Why are CYA levels lower when using ORP?

[woodsage]

I have a outdoor, 21,000 gal plaster pool, screened cage, no pool cover, solar heat, SWG, cartridge filter, sense & dispense system, Goldline controller in SW Florida and I like to keep the FC level at 3.5.

I don't understand why with an ORP sensor/probe system the recomended CYA level should be 30-50 ppm but without an ORP sensor/probe then the recommended CYA level is 70-80 ppm. If all things being equal, except for the use of an ORP sensor, why is there a difference in CYA levels? It was my understanding that a higher CYA level would cushion/blanket chlorine from sunlight/UV and therefore I could dial back the SWG.

Thanx for the help...

 

[JasonLion]

Higher CYA levels help the SWG work more efficiently, but interfere with ORP testing. To get the ORP probe to return useful results, the CYA level need to be fairly low.

CYA reduces the level of active chlorine in the pool, which lowers the ORP reading. Without any CYA in the water, ORP goes up very rapidly with increasing FC levels. As you add CYA, the ORP reading goes up far much more slowly for any given increase in FC level. Eventually, with rising CYA levels, the change in ORP level becomes so small that it can not be reliably distinguished from background noise in the system. For most ORP systems this starts to happen around a CYA level of 50 or 60. For reliable ORP operation, you should keep CYA below this level.

 

[woodsage]

Okay, I still don't get it. So with ORP I'm giving up SWG efficiency and I have to deal with higher FC levels.

You state "Without any CYA in the water, ORP goes up very rapidly with increasing FC levels," so then I should have high CYA levels (70-80 ppm) to lower FC levels, correct?

So if I want to have lower FC levels (3-4 ppm) then I should abandon ORP and continue with CYA levels in the 70-80 ppm range; if that's true, then what's the point of having ORP?

How can I get low (3-4 ppm) levels with ORP? I read on this forum that the rule of thumb for pools with SWG is FC levels that are 10% of CYA levels. Is that advice correct with ORP?

 

[JasonLion]

Lower CYA levels work with lower FC levels, but higher total chlorine usage. The SWG works harder to maintain the level, even though you are aiming for a lower level, because more chlorine is lost to sunlight. If your CYA is at 30 or 40, you can aim for an FC level of 2 to 3.

The point of having ORP is that you don't need to test the FC level very often, nor do you need to adjust the SWG percentage as the seasons change. The ORP controller takes care of adjusting chlorine production to respond to increased bather load, or reduce sunlight levels, or any other factors that come into play.

I have never found ORP automation to be very useful in outdoor pools. It can make a huge difference in an indoor pool, where chlorine usage varies dramatically depending on bather load. But in an outdoor pool, chlorine usage is fairly steady anyway, because most of it gets used by sunlight, which is fairly regular.

PH automation is far far more useful in an outdoor pool.

 

[woodsage]

Well I still don't get it. The Pool School link states that for pools that are using SWG's the CYA level should be between 60-80 ppm and preferably on the higher end and then goes on to say that the biggest mistake people make with SWG pools is that the CYA is too low.

So, all things being equal, except the use of ORP or not using ORP,then if I'm using ORP then I need a CYA level of 30-40 but if I'm not using ORP then I need a CYA level of 30-40? :?

The chart below is from the Chlorine/CYA Chart at Trouble Free School and to me it's stating the exact opposite of what your stating that, "Lower CYA levels work with lower FC levels, but higher total chlorine usage. The SWG works harder to maintain the level, even though you are aiming for a lower level, because more chlorine is lost to sunlight. If your CYA is at 30 or 40, you can aim for an FC level of 2 to 3."

The chart below states that to have a FC level of 3, then I need a CYA level of 60 not 30 - I don't understand the discrepancy, please help.

SWG Pools
CYA (Stabilizer) Minimum FC Target FC Shock FC
60 3 4 24
70 3 5 28
80 4 6 30

 

[Guest]

CYA has always been the big problem with ORP electrodes. They just don't work properly when the CYA is too high. They used to not work at all with CYA present but the newer ones are better and will work with CYA in the 30-50 range. IMHO, ORP control for an outdoor pool with a SWG is useless since, once ajusted and with weekly testing the pool will remain stable. It does have some value in indoor pools (but CYA levels are very low or non existent in indoor pools also.)
Higher CYA levels ARE better but they will not work with an ORP controller so compromises have to be made. It's really that simple. If there was an ORP controller that would work at the higher CYA levels it would be ideal but there isn't.

 

[JasonLion]

The chart below is from the Chlorine/CYA Chart at Trouble Free School and to me it's stating the exact opposite of what your stating

The chart you quoted doesn't list any CYA levels below 60, nor does it list what total chlorine usage will be, so it really doesn't tell you anything about what I said.

If you have a SWG and don't have ORP, then CYA should be around 70 or 80 and FC levels around 3 or 4. That will give you the longest possible cell life and the most stable chemical balance.

If you want to use ORP, then you must lower the CYA level below 50, or the ORP controller simply will not work. With CYA levels around 40 the FC level can be around 2 or 3, which isn't listed in the chart. However, the cell will need to work twice as hard, and last half as long, and it will be more difficult to maintain your chemical balance.

Nothing that I can remember at pool school says much of anything about ORP or running a SWG at low CYA levels, so I am not sure what you were looking at that gave you a different impression.

Keep in mind that the FC level and the total chlorine usage are not the same thing. At low CYA you maintain a lower FC level, but need to use a lot of chlorine to keep the FC level stable. At higher CYA levels you maintain a higher FC level, but don't use very much chlorine to keep the level high.

 

[woodsage]

So, would lowering the mV of the ORP controller from 650mV to 600mV or less reduce the "on" time of the SWG and therefore lower the FC level while maintaing a higher CYA level to reduce chlorine loss?

Is mV chlorine control of a SWG the same as a percentage control of chlorine w/o ORP?

 

[Guest]

So, would lowering the mV of the ORP controller from 650mV to 600mV or less reduce the "on" time of the SWG and therefore lower the FC level while maintaing a higher CYA level to reduce chlorine loss?

Is mV chlorine control of a SWG the same as a percentage control of chlorine w/o ORP?

No, the mv level is he measure of oxidation potential of the water. 650 mv is the accepted standard of water that is 'sanitized'. Realize that CYA is not the only thing that interferes with ORP measurements. MPS and Ozone also affect ORP readings.
There is a rough correlation between mv readings and FC levels but not any direct conversion. You are really talking about apples and oranges here.
Forget about lowering mv levels or raising CYA if you are using an ORP controller. If you want to run 80 ppm CYA then turn off the ORP controller and manually adjust your cell output to maintain the desired FC level.
If you want to usr ORP then you want to maintain a minimum 650 mv level and the FC level is not as important.

 

[learthur]

don't take this the wrong way but I think you are pursuing a pathway that exceeds your technical knowledge.

For you I would recommend you not do ORP control, however pH control would serve you well.

Making ORP work has a lot of variables in it that the average pool owner shouldn't and most won't desire to deal with.

Yes ORP works for me , but there are lots of reasons why which I don't have the time to explain. There is a lot of research and undiscovered science related to ORP use in swimming pools. I enjoy studing it with my test pool, but this is not for most people.
please focus on pH control.

Good luck

Lee

 

[Guest]

don't take this the wrong way but I think you are pursuing a pathway that exceeds your technical knowledge.

For you I would recommend you not do ORP control, however pH control would serve you well.

Making ORP work has a lot of variables in it that the average pool owner shouldn't and most won't desire to deal with.

Yes ORP works for me , but there are lots of reasons why which I don't have the time to explain. There is a lot of research and undiscovered science related to ORP use in swimming pools. I enjoy studing it with my test pool, but this is not for most people.
please focus on pH control.

Good luck

Lee

Well said!