Testing The Orp (oxidation Reduction Potential)

[Retep]

Hi everyone,

First time poster here - found this forum by googling .

Thought I share a bit of my experience here ( in case somebody actually really cares. .

Got one of those portable ORP meters and did some tests on my tub.

ph was at 7.5 ; FAC = 4.98 ; Total Cl2 = 5.03 ; ORP = 775 mV

Lowered the ph to 7.11 and the ORP went up to 828 mV.

24hrs after I tested the water again :

ph = 7.16 ; FAC dropped to 1.50 ppm ; Total Cl2 =1.73 and ORP was still at 779 mV.

750 mV should be sufficient to keep my water microbiologically safe - looks like I don't need more than 1.5 ppm FAC

providing I keep my pH between 7.1 - 7.2

Anyone else tested their water with an ORP meter ?

[quantumchromodynamics]

I think that 7.1 to 7.2 is a bit too low for pH. I recommend that it be kept at least 7.3. Are you using cyanuric acid?

[chem geek]

The ORP reading is going to depend a lot on the Cyanuric Acid (CYA) level because most of the chlorine binds to CYA (if present) and is not a strong oxidizer so effectively doesn't register with ORP. That is, ORP is mostly measuring the hypochlorous acid, though ORP is also affected by pH independent of chlorine level. Though a rough proxy, it is not enough to ensure bacteriological safety. Also note that ORP readings of the same water by different manufacturers often read differently unless you set their setpoints the same at the same FC/CYA ratio. Even then, some will diverge as the chlorine level changes because their "slopes" of mV per doubling of hypochlorous acid concentration varies. In short, they are fine for process control once you set the setpoint, but they aren't so great for "absolute" readings.

To give you some idea of the variability, an FC of 5 ppm with no CYA would read around 792 mV on a Chemtrol, 779 mV on an Oakton and 857 mV on a Sensorex while an FC of 5 ppm with 20 ppm CYA would read around 742 mV on a Chemtrol, 715 mV on an Oakton and 668 mV on a Sensorex. At a pH of 7.11 with an FC of 5 ppm and no CYA, the Chemtrol would read 821 mV, the Oakton 806 mV and the Sensorex 899 mV. All of these are at 104F and based on tables from the manufacturer in the case of Chemtrol and Sensorex and based on field measurments with the Oakton.

What kind of chlorine did you use? Was this fresh water where you added Dichlor? Based on the relatively large swing vs. pH it would seem that you had no CYA in the water and might have used bleach, though the fairly significant pH drop would be something more like Trichlor powder unless your TA level was very low.

Most heterotrophic bacteria have CT values (chlorine concentration in ppm times time in minutes) for 99% kill of around 0.08. We normally recommend having 4 ppm FC at 30 ppm CYA as a target, though the actual amount varies and swings as one adds more oxidizer after a soak and the level drops by the time of the next soak. 4 ppm FC at 30 ppm CYA has roughly the same hypochlorous acid concentration (at pH 7.5) as 0.6 ppm FC with no CYA which would kill 99% of bacteria with a CT of 0.08 in 60*0.08/0.6 = 8 seconds. The main reason for the higher chlorine level recommendation is for bacteria more resistant to chlorine and in particular to break up any early biofilms. As for preventing uncontrolled bacterial growth, even in the best-case of a generation time (doubling of bacteria population) of 15 minutes (normally it's 15-60 minutes), the 4 ppm FC with 30 ppm CYA would keep any bacteria at bay up to a 2-log CT (99% kill) of around 60 so is quite substantial. The 4 ppm FC with 30 ppm CYA at pH 7.5 would be an ORP of 723 mV on the Chemtrol, 691 mV on the Oakton and 597 mV on the Sensorex.

Why don't you try using Dichlor for a while and see what happens to the ORP reading at the same FC and pH levels? I think you will be in for a surprise as the CYA builds up and the ORP goes down substantially at first.

Richard

[Retep]

No - I am not using cyanuric acid ( and never will). Calcium hypochlorite 65% and muriatic acid to bring down the pH.

Thanks for the nice write up chem geek. If I understand correctly there is a difference between different manufacturers of ORP Meters.

Anyhow - as stated before I should be ok as long I am above 750mV. Naturally I try to keep my FAC never below 1 ppm.

After I posted my initial post I did go into the tub ( my son left after 20 min in there)- I stayed for about 30 min - turned jets on - water temp at 101F.

10 min after getting out of the tub I took another test and got the following results :

pH=7.18 , Free Cl2 = 0,16ppm , Monochloramine(DPD2 Tablet)= 0.28ppm , Total Cl2 = 0.5 ppm ; ClO2= 0.1ppm. ORP dropped down to 681mV.

[Dietitian]

No - I am not using cyanuric acid ( and never will). Calcium hypochlorite 65% and muriatic acid to bring down the pH.

Thanks for the nice write up chem geek. If I understand correctly there is a difference between different manufacturers of ORP Meters.

Anyhow - as stated before I should be ok as long I am above 750mV. Naturally I try to keep my FAC never below 1 ppm.

After I posted my initial post I did go into the tub ( my son left after 20 min in there)- I stayed for about 30 min - turned jets on - water temp at 101F.

10 min after getting out of the tub I took another test and got the following results :

pH=7.18 , Free Cl2 = 0,16ppm , Monochloramine(DPD2 Tablet)= 0.28ppm , Total Cl2 = 0.5 ppm ; ClO2= 0.1ppm. ORP dropped down to 681mV.

Why have you decided to never use CYA? What did you use to test for monochloramine, and how did you get such a detailed reading for free chlorine at 0.16ppm?

[Retep]

Why have you decided to never use CYA? What did you use to test for monochloramine, and how did you get such a detailed reading for free chlorine at 0.16ppm?

I don't get lots of sun in my spa to start with. If I would then I would keep it at minimum levels, since CYA makes your FAC less active ( to put it in simple terms).

Chem geek explaines it quite well on the 3rd posting :

Quote:

"Why don't you try using Dichlor for a while and see what happens to the ORP reading at the same FC and pH levels? I think you will be in for a surprise as the CYA builds up and the ORP goes down substantially at first."

In other words : You would need more FAC when using CYA in order to achieve the same ORP's.

In regards to your second question :

I am using a Photo Spectro Meter in order to give me exact readings.

Here I am using also DPD 1 ( for FAC), DPD3 (Total Cls) and a DPD 2 Tablet for Monochloramine.

This kit also has special drops for measuring Chlorine dioxid ( which I also use for my tub ). In this case the Chlorine dioxid acts as my oxidiser -

usually 0,2 ppm is sufficient ).

Additionally I am using a Turbidity meter - lowest level in my tub was 0.09 NTU.

For public swimming pools I use a flocculation / coagulation process (when using Sandfiltration) , Chlorine for sanitation and a modified chlorine dioxid process which prevents (re)formation of chlorite and chlorate.

With this water treatment process it is possible to achieve microbiological safe healthy water by using less than 1 ppm FAC, very low combined ( < 0,2ppm) and superior turbidity ( as low as 0,03 NTU ) , THM levels less than 15 ppb and ORP levels between 750 and up.

Spas are a bit more problematic though ,due to cartridge filtrations where proper flocculation/ coagulation can not really be used

[Dietitian]

I don't get lots of sun in my spa to start with. If I would then I would keep it at minimum levels, since CYA makes your FAC less active ( to put it in simple terms).

Chem geek explaines it quite well on the 3rd posting :

Quote:

"Why don't you try using Dichlor for a while and see what happens to the ORP reading at the same FC and pH levels? I think you will be in for a surprise as the CYA builds up and the ORP goes down substantially at first."

In other words : You would need more FAC when using CYA in order to achieve the same ORP's.

CYA binds to free chlorine, keeping a fixed ratio of "active" and "bound" chlorine. The ratio is determined by the FC and CYA levels in the tub. A spa with 4ppm FC and 30ppm CYA at 100 degrees F would have most of the chlorine bound to the CYA, and only about 0.65ppm unbound (able to oxidize waste/sanitize). This can be calculated as FC/CYA = unbound chlorine, and for spas above 100 degrees it would be approx 5*FC/CYA.

As the unbound chlorine is reduced (by waste/bacteria) the CYA releases more, keeping the level of active chlorine at around 0.65ppm until the "reserve" of 4ppm FC is exhausted. The drop tests we traditionally use to yield FC in ppm are showing both the CYA-bound and unbound chlorine.

CYA serves three functions -- to buffer the power of chlorine, protect the chlorine from UV degradation, and to reduce chlorine outgasing/evaporation rate. As I understand it the bound chlorine cannot "evaporate."

Without CYA in the tub, your cover and swimsuits will degrade faster, you'll have more chlorine in the air above the spa during use, you'll have more end stage chloramines, and bathers will be exposed to more active chlorine causing an increased likelihood of skin and eye irritation.

[chem geek]

Retep,

Basically, why do you think you need the active chlorine level in a spa to be so high? We're not talking about a lot of CYA in the water. 4 ppm FC with 30 ppm CYA at hot spa temperatures is roughly equivalent to around 0.6 ppm FC with no CYA in terms of the active chlorine (hypochlorous acid) concentration and should be sufficient for very decent sanitation as well as fast enough oxidation of bather waste. Of course, I'm talking about residential spas. Perhaps if you have commercial/public spas you need faster oxidation, but I'd rather do that with supplemental oxidation analogous to what you are doing with your chlorine dioxide in pools.

Richard

[Retep]

As the unbound chlorine is reduced (by waste/bacteria) the CYA releases more, keeping the level of active chlorine at around 0.65ppm until the "reserve" of 4ppm FC is exhausted. The drop tests we traditionally use to yield FC in ppm are showing both the CYA-bound and unbound chlorine.

CYA serves three functions -- to buffer the power of chlorine, protect the chlorine from UV degradation, and to reduce chlorine outgasing/evaporation rate. As I understand it the bound chlorine cannot "evaporate."

Without CYA in the tub, your cover and swimsuits will degrade faster, you'll have more chlorine in the air above the spa during use, you'll have more end stage chloramines, and bathers will be exposed to more active chlorine causing an increased likelihood of skin and eye irritation.

I was always under the impression that CYA has only one purpose , -> protecting the Chlorine from UV degration.

The other 2 functions (buffer the power of chlorine and reducing outgasing / evaporation rate ) are new to me.

Not saying that's not true - I guess I have to do more research on that.

My prevoius house had an outdoor pool , which was exposed to sunlight and I would keep a level of around 30 - 40 ppm CYA in the pool.

Mind you - this was a pool with about 14.000 gal with a very light bather load. I'd keep my FAC at 1-2ppm.

Hot tubs are much more "dangerous" than slightly used private pools in terms of keeping them microbiological safe.

We all know that if ( for example) 3 people enter the tub and stay in there for 30 min. the FAC level drops quite fast.

For that reason I prefer having about 3 ppm in the tub before people use the tub.

I rather have a bit of chlorine "reserve" in my tub then getting (amongst other things) legionella problems.

Again - for this reason it is (to me anyway) important to monitor my ORP's in the water.

If I got more than 750mV I am assuming that no cross contamination can take place, regardless how high ( or low) my FAC is in the tub.

Found an article about CYA , which demonstrates the relation between ORP and CYA ->

http://www.texaswaterworks.com/overstabilization.pdf

Disclaimer : We all know that not everything we read/find on the net may be true or accurate.

For my part I love to learn new things and I guess we learn every day.

[chem geek]

Why don't you start with the definitive scientific paper that determined the equilibrium constants for the chlorine/CYA relationship which you can find here? Basically, if you don't use any CYA at all, then your active chlorine (hypochlorous acid) level will be very, very high and since as you point out you want chlorine in reserve you end up with a lot of oxidation power that degrades hot tub covers, swimsuits, skin, hair, etc. and can lead to more nitrogen trichloride (see this post).

If you look on the web, you will find everything from "CYA doesn't matter" from chlorinated isocyanurate manufacturers (e.g. makers of Trichlor and Dichlor) to "CYA should never be used in indoor pools or in spas". These rules don't look at the real science behind chlorine/CYA. CYA absolutely, positively does not only protect chlorine from sunlight. It is a hypochlorous acid buffer and significantly moderates chlorine's strength. This is both a good and bad thing -- that is, overuse of CYA isn't good but underuse has issues as well.

As for the correlation of ORP with FC vs. CYA, you can see from the graphs in this post that it is the hypochlorous acid concentration that is more closely correlated with ORP than it is FC. You can read more about a variety of issues not generally taught in the pool/spa industry here.

[Retep]

Why don't you start with the definitive scientific paper that determined the equilibrium constants for the chlorine/CYA relationship which you can find here? Basically, if you don't use any CYA at all, then your active chlorine (hypochlorous acid) level will be very, very high and since as you point out you want chlorine in reserve you end up with a lot of oxidation power that degrades hot tub covers, swimsuits, skin, hair, etc. and can lead to more nitrogen trichloride (see this post).

If you look on the web, you will find everything from "CYA doesn't matter" from chlorinated isocyanurate manufacturers (e.g. makers of Trichlor and Dichlor) to "CYA should never be used in indoor pools or in spas". These rules don't look at the real science behind chlorine/CYA. CYA absolutely, positively does not only protect chlorine from sunlight. It is a hypochlorous acid buffer and significantly moderates chlorine's strength. This is both a good and bad thing -- that is, overuse of CYA isn't good but underuse has issues as well.

As for the correlation of ORP with FC vs. CYA, you can see from the graphs in this post that it is the hypochlorous acid concentration that is more closely correlated with ORP than it is FC. You can read more about a variety of issues not generally taught in the pool/spa industry here.

Thanks for all those links. Very , very interesting indeed. I just looked at them quickly and will read them through later on.

As I mentioned in my posting above - I love to learn new things.