Well, 0.2 off isn't as bad as I though it might be. I suggest you go ahead and add the CYA and once that is in you retest the PH as carefully as possible and then clean and then calibrate the PH sensor.
Autopilot total control?
- Thread starter andy
- Start date
OK, I added 3lb CYA last night 12-14-07 didn't do anything else.
Test results BEFORE adding CYA last night:
FC .4
CC .6
PH 7.2
TA 120
CH 375
CYA 0
Test results tonight:
FC 3.2
CC .8
PH 7.2
TA 130
CH 380
CYA 0
AutoPilot reads:
ORP 651
PH 7.37
Note***My Taylor K-2006C test kit doesn't read CYA below 30ppm. Any suggestions where/what kit to get one that reads lower?
Where do I go from here? Should I breakpoint shock now to try and get CC down?
Thanks, Andy
Test results BEFORE adding CYA last night:
FC .4
CC .6
PH 7.2
TA 120
CH 375
CYA 0
Test results tonight:
FC 3.2
CC .8
PH 7.2
TA 130
CH 380
CYA 0
AutoPilot reads:
ORP 651
PH 7.37
Note***My Taylor K-2006C test kit doesn't read CYA below 30ppm. Any suggestions where/what kit to get one that reads lower?
Where do I go from here? Should I breakpoint shock now to try and get CC down?
Thanks, Andy
Andy,
As expected, your controller is ramping up the FC to maintain the ORP level. It may very well end up overshooting (it looks like that's what it is doing) in which case you'll need to lower the setpoint for the ORP level or whatever it is you do to be able to get a lower FC level. You'll only need an FC of 2 ppm if your CYA is 20 ppm.
You can either get the full test kit at tftestkits.com or get the Taylor K-1720 kit, both of which will measure CYA down to 20 ppm. Or you can just guestimate as the water should be cloudy, but the black dot won't be gone at the 30 ppm mark, but will certainly be dulled in appearance.
I'd first get your FC level lower and stable by adjusting the setpoint. Then wait a bit to see what happens with the CCs as they might drop over time with the lower disinfecting chlorine level. If not, then you can go to the shock and airing out approach, but let's go slow and take one step at a time. There's no rush to lower the CCs -- they aren't causing damage. Getting the CYA in the water and getting the FC at a reasonable level for that CYA level is the most important thing right now. We also want to see how the corrosion goes, though that obviously is something that will take some time to see (or not see, hopefully).
Richard
As expected, your controller is ramping up the FC to maintain the ORP level. It may very well end up overshooting (it looks like that's what it is doing) in which case you'll need to lower the setpoint for the ORP level or whatever it is you do to be able to get a lower FC level. You'll only need an FC of 2 ppm if your CYA is 20 ppm.
You can either get the full test kit at tftestkits.com or get the Taylor K-1720 kit, both of which will measure CYA down to 20 ppm. Or you can just guestimate as the water should be cloudy, but the black dot won't be gone at the 30 ppm mark, but will certainly be dulled in appearance.
I'd first get your FC level lower and stable by adjusting the setpoint. Then wait a bit to see what happens with the CCs as they might drop over time with the lower disinfecting chlorine level. If not, then you can go to the shock and airing out approach, but let's go slow and take one step at a time. There's no rush to lower the CCs -- they aren't causing damage. Getting the CYA in the water and getting the FC at a reasonable level for that CYA level is the most important thing right now. We also want to see how the corrosion goes, though that obviously is something that will take some time to see (or not see, hopefully).
Richard
chem geek said:
What is causing my controller to ramp up the FC? Adding the CYA? Ever since I've been testing my FC has always been .2-.6 range and the controller has never been changed along with the ORP reading always being steady(the same as it is now).
It can sometimes take a few days (or even a week) for the full CYA level to show up in the test. In the mean time the CYA level will have been coming up, even though it is still too low to register on the test. As the CYA level comes up the FC level needs to increase to maintain the same ORP level. FC of 3.2 does sound a little higher than I would expect, but I doubt that it will be a problem. Once you can measure a CYA level it will be easier to tell what is really happening.
TF Test Kits sells a CYA test that goes down to 20 as does Taylor. It isn't possible to measure CYA reliably below 20, even though a few kits from other companies will claim to do so. Even around 20 the readings are fairly speculative, since the precision of the test is around +-10 or +-15.
TF Test Kits sells a CYA test that goes down to 20 as does Taylor. It isn't possible to measure CYA reliably below 20, even though a few kits from other companies will claim to do so. Even around 20 the readings are fairly speculative, since the precision of the test is around +-10 or +-15.
Jason, in this post Andy said he pre-dissolved the CYA in a bucket of water so it will probably not take a week for the CYA to register (assuming the CYA was completely dissolved in the bucket) and I'd expect it to be reasonably distributed within a day. Andy, what exactly does the CYA test look like now? Is there noticeable cloudiness, but the black dot does not completely disappear at the 30 ppm line? Is it only partially visible or is is still pretty clearly visible? Look at this link from Taylor. If the black dot is less clear than in the "mid-test", then you are probably in the neighborhood of 20 ppm. If it's like the mid-test, then it's probably a bit less (perhaps 15 ppm) and if it's more clear than the mid-test then it may be 10 ppm or less. As Jason points out, this is rather rough. My guess is that if you measured correctly and you pre-dissolved, then you are probably near 20 ppm already, but I'd like to know how the CYA test looks.
Your pool without CYA was essentially being over-chlorinated because the controller was trying to maintain a very low FC level (which is hard to do) since it takes very little FC without CYA to get to a high ORP. It's also not a good idea to try and maintain 0.2 or 0.4 ppm FC in a pool since local usage from bather demand can readily consume that amount of chlorine leaving the water unsanitized at least locally. So one wants a higher FC level (usually at least 1 ppm) but then if you don't have CYA in the water the chlorine is too strong. So when you added the CYA it cut down chlorine's effectiveness (technically reducing the hypochlorous acid concentration) by a factor of around 20 and since the ORP sensor is roughly measuring the hypochlorous acid concentration, it is ramping up the FC level to compensate. The ORP sensor is probably set too high, but if it stops at 3.2 ppm FC then this is not terrible. If you set the ORP mV target 10 mV lower, then it may be closer to a target of 2 ppm FC (assuming that 3.2 is where it currently stops -- it may still be going up so let us know if it's stable or not).
With 3.2 ppm FC and (assuming) 20 ppm CYA, then this has the same amount of disinfecting chlorine (hypochlorous acid) as 0.16 ppm FC with no CYA. That is lower than your typical 0.2 - 0.6 range and certainly far lower than the level when you shocked the pool with chlorine. The hope is that at this lower level of disinfecting chlorine, the rate of corrosion of your aluminum rails will be slowed. Also, constant exposure to the higher chlorine levels of 0.4 ppm or more would be more disinfection than is needed and would also be harsher on skin and hair and swimsuits. Think of CYA as a buffer for chlorine -- it reduces the effective chlorine concentration (which is what ORP sort of measures) but keeps the chlorine in reserve and releases it quickly as needed (this reserve plus the effective chlorine is what FC measures).
Richard
Your pool without CYA was essentially being over-chlorinated because the controller was trying to maintain a very low FC level (which is hard to do) since it takes very little FC without CYA to get to a high ORP. It's also not a good idea to try and maintain 0.2 or 0.4 ppm FC in a pool since local usage from bather demand can readily consume that amount of chlorine leaving the water unsanitized at least locally. So one wants a higher FC level (usually at least 1 ppm) but then if you don't have CYA in the water the chlorine is too strong. So when you added the CYA it cut down chlorine's effectiveness (technically reducing the hypochlorous acid concentration) by a factor of around 20 and since the ORP sensor is roughly measuring the hypochlorous acid concentration, it is ramping up the FC level to compensate. The ORP sensor is probably set too high, but if it stops at 3.2 ppm FC then this is not terrible. If you set the ORP mV target 10 mV lower, then it may be closer to a target of 2 ppm FC (assuming that 3.2 is where it currently stops -- it may still be going up so let us know if it's stable or not).
With 3.2 ppm FC and (assuming) 20 ppm CYA, then this has the same amount of disinfecting chlorine (hypochlorous acid) as 0.16 ppm FC with no CYA. That is lower than your typical 0.2 - 0.6 range and certainly far lower than the level when you shocked the pool with chlorine. The hope is that at this lower level of disinfecting chlorine, the rate of corrosion of your aluminum rails will be slowed. Also, constant exposure to the higher chlorine levels of 0.4 ppm or more would be more disinfection than is needed and would also be harsher on skin and hair and swimsuits. Think of CYA as a buffer for chlorine -- it reduces the effective chlorine concentration (which is what ORP sort of measures) but keeps the chlorine in reserve and releases it quickly as needed (this reserve plus the effective chlorine is what FC measures).
Richard
chem geek said:
There have been a number of reports of people putting CYA in a sock and having it vanish from the sock within 24 hours and still not show up in the test for several days. I have no idea what is really happening but it does seem like CYA doesn't "fully" dissolve into the water in an active form for several days even if it does get into small enough bits to appear to be dissolved. Likewise, there are many reports of people being able to fully dissolve CYA in a bucket in any plausible amount of time (and in one case it didn't dissolve even when the water was heated).
Guessing wildly, I wouldn't be surprised if some enterprising manufacturer makes a version of CYA where very small crystals of CYA, that are not easily seen individually, are bound together into clumps. The clumps could then "dissolve" without the CYA going fully into solution for some time. That would dramatically increase the surface area, so the CYA would dissolve faster than otherwise, but it could still take far longer to dissolve than the amount of time it would take to vanish from a sock or into a bucket of water.
Like I said, I am guessing between the lines of the various reports. I can't say what really happened in this particular case. But I wouldn't be the least bit surprised if the CYA level tested higher in a couple of days than it did after 24 hours.
Jason,
There actually is a form of Cyanuric Acid that dissolves quickly (it's already a concentrated liquid) and is described here. I've corresponded with the inventor of this product and it's a monosodium salt of cyanuric acid combined in a slurry that keeps it well dissolved (it took him years to develop to perfect the slurry that keeps things liquid and well-mixed). It's not cheap at $23.45 per gallon here where one gallon adds 40 ppm to 10,000 gallons, but it's a very convenient alternative. This link compares the cost of CYA from various sources. Dichlor also dissolves quickly and adds CYA, but it also adds chlorine (so fine for an outdoor pool where you might be adding 3-4 ppm FC every day in full sunlight, but not so fine for an indoor pool).
Anyway, the delay in CYA reporting is interesting and unexplained. Andy can wait a few days and remeasure the CYA level.
Richard
There actually is a form of Cyanuric Acid that dissolves quickly (it's already a concentrated liquid) and is described here. I've corresponded with the inventor of this product and it's a monosodium salt of cyanuric acid combined in a slurry that keeps it well dissolved (it took him years to develop to perfect the slurry that keeps things liquid and well-mixed). It's not cheap at $23.45 per gallon here where one gallon adds 40 ppm to 10,000 gallons, but it's a very convenient alternative. This link compares the cost of CYA from various sources. Dichlor also dissolves quickly and adds CYA, but it also adds chlorine (so fine for an outdoor pool where you might be adding 3-4 ppm FC every day in full sunlight, but not so fine for an indoor pool).
Anyway, the delay in CYA reporting is interesting and unexplained. Andy can wait a few days and remeasure the CYA level.
Richard
Test results 12-16-07
FC 2.8
CC .8
PH 7.2
TA 130
CH 400
CYA 0
AutoPilot Total Control
ORP 656
PH 7.37
**CYA test. I filled the small comparator tube to the 30ppm mark with regular pool water and could see the black dot very clear and bright
I then did the test and filled the comparator to the 30ppm mark with the solution and could still see the black dot good but it was dull.
I'll keep testing each day and see where things level off at. Unless you tell me to do something different.
Thanks, Andy
FC 2.8
CC .8
PH 7.2
TA 130
CH 400
CYA 0
AutoPilot Total Control
ORP 656
PH 7.37
**CYA test. I filled the small comparator tube to the 30ppm mark with regular pool water and could see the black dot very clear and bright
I then did the test and filled the comparator to the 30ppm mark with the solution and could still see the black dot good but it was dull.
I'll keep testing each day and see where things level off at. Unless you tell me to do something different.
Thanks, Andy
12-17-07
Only checked FC, CC and CYA. Let me know if I should be checking others
FC 2.4
CC .8
CYA same as yesterday
AutoPilot Total Control
ORP 656
PH 7.37
**CYA test. I filled the small comparator tube to the 30ppm mark with regular pool water and could see the black dot very clear and bright
I then did the test and filled the comparator to the 30ppm mark with the solution and could still see the black dot good but it was dull.
Only checked FC, CC and CYA. Let me know if I should be checking others
FC 2.4
CC .8
CYA same as yesterday
AutoPilot Total Control
ORP 656
PH 7.37
**CYA test. I filled the small comparator tube to the 30ppm mark with regular pool water and could see the black dot very clear and bright
I then did the test and filled the comparator to the 30ppm mark with the solution and could still see the black dot good but it was dull.
It looks like your controller is getting more stable and perhaps the CYA is mostly dissolved even though it does not all show up on the test. You can just check the CYA in a few days -- you don't need to check it every day (save your reagents). At least the lower 656 mV reading represents a lower disinfecting chlorine level than you had when it was at 690 mV at one point. Hopefully, this lower effective chlorine level will be less corrosive to your rails (it would if they were immersed in the water, but I'm not sure how much it will help with splash-out and evaporation).
As for the CC, if you want to try and tackle that now, then I would first see what uncovering the pool and having good air circulation does (i.e. to see if it reduces CCs). Of course, if there were any way to get UV rays of sunlight (or UV lamps) on the pool, then that would be great, but may not be practical or possible in your situation. With the lower disinfecting chlorine level, you should be producing fewer persistent chlorine (such as nitrogen trichloride) but will need to outgas what's already there and get it out of your pool and room.
One other important factor is to see if you can get the pH up to at least 7.5 and stable there. The higher pH not only helps reduce any corrosion effects, but also produces far fewer disinfection by-products including nitrogen trichloride that is hard to get rid of unless you air out the water (it's volatile and smells bad). The effect of pH on this ammonia breakpoint reaction is quite strong with the difference between a pH of 7.2 and 7.5 being a factor of at least 3 or higher in DPB production.
Richard
As for the CC, if you want to try and tackle that now, then I would first see what uncovering the pool and having good air circulation does (i.e. to see if it reduces CCs). Of course, if there were any way to get UV rays of sunlight (or UV lamps) on the pool, then that would be great, but may not be practical or possible in your situation. With the lower disinfecting chlorine level, you should be producing fewer persistent chlorine (such as nitrogen trichloride) but will need to outgas what's already there and get it out of your pool and room.
One other important factor is to see if you can get the pH up to at least 7.5 and stable there. The higher pH not only helps reduce any corrosion effects, but also produces far fewer disinfection by-products including nitrogen trichloride that is hard to get rid of unless you air out the water (it's volatile and smells bad). The effect of pH on this ammonia breakpoint reaction is quite strong with the difference between a pH of 7.2 and 7.5 being a factor of at least 3 or higher in DPB production.
Richard
Is the pool being kept covered or did you try uncovering it and getting air circulation and fresh air above it? You had previously said you kept it covered most of the time. I know that uncovering it makes things humid inside and that circulating in fresh air makes it cold, but that is worth trying first.
If you have an adjustment for ORP and pH, try setting the ORP target down 10 mV which may get the FC level to shoot for something closer to 2.0 ppm. And set the pH target to 7.5 -- it looks like it's moving towards that.
Poolsean, have you ever seen a pool, even an indoor pool, with an SWG having persistent Combined Chlorine (CC)? One would think that the superchlorination in the cell would take care of that.
Richard
If you have an adjustment for ORP and pH, try setting the ORP target down 10 mV which may get the FC level to shoot for something closer to 2.0 ppm. And set the pH target to 7.5 -- it looks like it's moving towards that.
Poolsean, have you ever seen a pool, even an indoor pool, with an SWG having persistent Combined Chlorine (CC)? One would think that the superchlorination in the cell would take care of that.
Richard
I have not left it uncovered yet. How long are you suggesting I uncover it for? If that helps lower the CC are you saying I would need to leave it uncovered often or just to get the CC lower the first time?
I will adjust the PH setpoint on my Total Control until I get a 7.5-7.6 reading using the Taylor test kit. Then I will calibrate the Total Control to match that. I believe right now the Total Control PH sensor is reading about .1 - .2 low but I'll have a more accurate reading when I get to the 7.5-7.6 color.
I'll also drop the ORP 10mv as you suggested if possible. I'll have to read the manual.
I will adjust the PH setpoint on my Total Control until I get a 7.5-7.6 reading using the Taylor test kit. Then I will calibrate the Total Control to match that. I believe right now the Total Control PH sensor is reading about .1 - .2 low but I'll have a more accurate reading when I get to the 7.5-7.6 color.
I'll also drop the ORP 10mv as you suggested if possible. I'll have to read the manual.
Richard,
That's confusing me too. There's no reason why, even with the cover on, the cc shouldn't be already oxidized by the cell, unless the ORP has been high and not allowing the cell to activate.
Andy, you may want to consider putting the Pool Pilot in a boost mode, or using Potassium Monopersulfate to oxidize your pool, and leave the cover off to offgas the cc's.
To lower the ORP,
From the normal display, press the up or down button when the display is showing the ORP readings. This will get you into the program mode.
Lower the ORP 10 mv, as Richard suggested, then press SELECT.
Keep up posted on your results.
That's confusing me too. There's no reason why, even with the cover on, the cc shouldn't be already oxidized by the cell, unless the ORP has been high and not allowing the cell to activate.
Andy, you may want to consider putting the Pool Pilot in a boost mode, or using Potassium Monopersulfate to oxidize your pool, and leave the cover off to offgas the cc's.
To lower the ORP,
From the normal display, press the up or down button when the display is showing the ORP readings. This will get you into the program mode.
Lower the ORP 10 mv, as Richard suggested, then press SELECT.
Keep up posted on your results.
Poosean wrote:
Hey, guys,
It's not completely clear to me just how a pool rids itself of excess CC's. I know you must have additional FC and sunlight and air play a part but how the Chloramines are actually eliminated (reduced) is a puzzle to me. I would appreciate an explanation aimed at us laymen......please type s-l-o-w-l-y!! :lol:
Hey, guys,
It's not completely clear to me just how a pool rids itself of excess CC's. I know you must have additional FC and sunlight and air play a part but how the Chloramines are actually eliminated (reduced) is a puzzle to me. I would appreciate an explanation aimed at us laymen......please type s-l-o-w-l-y!! :lol:
Initially I would leave it uncovered for at least several hours with good air circulation and possibly longer (a full day) for this first time. After that, then having the cover open for at least 30 minutes up to around one hour after you use the pool would be good. This gives the combined chlorines a chance to outgas and to achieve breakpoint (i.e. to for chloramine to breakdown to nitrogen gas).andy said:
Richard
I'd first like Andy to try just airing out the pool before trying more shocking with either chlorine or MPS.duraleigh said:
Combined Chlorine (CC) is a generic term representing any compounds where disinfecting chlorine (hypochlorous acid) has combined with them strongly enough to not measure in the Free Chlorine (FC) test, but weakly enough to be measured in the Combined Chlorine (CC) test. Essentially, there is a range of chemical bond strength between chlorine and various other substances.
There are three broad categories based on the type of substances and I'll talk about each.
CHLORAMINES (Chlorine combined with ammonia) -- measures as CC
Monochloramine, NH2Cl
Dichloramine, NHCl2
Nitrogen trichloride (aka Trichloramine), NCl3
CHLORINATED CYANURATES (Chlorine combined with CYA) -- measures as FC
6 different related substances -- CYA combined with 1, 2 or 3 chlorine plus dissociated acid relatives
Other organic compounds that mimic CYA such as Glycoluril and maybe Dimethylhydantoin (DMH)
OTHER CHLORO-ORGANICS (Chlorine combined with organic compounds more strongly than CYA) -- measures as CC
Many compounds including byproducts that are possible carcinogens including Trihalomethane (Trichloromethane aka Chloroform) and many others (many are relatively innocuous while others are not). The two most common reactions with chlorine are substitution reactions where chlorine replaces a hydrogen attached to a carbon or nitrogen and addition reactions with chlorine (and/or oxygen) attaching to carbons that had a double bond (i.e. unsaturated hydrocarbons as found in oils like suntan lotion).
Urea is a special case since it is an organic that can behave like ammonia since it can breakdown (oxidize) into carbon dioxide and ammonia, though generally does so slowly.
Free Chlorine (FC) will measure the truly free chlorine that hasn't combined with anything, namely hypochlorous acid and hypochlorite ion, but will also measure chlorine loosely bound to organics such as Cyanuric Acid (CYA). Combined Chlorine (CC) will measure the chloramines and chlorine combined with most other organics.
We've talked about chlorine and CYA ad nauseam in many threads so let's start with the chloramines. Chlorine combines with ammonia to form monochloramine very quickly, with 90% conversion in about 30 seconds at our normal FC/CYA levels and even within 5 minutes at low chlorine levels (i.e. 1 ppm FC with 100 ppm CYA). However, further combination of monochloramine with ammonia to get broken down completely to nitrogen gas and water takes longer -- at our usual FC around 10% of CYA level, it takes about 2 hours for 90% completion. During this process, several things can happen, but generally the simplified reactions are as follows.
NH3 + HOCl --> NH2Cl + H2O
Ammonia + Chlorine --> Monochloramine + Water FAST
NH2Cl + HOCl --> NHCl2 + H2O
Monochloramine + Chlorine --> Dichloramine + Water MODERATE
NHCl2 + NH2Cl --> N2(g) + 3H+ + 3Cl-
Dichloramine + Monochloramine --> Nitrogen gas + Hydrochloric Acid SLOW
NHCl2 + HOCl --> NCl3 + H2O
Dichloramine + Chlorine --> Nitrogen trichloride + Water SLOW
The last two reactions compete and which one occurs determines whether you end up breaking down the ammonia completely into nitrogen gas vs. whether you end up with Nitrogen trichloride which doesn't break down readily and is very smelly and irritating (and volatile). It turns out that higher levels of chlorine and lower pH favor the production of Nitrogen trichloride so this is yet another reason to use at least some CYA in an indoor pool and keep the pH at 7.5 or above. This is also why I want Andy to air out the pool since IF the Combined Chlorine is Nitrogen trichloride, then airing it out will help since this is a volatile chemical.
You can also see that the Dichloramine is a temporary intermediate chemical in the process and it is also smelly and volatile, more than monochloramine but not as bad as Nitrogen trichloride. So having good air flow during the above process helps ensure that the breakpoint will be complete as it helps reduce the smellier byproducts and sweeps away whatever Nitrogen trichloride ends up getting produced.
Technically speaking, the above reactions are always occurring when ammonia and chlorine are both present. It is not necessary to shock the pool to have these reactions occur. Shocking just has them go faster which is not always better as I indicated above since it can end up actually producing more Nitrogen trichloride. Also, the 10x rule isn't correct, but I won't get into that here.
As for other organics that have combined with chlorine, things are not so neat and simple. Some of these chloro-organics as well as Nitrogen trichloride can be persistent Combined Chlorine. At least with the Nitrogen trichloride you can remove it by airing out the water since it is so volatile, but with non-volatile chloro-organics there isn't much you can do. You can try using a non-chlorine shock (MPS) that might help, but usually such non-chlorine shock is better to use BEFORE chloriine combines with organics. That is, MPS oxidizes organics and ammonia before chlorine gets a chance to combine with them. So MPS is more like a Combined Chlorine prevention method. It is not without it's own side effects, however, as it is still a strong oxidizer so some people have skin sensitivities to it and it might accelerate corrosion of metal so I'm particularly concerned in Andy's environment so I don't want him using it regularly (but one time is fine and we may do that if the airing out doesn't help).
Though I talked about airing out the volatile Combined Chlorine, such as Nitrogen trichloride, I didn't talk about sunlight. The UV rays of sunlight help break down various chlorine products, but its unclear how quickly this occurs. We know it breaks down chlorine rather quickly, less so when CYA is present, and Nitrogen trichloride is also relatively unstable in sunlight. Monochloramine may persist for a while though does slowly break down in sunlight. I don't know about the chloro-organics in terms of how they react in sunlight.
So the bottom line is that uncovering a pool and letting it air out (circulating the air above it with fresh air) helps to remove any volatile Combined Chlorine such as Nitrogen trichloride. Exposing the pool to sunlight (or even better, to UV lamps) can break down at least some Combined Chlorine. As for shocking with chlorine to reduce CC -- that really doesn't make sense if you've already got chlorine in the water and the CYA isn't too high (with low FC) unless you aren't patient and want to speed up the reaction. A non-chlorine shock, such as potassium monopersulfate (MPS), is best used as a preventative to stop the formation of Combined Chlorine in the first place.
Richard
Let me try a no chemistry explanation, hopefully more or less true, but leaving out all the details.
Chlorine reacts with many contaminates in more than one step. In the first step chlorine reacts with contaminates to form combined chlorine. The first step generally happens quite quickly. Subsequent steps require additional chlorine (or sunlight) to react with the CC and break it down further into things which are harmless. These secondary steps usually happen much more slowly than the first step. Because of this there is a risk that CC can accumulate, the first step producing CC more quickly than the secondary steps can eliminate it.
CC accumulates in the pool when the secondary breakdown steps happen more slowly than the rate at which the first step is producing CC. A number of factors enter in here as there are many different reaction paths and quite a few things can influence the various reaction rates.
In general an outdoor pool will have the second step speed up enough by sunlight that CC does not accumulate. In an indoor pool the secondary steps can be speed up by the super chlorination inside of a SWG and that is usually sufficient. Adding MPS slows down the first step enough to prevent CC from accumulating, since the MPS usually combines with the contaminates before the chlorine can get to them to form CC. Adding large amounts of additional chlorine (shocking) speeds up both steps, however you usually run out of contaminates quickly enough that the secondary steps can then run to completion.
That isn't quite true in every technical detail, but I think that it catches the essential elements of what is going on in a way that is much simpler for most people to follow than Chem Geek's answer.
Chlorine reacts with many contaminates in more than one step. In the first step chlorine reacts with contaminates to form combined chlorine. The first step generally happens quite quickly. Subsequent steps require additional chlorine (or sunlight) to react with the CC and break it down further into things which are harmless. These secondary steps usually happen much more slowly than the first step. Because of this there is a risk that CC can accumulate, the first step producing CC more quickly than the secondary steps can eliminate it.
CC accumulates in the pool when the secondary breakdown steps happen more slowly than the rate at which the first step is producing CC. A number of factors enter in here as there are many different reaction paths and quite a few things can influence the various reaction rates.
In general an outdoor pool will have the second step speed up enough by sunlight that CC does not accumulate. In an indoor pool the secondary steps can be speed up by the super chlorination inside of a SWG and that is usually sufficient. Adding MPS slows down the first step enough to prevent CC from accumulating, since the MPS usually combines with the contaminates before the chlorine can get to them to form CC. Adding large amounts of additional chlorine (shocking) speeds up both steps, however you usually run out of contaminates quickly enough that the secondary steps can then run to completion.
That isn't quite true in every technical detail, but I think that it catches the essential elements of what is going on in a way that is much simpler for most people to follow than Chem Geek's answer.
Thanks Jason. I agree it's much easier to understand and gets the essence of what is happening.