in columns B and C since spas are much hotter than pools.

Also, ORP is affected by the pH independent of hypochlorous acid level (it varies with pH even with no chlorine), yet there is no scientific basis for a corresponding faster killing of pathogens at lower pH other than that completely accounted for by the hypochlorous acid level. That is, the ORP is affected by parameters (including temperature and pH) that are not necessarily correlated with disinfection in the same amount or direction (e.g. ORP drops at higher temperature yet disinfection rates actually increase as oxidiation/disinfection increases at a faster rate than the reproduction rate of pathogens increases as temperature rises). And again, any "absolute" ORP number is close to meaningless since the different probes read differently, not only in absolute terms, but even their "slopes" of mV per doubling of hypochlorous acid concentration vary. Take a look at this post and note that even Clifford White's classic Handbook of Chlorination was inconsistent in its readings measuring the same waters under the same conditions.

If you know the FC and CYA levels and are in a reasonable pH range then you know your sanitation level. You don't need to use ORP. It is useful as a setpoint and automated adjustment tool, but not very much as any sort of absolute guide except VERY rough -- not nearly as good as measuring FC to within 0.2 ppm (using a Taylor K-2006 with FAS-DPD chlorine test, for example) and CYA to within 10 ppm. If you add an oxidizer such as non-chlorine shock (MPS), it will read high in ORP, but will not be that high in disinfection rates (except with silver ion at high spa temperatures). Hypochlorous acid (HOCl) doesn't just kill quickly because of its oxidation potential, but rather because it is a neutral polar molecule that looks very much like water so that it slips fairly easily into cells where it can then combine with nitrogenous compounds interfering with them -- even if it doesn't get to actually oxidizing them completely. There are multiple routes/paths of HOCl destruction of pathogens, but it isn't just related to its ORP. To the extent that ORP accurately shows HOCl, then that's great, but it normally doesn't. Also, ORP has little to do with actual reaction rates -- ORP is a thermodynamic measurement so again it is only a roughly correlated proxy.

There are specialized membrane sensor products that can measure hypochlorous acid more directly, but these have not been very reliable.

[Retep]

Very interesting.

Thanks again for all those links.