Having operated a SWCG pool since May of 1998, I thought I would give my dollarsworth of SWCG experience over the years, especially for the new SWCG owners on the forum..
Living in Central Florida allows me to run the Diamondbrite 20,000 gallon pool 8 - 12 hours a day depending on use and water temperature. The SWCG was initially an AutoPilot LS1000 with a R-15 cell. It was upgraded with a Pool Pilot DIG-220 and an SC-48 cell about 2 years ago when the R-15 cell started to die.
I had to replace the R-15 once only, and am still running quite happily on the SC-48 cell.
SWCG Operation
I had previously described in the old forum (now lost) how the cell creates the products needed to sanitize the pool and am resurrecting that procedure, with some additional explanations. There are some corrections since this was originally created, and I have to thank Chem_Geek for bringing them to light.
Salt water passing through the cell is dissociated using DC electricity applied to the membranes in the cell. This creates
Hydrogen gas, H2
Chlorine gas, Cl2
The Chlorine gas reacts with the pool water to make Hypochlorous Acid, which is the component that destroys the bacteria and chloramines in the pool.
Cl2 + H2O --> HOCL +H+ + Cl-
The Hypochlorous Acid further dissociates into Hydrogen ions and Hypochlorite ions.
HOCl <--> H+ + OCl-
As this reaction can go both ways, it depends on the pH. The higher the pH , the further it goes to the right or more dissociation. The lower the pH, the more it goes to the left, creating more Hypochlorous Acid, and making it more effective as a sanitizer.
This is why you need to keep the pH between 7.2 - 7.6 for best sanitization.
To summarize then for the non-chemists, the cell creates the components mentioned at the beginning, reacting to make Hypochlorous Acid, the main sanitizing agent. The chloramines are instantly destroyed, leaving only nitrogen and hydrogen coming from the returns, and retaining chlorine for use in the pool because of the high CYA content.
Lessons Learned
pH - Maintain between 7.2-7.6 using muriatic acid. Most of the other manufacturers also recommend this pH range. If you decide to use dry acid, then you will have to add salt to maintain the salt level. The effect of dry acid is to raise the TDS level as well as lower the pH and is not part of the process. I prefer to keep it simple with Muriatic Acid.
Salt Level - Different manufacturers call for different salt levels for their products. The Poolpilot calls for 2500 - 3500 ppm, Watermaid for 6000 ppm, and Aquarite from 2700 - 3400 ppm. For the Poolpilot, maintaining 3000 - 3500 ppm seems to work fine for me.
BTW, when adding salt in granular form to a plaster pool that has a concrete coping, be very careful not to spill any salt granules on the concrete. It will eventually create a hard deposit that will have to be removed with hammer and chisel. Wash any areas with pool water using a bucket back into the pool or use a garden hose.
Flow through the Cell - Several factors can reduce the flow through the cell, such as high differential pressure through the filter, which may need to be cleaned or backwashed. The use of solar or other forms of heating too can have an effect, as can vacuuming. With the Autopilot or Poolpilot, there is a springloaded check valve that helps to maintain flow through the cell at low flows. Other factors are scale buildup on the trisensor affecting both flow and salt concentration. This requires cleaning the unit with dilute Muriatic acid, even though the cell might not have scale. Whenever I clean the cell the trisensor is included. I use a Muriatic acid concentration of 1 part acid to 4 parts water. Always add the acid to the water, never the other way round.
Calcium Hardness - Keeping the CH at no more than 300 ppm for a plaster pool helps to keep the cell clear of scale. Check the cell once a month for any scale buildup, and clean according to the manufacturer's instructions, or the method mentioned above. A vinyl pool has to have no more than 120 ppm, but this may be different for other types of non-plaster pools.
Refill Water - Use of softened city water as refill helps to minimize the buildup of calcium hardness as well. Use of well water especially in Central Florida should be avoided, as if it contains Hydrogen Sulfide gas, it can destroy the active coating on the cell fairly rapidly. If it has to be used, either find a way to bypass the cell, or shut the circulation down while refilling to allow the gas to escape. A further help would be to use a filter that can remove iron, as this a major problem here in Central Florida.
Stabilizer (CYA) - Most of the manufacturers recommend 60 - 80 ppm, but I have been able to run with nearly 0 ppm in winter to about 50 ppm in the summer, with a maximum of 60 ppm. These figures have worked for me, but some units may require more. The reason the manufacturers require a high CYA concentration is because the cell produces a high concentration of chlorine at the cell membranes, and most of it is used in instantly destroying the chloramines. By keeping the CYA level high, it maintains the chlorine needed in the body of the pool.
Testing - Having a very good testkit is paramount for checking SWCG operation. Besides the regular chemicals to be tested, a drop type testkit for salt is highly recommended as a backup for pH meters. These units have to be calibrated frequently for reliable operation, no matter who tells you different. Ben's PS234 testkit is the choice of record for this in my opinion. Test strips should ONLY be used if there is nothing else, and are not recommended for SWC or regular bleach operation in my opinion.
Startup and Upsets - If the pool system has to be shut down for the winter, then on Spring startup, it is my recommendation and the manufacturer's not to turn on the SWCG until the system has been stabilized using bleach, and only bleach, as well as the other chemicals including salt to the manufacturer's specifications.
Similarly for upsets, there is no need to increase the SWCG loading, as this reduces cell life unnecessarily. Add bleach as needed to bring the sanitization level back to normal for your system. The SWCG is designed to maintain a steady chlorine level only, provided the water chemistry is monitored frequently.
For a new plaster pool, some plaster vendors recommended not adding salt for a SWCG for at least a month, until the plaster has had time to cure. Most of the time the salt can be added straightaway.
Also make sure that all leaves and debris are removed from the pool, skimmers and pool pump basket on a regular basis, otherwise the cell is working longer to remove this debris.
Cable Connections - need to be checked both to the cell and control sensors for good electrical contact on a regular basis, as this seems to be the weakest area when a control unit is not functioning properly. Scaling of the sensors can be another problem and have to be checked if there is scale buildup on the cell.
Pros
The unit saves having to lug Liquid Chlorine or Bleach and pucks, which have to be kept in a cool environment tightly sealed.
The great advantage is that it is a constant source of Chlorine, assuming the rest of the pool chemistry is maintained properly.
Cost wise I have found it is equivalent to running a 220 watt bulb for whatever hours of the day the unit is running. Based on the PoolPilot I have, it is about $22 per month for an 8 - 12 hour day, and that includes the cost of the cell over a 5 year life.
As the price of oil goes up, so will the cost of electricity and the cost of manufacturing bleach. SWCGs will start to pay for themselves in the long run. The initial cost will offset any price increases in bleach later on.
Cons
According to most AG pool manufacturers, they are not recommending these units for their pools and you will have to check the warranty. Similarly, SWCG manufacturers are not recommending them for AG pools and will give fair warning if you do. Check with both types of manufacturer bofore investing in an AG pool/SWCG combination.
The initial cost can be the problem here, but once set up and running, the return in ease of maintenance will be well worthwhile. Prices from pool equipment suppliers on the Internet give you a lot more choices for shopping around for the best and lowest prices.
I have found no real problems other than cable and sensor connections. Electricity has a way of finding the weakness in any electrical circuit.
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Any input from the "seasoned" SWCG owners would be appreciated, including manufacturer's reps.
I did this mainly as a general explanation for the new SWCG owners, to help them operate their particular setup.
Hope this all helps.
Pat
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