chem geek

For swimming pools, it is more common to use sodium carbonate aka soda ash or washing soda to raise the pH. Sodium bicarbonate aka baking soda doesn't raise the pH as much unless the pH is quite low. Baking soda is normally used to raise the TA precisely because it doesn't raise the pH as much. Soda Ash raises both the pH and the TA since it is technically the same as a combination of lye (sodium hydroxide) and baking soda (sodium bicarbonate). The reason that lye itself is not normally used is that it is fairly hazardous to skin and is very hygroscopic (absorbs water) so isn't a convenient powder as with soda ash or baking soda. As was pointed out by Théière, higher TA causes more carbon dioxide outgassing which raises the pH. Low pH and higher TA result in more over-carbonation of the pool water. This is shown in [u][url=http://www.troublefreepool.com/~richardfalk/pool/CO2.htm]this table[/url][/u].

See the Pool School article "Water Balance for SWGs" at Trouble Free Pool: http://www.troublefreepool.com/pool-school/water_balance_saltwater_generator The basic rule is that the Free Chlorine (FC) should be at least 5% of the Cyanuric Acid (CYA) level. Also, with most saltwater chlorine generator (SWG) pools, they tend to rise in pH so to mitigate that you keep your Total Alkalinity (TA) lower at around 70 ppm and you have a somewhat higher pH target of 7.7 if needed. If you have access to boric acid (not always easy to get in Europe) you can use 50 ppm borates (as ppm Boron) in your pool. You can use The Pool Calculator: http://www.thepoolcalculator.com for calculating dosages.

[url=http://www.troublefreepool.com/bernoulli-vs-the-pool-guy-t26694.html#p220462]This post[/url] and [url=http://www.troublefreepool.com/bernoulli-vs-the-pool-guy-t26694-60.html#p221533]this post[/url] detail the main piping head losses in the system BEFORE I got a variable speed pump.  The solar added 150 feet of round-trip 2" pipe to and from the starting point on the roof which is one story but pitched.  The solar goes the entire length of the roof with zig-zag roof hips and probably at least 120 feet round-trip of again 2" pipe.  The head loss from the panels at 2 feet of head is almost negligible compared to the overall system losses mostly from the very long pipe runs.  The piping for the solar is over half the total loss. With a separate panel system, either glazed panels or evacuated tubes, the circulation through the panels can be at a relatively slower flow rate since the water getting hotter is not a problem for these panels and therefore results in a very low pump electricity cost for that part of the system.  So one can locate the heat exchanger closer to the pool pump circulation system and of course one can use larger piping for more efficiency as well.  The main pool pump circulation system could then operate at a lower flow rate for additional savings.  One may still need periodic higher flow rates for decent skimmer action, but that's about it.

For glazed panels and evacuated tubes, one must include not only the price of the panels, but that of the heat exchanger system since pool water is not supposed to go directly through these panels (which to me is ironic since chlorinated tap water has a higher active chlorine level than pools with cyanuric acid in them -- perhaps the issue is to avoid the risk of higher chlorine from excessive shocking or low pH from improper maintenance).  I presume one was looking at complete system costs including any differences in installation labor. If the purchase price of the glazed panels and evacuated tubes has really come way down to be closer to black flat mat, then I'll certainly be looking at those when I eventually replace my panels (they are currently 9 years old and going strong -- no leaks).  9 years ago in the U.S., these other alternatives would have added tens of thousands of dollars to the system.  Our roughly $1500 per year in gas heating if completely eliminated would have taken 10 years or so to pay back.  It sounds like the economics have drastically changed since then.  I also wish that we had insulated the pool and piping -- the pool builder didn't think that was necessary, but I don't think he understood how we wanted the pool warm for pool water therapy (88ºF) and that we would be using it not just for 5 months but for a 7-8 month season. Another consideration that was touched upon earlier is the increase in pump electricity costs if the panel has a lot of head (pressure) resistance to flow.  My Fafco panels at 4 GPM per panel (48 GPM for 12 panels) has 2 feet of head (0.87 PSI), but far worse is the loss from very long runs to/from the panels and between them, though they are at least 2" pipe (would be better to be larger, say 2.5").  At least with my variable speed pump I have cut my pump electricity costs in half, mostly from running the pump slower when the solar isn't on as well as tuning the rate when the solar is on -- my pump electricity costs went from around $1500 to $750 per year. By the way, our marginal electricity rate is high (I live in California) at around $0.33/kWh while the equivalent cost for gas is only $0.05/kWh.  That makes a heat pump less economical than in areas with lower electric rates.

This argument about panel types isn't so simple as one is always better than the other or more economical.  It depends on how you will use them and the usual weather where you live.  [url=http://www.troublefreepool.com/solar-panel-technology-comparisons-t9648.html]This thread[/url] shows a technical comparison of different panel technologies (add 13% to all evacuated tube values in the table for the most efficient tubes now available). [url=http://eosweb.larc.nasa.gov/cgi-bin/sse/retscreen.cgi?email=rets%40nrcan.gc.ca&step=1&lat=48.8742&lon=2.3470&submit=Submit]This link[/url] shows the solar insolation in Paris, France.  (submit your own location using [url=http://eosweb.larc.nasa.gov/sse/RETScreen/]this link[/url] but you need to enter latitude/longitude since the geographic map link doesn't work).  Your summer sun is similar to my August sun.  I have black flat mat panels and they heat our pool to 88ºF adding 15ºF-20ºF to the water temp compared to no solar or cover. [url=http://www.troublefreepool.com/what-does-it-cost-to-run-a-pool-heater-anyway-t18542.html#p151853]This post[/url] shows the situation with my pool.  Note that without a pool cover and no solar or gas heating, the pool water temperature will get to slightly above the average day/night air temperature (it gets higher in more humid environments where there is less heat loss from evaporation so the sun's heating is a net positive).  A pool cover can increase the temp by 10-15ºF (my cover is an electric safety cover so while it eliminates evaporation, it is not as insulating against conductive losses so raises temps by around 5-8ºF).  The solar system adds another 10-15ºF on top of that. So what's the bottom line here?  If you are only going to be swimming when the weather is somewhat warm with mostly clear sunny skies most days with little wind, then a black flat mat should work well for you and will be far less expensive up-front cost.  If you instead intend to swim during the time of year when the days are partly or mostly cloudy or there is cold wind, then glazed panels or evacuated tubes would cost more up-front, but save more energy when in use. In the U.S., the glazed panels cost around 3 times as much as the black flat mats (for the same gross area of panel) while the evacuated tubes cost around 2 times that of the glazed panels so 6 times that of the black flat mats.  With that sort of price differential, the payback time is longer unless one would otherwise be using gas heating extensively during cloudier or cooler days.  Some argue that you need less surface area with the more efficient panels, but then you get slower heating on sunny days which may or may not be important depending on your situation. On a sunny day where the daytime air temperature is warm and close to the pool water temperature, then the black flat mat panels are the most efficient at around 80% or so.  However, with a 10ºF or higher air vs. water temperature difference, the glazed panels start to become more efficient (probably at a higher temp difference since the table does not account for heat exchanger losses since you normally cannot pump pool water directly into the glazed or evacuated tube panels).  At a 20ºF or higher air vs. water temp difference, the evacuated tube panels start to become more efficient. Now it's for cloudy/overcast days where the situation shifts even more.  Technically, if the air and water temps are the same, then the flat black mat is still the most efficient though the total amount of heating is lower for all panels (about 1/3rd) due to less sunlight.  However, if there is even the smallest temperature difference between air and water, the glazed and evacuated tube panels start to shine.  With a 27ºF temperature difference between air and water, the flat black mat doesn't heat at all, the glazed panel is 51% efficient, and the best evacuated tube is 48% efficient.  With an 85ºF temperature difference between air and water, the glazed panels don't heat at all while the evacuated tubes are at more than 20% efficiency. If you want to use the system for domestic hot water heating, then hands-down the evacuated tube is most efficient (except in hot summer) since the temperature difference is much greater since domestic hot water is in the 120-140ºF range compared to a pool in the 80-90ºF range.

Assuming you are using a hypochlorite source of chlorine (and it sounds like you are via an automated dosing system), you do NOT want your Total Alkalinity (TA) to be very high -- probably no higher than 80 ppm.  Basically what is happening is that you are adding baking soda which makes the TA rise and to some extent the pH as well, but the higher TA causes faster carbon dioxide outgassing that has the pH rise.  This is triggering your pH controller to added acid which lowers both the pH and the TA.  You then add more baking soda and are in a vicious cycle. When using a hypochlorite source of chlorine, you generally want the TA to drop as needed until the pH becomes more stable with less of a rise, though I wouldn't go below 50 ppm in any event.  If your TA does end up on the low side, you can increase the Calcium Hardness (CH) to compensate for the calcite saturation index or you can increase your pH target a little on your controller (but probably not above 7.8 at the most). This chart shows how far out of equilibrium pool water is in terms of being over-carbonated with respect to air at various TA and pH levels.  I'll bet you find that you use a lot less acid at a lower TA level. As for hydrochloric acid vs. sulfuric acid, they are identical in their acidic effects and differ only in what is leftover -- chloride with the former, sulfate with the latter.  I've never heard of hydrochloric acid leaving crystalline deposits in valves.  What is the chemistry behind that?

I should have said that a "large puck" was about 200 grams and about 7-1/2 centimeters in diameter.  Sorry about that.  Also, if you see "little blue bits" in pucks, then that is not CYA.  It's probably something else like copper or some other additive.  Standard Trichlor is a white solid and the chlorine and CYA are not separate -- they are combined in a single chemical (i.e. the chlorine is attached to the CYA).

1) Chlor choc is Trichlor and adds Cyanuric Acid (CYA) aka stabilizer or conditioner to the water.  For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases CYA by 6 ppm.  Chlor choc is quite acidic so needs pH Up or equivalent chemicals and it also lowers Total Alkalinity (TA) over time, though pH Up will raise that (i.e. compensate) as well.  Chlor choc dissolves slowly, usually a large puck lasting around 5 days or so in a floating feeder, so it's main advantage is dosing convenience.  It's disadvantage is primarily the buildup of CYA over time. 2) If you have CYA in the pool and add javel (chlorinating liquid or bleach) to the pool, the CYA already in the pool will help protect the chlorine from breakdown from the UV in sunlight.  So as you have surmised, you don't need to use Chlor choc and can use separate CYA and javel instead.  Since the CYA doesn't go away except by water dilution, once you are at the proper level then javel is the best choice though must be added regularly (usually every day or two in the summer).  To keep the pH more stable, when using javel you want a lower TA, usually no higher than 80 ppm.  TA is a source of pH rise due to carbon dioxide outgassing -- it essentially is a measure of the over-carbonation of the pool. 3) If you have some water dilution, such as from backwashing a sand filter, then you may not get too much CYA rise from using a few gallets each month.  Also, you can always increase your FC target level as the CYA level rises to keep the FC/CYA ratio constant.  If you keep the FC at least as high as 7.5% of the CYA level, then you should be able to prevent algae growth regardless of phosphate level and without the need for any additional algaecides.

There is a slow oxidation of CYA by chlorine so over months the loss of CYA can be noticeable, but this is on the order of 2-3 ppm CYA per month during the warmer pool water in the summer.  Some people report higher losses of around 5 ppm CYA per month and that is also the typical rate of loss in hot spas (say, using the Dichlor-then-bleach method).  As for winter losses, most people who maintain chlorine in the water don't find a loss beyond that explained by water dilution from rain overflow, but some report an additional loss.  For those who let their pool go over the winter (i.e. let the chlorine level get to zero at some point), anaerobic bacteria can grow and consume the CYA and either convert it to ammonia creating a HUGE chlorine demand upon spring opening where every 10 ppm CYA loss can create 30 ppm FC demand or additional bacteria can use the ammonia and convert it to either nitrates or to nitrogen gas.  This is described in more detail in this thread. Unfortunately, there is no easy way to force the bacteria to do what one might want such as oxidizing the CYA to nitrogen gas and carbon dioxide gas.  If there were, then that would be a way to reduce CYA levels.  As you pointed out, life is not as easy as one might hope.

Whomever initially balanced your pool water chemistry might have added too much Cyanuric Acid (CYA) aka stabilizer or conditioner to start with.  That would make any chlorine you subsequently add less effective and allow algae to grow.  If you have a test kit that can check the CYA level, I suggest you do that.  If it's high, then a partial drain/refill will be needed eventually so doing it sooner would let you shock more effectively. By the way, when one refers to "Chlor Shoc", what is that?  I certainly hope it isn't Dichlor which would only exacerbate the problem by increasing the CYA level.

Cryptosporidium parvum is not a bacteria, but is a protozoan oocyst.  So bacterial tests won't check for it.  It is only introduced into swimming pool water from an infected person who has a loose diarrhea accident.  You can read more about this parasite at the Centers for Disease Control (CDC) website.  Quite frankly, in a residential swimming pool, I wouldn't worry about it.  Commercial/public pools, on the other hand, don't always have tight control over who swims in them and they don't always have the supplemental systems needed for killing or removing Crypto (UV, ozone, coagulation/filtration).

The degradation of CYA by bacteria requires living bacteria to occur and that should not be present in a pool with virtually any measurable chlorine in it.  The other pathway is from a slow oxidation of CYA by chlorine itself.  We see this in hot spas at around 5 ppm CYA loss per month at roughly a 2-4 ppm FC with 20-30 ppm CYA level.  In pools, it is usually lower -- in my own pool it seems to be around 2-3 ppm CYA per month (I have minimal water dilution in my pool with an oversized cartridge filter cleaned only once a year and with no summer rains).  There are some people who report somewhat higher losses, but not usually more than 10 ppm CYA per month unless the pools are small and there is significant water dilution from backwashing and rain overflow. Is only your CYA level dropping or are you finding other levels dropping as well such as Calcium Hardness (CH) and salt (TDS)?  Maybe you have a leak in the pool where auto-fill is preventing it from being obvious.

If you let the chlorine test sit, then there can be bleed-through of Combined Chlorine (CC) reading as Free Chlorine (FC) and there can be other reactions as well.  For the FC test you read it soon after mixing, but not more than one minute later.  I don't think the temperature affected the chlorine reading, but elapsed time probably did. As for pH, it actually drops a little as temperature increases, but in your case I suspect that your Total Alkalinity (TA) may be on the high side and having the small sample sit exposed to the air had carbon dioxide outgassing increase the pH over time.

If the FC is truly low and the TC higher, then that implies Combined Chlorine (CC) higher and could very well be bacterial conversion of CYA into ammonia.  You can do a bucket test to see how much chlorine it will take before you get an FC reading.  1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC.  Or for 10% Javel this is 1 ml in 10 liters for 10 ppm FC.

One more annoying fact specifically about TA is that since the units are ppm CaCO3 equivalent, the TA is about carbonate equivalent, not bicarbonate, and carbonate can accept TWO protons (hydrogen ions) so if you are converting from sodium bicarbonate weight to ppm CaCO3, you still need to divide by 2 to calculate the TA change because it would take half as much CaCO3 as bicarbonate (in moles) for the same TA change.  I know this is very confusing, but that's the way it's defined. For CH, no such factor of 2 is needed since there is only one calcium in CaCO3, the same as in calcium chloride (CaCl2).