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Technical Question About Chlorine Generators


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I am a first-year pool owner, but a senior electrical engineer so, I'm interested to know how this system works. Does anyone know how the chlorine generator control boxes work? Do they just apply a voltage to the cell? Is it AC or DC? Can I apply my own voltage from an external power supply and get the same result?

Also, are the cells any more complicated than just an electrode with two wires? What signals are carried in the cable from the cell to the control unit?

More impotantly, has anyone made a homemade device that does the same thing......I'm sure there are a few experimenters out there?

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It's basically a DC voltage applied to plates where there is water flow from the pump. It's basic electrolysis. However, the "secret sauce" is in the coatings on the plates that maximize efficiency by limiting side reactions such as the production of oxygen gas instead of chlorine gas and in increasing plate life. There is also proprietary construction of the cells to make water flow efficient through the gaps in between the plates. The chemical reactions are described in the latter part of this post. The DC voltage is typically reversed every hour or so in order to reduce the amount of scale buildup at the cathode (where hydrogen gas is generated and that gets very high in pH causing scale to form). Also, most SWG cells use bipolar plates which has multiple plates that are not electrically connected and are in-between the two plates at the ends. A higher voltage is applied and the interior plates increase the effective surface area and amount of output without increasing the current.

There was someone making their own SWG cell but buying the special plates from a manufacturer and talked about this at Trouble Free Pool, but I can't seem to find that thread.

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  • 2 weeks later...
I am a first-year pool owner, but a senior electrical engineer so, I'm interested to know how this system works. Does anyone know how the chlorine generator control boxes work? Do they just apply a voltage to the cell? Is it AC or DC? Can I apply my own voltage from an external power supply and get the same result?

Also, are the cells any more complicated than just an electrode with two wires? What signals are carried in the cable from the cell to the control unit?

More impotantly, has anyone made a homemade device that does the same thing......I'm sure there are a few experimenters out there?

I thought that when realizing that the cells are just powered up by DC voltage. I think the system that I have (jandy) has to much packed in the control box to do a basic thing like switching a ground from one pin to another. It has 3 pins on the cell. The thing IS - it monitors salt level/water temp/controls % of chlorine produced. If you can regulate DC voltage to about 25 volts or so to power a cell, and have it on a timer, and test the salt and chlorine level with a test kit. Same difference without circuit boards. Being a electrical engineer this would be easy. Jandy has electrical and install manuals on their website.

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The less expensive salt cells just measure the current and determine an implied conductivity from that and display a salt reading. Some adjust the voltage to get the same current (chlorine production is proportional to current). More expensive units measure temperature so compensate for the conductivity differences with temperature thereby giving a more accurate salt level reading.

The control box varies the % on-time in order to control output. That is, there is no variation in chlorine production output when the unit is on, except from varying conductivity (which doesn't vary much during summer use assuming salt levels and pool temps are fairly consistent). The unit is just switched on a fraction of the time in order to cut down output. Obviously, varying the pump run time influences actual output per day.

The SWG will also have auto-shutoff sensors if the flow rate stops so that one does not build up hydrogen gas in the cell. There are various ways such sensing can be done -- not all use water flow sensors.

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The less expensive salt cells just measure the current and determine an implied conductivity from that and display a salt reading. Some adjust the voltage to get the same current (chlorine production is proportional to current). More expensive units measure temperature so compensate for the conductivity differences with temperature thereby giving a more accurate salt level reading.

The control box varies the % on-time in order to control output. That is, there is no variation in chlorine production output when the unit is on, except from varying conductivity (which doesn't vary much during summer use assuming salt levels and pool temps are fairly consistent). The unit is just switched on a fraction of the time in order to cut down output. Obviously, varying the pump run time influences actual output per day.

The SWG will also have auto-shutoff sensors if the flow rate stops so that one does not build up hydrogen gas in the cell. There are various ways such sensing can be done -- not all use water flow sensors.

That being said...what is your opinion on the best system that doesn't require parts being replaced often, and is durable? How long is the cell expected to last on average? Just curious.

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I don't have experience with SWG brands to be able to tell you that. Others can tell you brands they like. As for cell life, it seems to be around 5 years these days -- used to be less. However, this assumes you get an oversized cell -- if you get one that is too weak so is on much longer, then it won't last as long. So there is a tradeoff between capital cost and cell life, but it's not linear so getting a somewhat oversized cell is usually worth it.

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Amen to that, Richard. A local service rep is coming out next week to troubleshoot the comm problem between my EasyTouch and IC40. I've talked to 3 different Pentair reps over the past 2+ weeks, and 2 of the 3 asked why my builder equipped my ~16,500Gal pool w/ an IC40 designed for pools up to 40,000Gal. I replied that I had no idea, but hopefully it would last longer, and both said it definitely will. Finally seeing a silver lining to the nightmare we had in late 2007, when we the first pool had so many problems that the contractor reluctantly agreed to excavate the whole mess and install a second pool.

BTW, the 2 "Pool Cooler" kits I bought last week definitely work - installed and ran the pool overnight during our recent heat wave and the water temp dropped 8F. Naturally, the heat wave broke a day later, but the water was perfect this evening, 85F.

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While I don't remember where, there was another fellow a couple of months ago that was making his own SWG - and running into all the problems that SWG mfg have already been thru. Unless you're planning on starting your own line, or if it's just something to do, stick with one of the major brands. My preference is Goldline, for a variety or reasons, and since they've been in the business much longer than the others.

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  • 1 year later...

I think somebody has already pointed out that more expensive units change the polarity in the cell every so often to prevent the build up of scale on the plates. If this is so why not use AC? is there something about the characteristics of the plates that prevents this, or is it due to the needs of monitoring circuits. My unit uses two IXYS mcc 44-12 io1B thyristors with all connections used, and if you care to study the data sheet you might see what I mean about monitoring, I don't know whether there are triacs available to provide same polarity monitoring outputs. Unfortunately it's high summer here so can't afford to disconnect it at the moment long enough for a good study of the circuit.

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Electrolysis does not work [EDIT] efficiently [END-EDIT] when you use AC because the products that are produced are not built up enough to get concentrated and swept away so reversing polarity too quickly doesn't give them a chance to fully react and get over the overvoltage (activation energy) hump. Basically, you start to generate something and then reverse it pulling it back in before the reaction completes. It's terribly inefficient. You want DC and don't want to reverse the plates every 1/60th of a second as with AC.

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Electrolysis does not work when you use AC because the products that are produced are not built up enough to get concentrated and swept away so reversing polarity too quickly doesn't give them a chance to fully react and get over the overvoltage (activation energy) hump. Basically, you start to generate something and then reverse it pulling it back in before the reaction completes. It's terribly inefficient. You want DC and don't want to reverse the plates every 1/60th of a second as with AC.

Sorry but don't fully understand "overvoltage hump" or the need to "get concentrated". AC will generate chlorine at opposite plates consecutively but I imagine at some frequency your supposition will become true. I would like to know at approx what frequency that is.

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I should have added that the DC used is unsmoothed rectified AC so every 120 ? times a sec the voltage (and current) is reduced to zero, not too different from normal AC.

"not too different from normal AC" -- no way. Going from positive voltage to zero and back on one plate is never going negative. Similarly, the other plate is never positive. So there is no reversal of chemical reactions. With fully reversing AC the chemical reactions reverse as well, at least to some extent (depending on frequency).

At the anode (positive plate), the following reaction normally occurs when doing DC electrolysis of salt water:

2Cl- ---> Cl2(g) + 2e- ..... E0 = -1.36V

At the cathode (negative plate), the following reaction normally occurs when doing DC electrolysis of salt water:

2H2O + 2e- ---> H2(g) + 2OH- ..... E0 = -0.83V

The negative potentials are why a voltage is required in order to make these reactions occur and technically a greater voltage than indicated is needed (that's the "overvoltage") because of the activation energy of the reactions. In practice, much higher voltage is used, usually around 5V (SWGs that use 20-25V are bipolar so have intermediate plates where the voltage between the plates is still around 5V). Activation energy is the energy hurdle (hump) required to make a reaction complete. When two molecules collide to react, they repel or resist completing their reaction so enough kinetic energy (which is related to temperature) is needed to get over this energy hurdle. In the case of electrolysis, it is the hurdle relating to having an electron added to or removed from a chemical species, but the principle is the same.

Anyway, with DC electrolysis there is a buildup of products and charges at the plate and ultimately this slows down the electrolysis. Diffusion and circulation of water remove such products and distribute charges so that the electrolysis becomes more efficient. However, with AC electrolysis instead of having the reaction you want to occur, some of the electrolysis is wasted on simply reversing the reaction you just made. That is, the following occurs to some extent:

2Cl- ---> Cl2(g) + 2e- ..... E0 = -1.36V

then electrode voltages reverse and the following occurs which exactly undoes what you just produced above:

Cl2(g) + 2e- ---> 2Cl- ..... E0 = +1.36V

because this is more favored to occur than what you really wanted which was:

2H2O + 2e- ---> H2(g) + 2OH- ..... E0 = -0.83V

Similarly, at the other plate the following occurs to some extent:

2H2O + 2e- ---> H2(g) + 2OH- ..... E0 = -0.83V

then electrode voltages reverse and the following occurs which exactly undoes what you just produced above:

H2(g) + 2OH- ---> 2H2O + 2e- ..... E0 = +0.83V

because this is more favored to occur than what you really wanted which was:

2Cl- ---> Cl2(g) + 2e- ..... E0 = -1.36V

In practice with typical 60 Hz AC, you do get some of what you want, but you also get the reversal reactions which you don't want so your efficiency is significantly lowered if you use AC. The higher the frequency of AC, the less efficient. I don't know the precise efficiency vs. frequency, but it must be significant enough to have all SWG systems use DC that reverse over time (usually hours) rather than to simply use voltage-reduced line AC which would obviously be less expensive since no rectifier would be needed.

At some point, a much higher frequency might lead to greater current flows (at the same applied voltage) due to the effective capacitance of the water (as opposed to its effective DC resistance) and this might improve output in spite of the reversed reactions, but changing the frequency of AC is likely to be quite a bit more costly than a rectifier that turns AC into unsmoothed DC. By the way, you are assuming that SWG systems only use a rectifier to produce unsmoothed DC, but they probably also have capacitors (after the rectifiers) to create more smoothed DC since that would be more efficient to keep the voltage more constant.

Richard

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I'm presuming that the voltages are low as with DC (i.e. a transformer is still used to lower the voltage) and such voltages and currents are contained within the cell. Yes, AC can shock more readily than DC, but I don't think it's enough to be a safety hazard assuming the same grounding requirements as with a normal SWG (i.e. the only voltages applied are to the plates in the cell).

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The same is true for DC as well as AC and electricity will take the path of least resistance which would be to the plate that is very close. The amount of stray current will be negligible. Where there are problems with current and shocking is when a higher voltage of ONE polarity drops into the pool and causes current to flow towards a ground.

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