Raising Ta

[AFiremanFirst]

Waterbear! Great thread on lowering TA, any advice on raising TA. I thought I'd start a new thread rather than treading on your old thread in a different direction. There are a few guys with in my (fire) department that are techs. A few of us are out on our own, and a few work for other companies. But the common problem seems to be low TA's with good solid stable pH. Pentair recently blamed a low TA reading for destroying a heat exchanger in a six month old pool. Yes ultimately Pentair did the right thing and replaced the heater under warranty. But I have read low TA is as corrosive as low pH, however it seems impossible to avoid a similar marry go roud. Sodium bicarb one week and MA the next. Any advice/knowledge will be greatly appreciated.

[waterbear]

Waterbear! Great thread on lowering TA, any advice on raising TA. I thought I'd start a new thread rather than treading on your old thread in a different direction. There are a few guys with in my (fire) department that are techs. A few of us are out on our own, and a few work for other companies. But the common problem seems to be low TA's with good solid stable pH. Pentair recently blamed a low TA reading for destroying a heat exchanger in a six month old pool. Yes ultimately Pentair did the right thing and replaced the heater under warranty. But I have read low TA is as corrosive as low pH, however it seems impossible to avoid a similar marry go roud. Sodium bicarb one week and MA the next. Any advice/knowledge will be greatly appreciated.

First some background (it gets a bit technicals so be warned). If you want to skip it and get right to the howto just scroll down to the blue text at the end.

Low TA is not corrosive. Low pH is. Period. Saturation index (as used in the pool industry,which is a modified Langelier index) does NOT predict corrosion of metal. It predicts scaling tendencies.

Bicarbonate (which is what we measure when we test TA) is the primary alkaline ion in poolwater, followed by cyanurate and borate if used, and the first two are the only ones we look at when computing saturation index in this industry. At normal pool pH there is practially no carbonate ion present. Just bicarbonate (and carbonic acid). We also look at calcium hardness, temperature and pH. However, to predict corrosion of metals you have to take into account sulphate, chloride, dissolved oxygen, exposure time, and other factors. There are other indices such as the Singley index that are more useful in predicting corrosion that do consider these other factors such as chlorides, sulphates, alkalinity, oxygen dissolved, buffer capacity, calcium, water velocity, and, exposure time. There is also the Larson index that includes chlorides and sulphates. However, the use of these is really beyond the scope and testing abilities of pool techs and home owners and, IMHO are not necessary, just monitoring pH is, as I will explain in a bit.

Chloride and sulphate concentration seems to play an important part in metal corrosion which is a good case against the use of dry acid and MPS since both add sulfates to the water, particularly with a salt system where the cloride levels are much higher than in a manually chlorinated pool. (This could be the unlooked at factor as to why some have such severe corrosion problems with salt pools and others do not.)

I said I would explain why pH is our limiting factor in pH corrosion. First, let us look at the LSI as we commonly apply it:

SI = pH + logCH + logALK + TC - CF

pH is self explanatory and is the only direct measurement in the formula so small changes in pH will have the biggest impact on the final saturation index. This is an important point because pH is a factor that we can easily control.

logCH and logALK are the base 10 logarithms of the measured CH and ALK. For those of you without the mathematics background to understand the significance of this it means that is takes VERY LARGE CHANGES IN EITHER CH or ALK to make changes in the saturation index. While we can control ALK in both upward and downward directions often we cannot readily change CH downward.

CF is a constant that is an approximate correction for the ionic strength (TDS). The actual formula to compute ionic strength is based on the logTDS so once again it takes very large changes in TDS to have a very small effect on the correction factor, which is why a constant is substituted since it is more than close enough in the real life conditions we see in pools and spas.

TC is the temperature correction factor and works out to a change of .1 for about every 10 degrees of temperature change for the range of 66 to 105 degrees (which is the temperature range we work with in pools and spas). This is not completely linear but it is very close.

This makes temperature the second most significant factor in calculating the index but once again this is often a factor we cannot readily control.

We then see that pH is the MOST IMPORTANT FACTOR IN PREDICTING SCALING AND ONE WHICH WE CAN CONTROL with temperature being the second AND ONE WE CANNOT READILY CONTROL IN MOST CASES.

It is also known that low pH is predictive of corrosion in copper and steel, particularly in an oxidative environment (and our pools and spas are oxidation, are they not?). i won't go into the chemistry here because it is a bit beyond the scope of this forum.

We can look at the LSI another way.

LSI = pH - pHs where pH is our measured pH and pHs is the pH at saturation of calcium carbonate (at our present temperature, CH, ALK, and TDS)

IF the pHs is higher than our present pH (negative SI) then our water is aggressive (can dissolve calcium)

IF the pHs is lower than our present pH (positive SI) then our water is scaling (can precipitate calcium as calcium carbonate)

IF they are the same the water is balanced (neither scaling or aggressive.

AS far as low TA destroying a heat exchanger, it can do so indirectly by allowing pH to drop too low because of not enough buffering action when an acid source is added to the pool (from addition of acid, trichlor, dichlor, MPS, or bromine since all are acidic) or even from the reaction that hypocholorous acid undergoes when it santizes (which is also acidic).

However (AND THIS IS THE IMPORTANT POINT) the damage to the heat exchanger was not due to low TA but rather low pH that happened as a result.

Now, on to how to raise TA.

It's pretty simple, just add baking soda!

Yes, it will cause the pH to rise, sometimes quite high, initially, BUT if you let the pool stabilize for about 24-48 hours after addition and then test the pH and adjust it downward you will now have your higher target TA and the pH will be on target too.

Here is an example. You have a 10k gallon pool running on trichlor so you want the TA at 120 ppm. Currently it is at 70 ppm and the pH is 7.4. This TA is too low for trichlor (which should be at least 100 ppm)

We adjust the TA upward with bicarb to bring it up to 120 ppm, which is a good place for the TA but now the pH has climbed to 8.2. We need to adjust this downward with acid. It will take about 1 pint of 20 baum muriatic acid to lower the pH down to 7.6 (given a TA of 120 ppm) but will this also lower the TA? Yes, by a whopping 6 ppm!. Your TA is still very close to desired range. In fact, if you are testing with a Taylor drop test the change in TA is LESS THAN THE PRECISION OF THE TEST so your TA is essentially unchanged as far as your testing method can show.

To summarize, when TA is too low and pH is on target or high adjust TA to desired target and then adjust pH downward afterwards.

When TA is too low AND pH is too low you need to bring pH up first using either soda ash (which can cause TA to go too high if you overdose) or borax (which you dose at twice the weight of soda ash for the same pH rise), once pH is in target range retest TA and if still low raise with baking soda and then adjust pH downward afterwards.

[chem geek]

But the common problem seems to be low TA's with good solid stable pH.

What was the "good stable pH" when the heat exchanger failed? Was it a saltwater chlorine generator (SWCG) pool and if so, what was the salt level?

Also, how did low TA's become a common problem? Usually the TA gets lower because of acid addition, including the continued use of Trichlor tabs/pucks which are very acidic. These usually drive the pH lower and if used in the skimmer can corrode heat exchangers. Is that what happened? How was the pool with the failed heat exchanger chlorinated -- was it using bleach or chlorinating liquid or was it using Trichlor pucks/tabs (if so, where were they added) or was it an SWCG pool?

[AFiremanFirst]

I appreciate the post... First... The pool was/is a swcg pool! 3000-3200 salt level.

waterbear. I most appreciate the detailed explanation, and it appears well on paper, but I haven't had the same experience in reality. History... Tech times 4 yrs. 28 contract pools and one of my own, self employed. The merry go round seems to be 10lbs of bicarb one week then 1 gallon of MA the next. Broad casting the bicarb and walking the acid (17k pool-mine). I nearly went broke buying bicarb and MA. I would use nothing less than the taylor k-2006, competitors in my area even poke fun as they did their test strips, whatever! I have found that I can easily maintain 7.4-7.6 with a TA of 40. How is that? I understand aeriating and it's affect on pH, I have no fountain, and no water falls. My hot tub is at deck level and only sits 1-2" above my pool so I don't believe this to be a factor. Is it possible/likely, my approach is incorrect? I understand I should mix my MA 5 to 1 in water before walking, now do that 29 times a week, okay, maybe 10, but you get my point. My goals are 2ppm FC (swcg), 7.5 pH, 80 TA, 300 CH, 50 CYA, 3100 salt, 50 borates.

Thanks again.

[chem geek]

Ah now I understand what you are talking about. It's SWCG pools where the pH tends to rise so you add acid and that lowers the TA as well but at some point when the TA gets to 40 ppm the pH stabilizes. So yes, that is explained. The SWCG aerates the water when it runs as it produces hydrogen gas bubbles (you can easily see them at night with a pool light). Now technically, that shouldn't be enough aeration to explain all the pH rise but the fact that your pH becomes stable at a TA of 40 ppm would indicate that total carbon dioxide outgassing is indeed the main source of pH rise in your example. Now in many SWCG pools, even getting the TA very low doesn't completely stop the pH rise and that may be from chlorine gas not complete dissolving so that some of it outgasses since that would make the pH rise as well (the TA also rises in this particular case).

So the good news is that you can achieve pH stability albeit with low TA levels. As for corrosion of the copper heat exchanger, the higher salt levels can accelerate that, but there is probably another issue making that worse such as stray current/voltage on the bonding wire assuming that heat exchanger is connected to the bonding wire. You might try connecting a zinc sacrificial anode (buried in moist soil) to the bonding wire to reduce corrosion elsewhere. Also, make sure the flow rate through the gas heater isn't too high (use a bypass if it is) since erosion corrosion is another possible source for the problem. I assume that the SWCG comes after the gas heaster; if not, then that's the most likely cause for corrosion since part of the stream of output from the SWCG is very low in pH and high in chlorine and very corrosive (by the time it gets mixed with other parts of the stream and hits the bulk pool water, the pH is just a little higher than it started and the FC is a somewhat higher than it started). I also assume that acid addition was not done by pouring into the skimmer since that is also very corrosive to copper heat exchangers (same is true with Trichlor pucks in the skimmer with pump cycling).

For plaster pools, you can have a higher CH level to compensate for the low TA level to keep the saturation index near zero.

[waterbear]

I appreciate the post... First... The pool was/is a swcg pool! 3000-3200 salt level.

I assume either a Goldline or Autopilot unit?

Broad casting the bicarb and walking the acid (17k pool-mine).

Why walk the acid. It does not matter HOW you add the acid. Acid will lower pH and TA. Period. There is a myth in the industry that refuses to die that walking acid lowers pH and slugging acid lowers TA. It is just not true. How you put the acid in makes no difference, How much you put in does. It takes a LOT more acid to lower TA than to lower pH. Period!

7.4-7.6 with a TA of 40.

Too low. Maintain pH at 7.6-7.8 with a TA of 70.

Is it possible/likely, my approach is incorrect?

Yes.

I understand I should mix my MA 5 to 1 in water before walking

Why? Just pour it slowly into the stream of a return.

My goals are 2ppm FC (swcg)

Too low. With the parameter I will suggest for best pH stability you want the FC between 4-5 ppm.

7.5 pH

Too low. The lower you put the TA the faster it rises because it outgasses CO2 faster because the lower the pH the more bicarbonate converted into carbon dioxide in the water.

After adjusting TA to 70 ppm adjust pH to 7.6. Use the acid demand test and measure the acid. Monitor pH. When it rises above 7.8 lower it. With the borates and other adjustments I am suggesting you will find it stays right around 7.7 for a long time.

80 TA,

Drop this to 70

300 CH

Too low if you have a plaster pool. The water is not going to be balanced with the other adjustments I am suggesting and with the borates. Bring this up to between 350 and 400 ppm. IF you have a vinyl pool CH is not important. If you have a fiberglass pool there is some empirical evidence that higher CH prevents or slows staining and cobalt spotting so 300-400 ppm.

50 CYA

Too low, bring it up to 80 ppm or the manufacturer recommend MAXIMUM level, which ever is higher. Higher CYA translates into less cell on time, which translates into less aeration of the water, which directly translates into less pH rise and acid demand.

The first thing you need to do is adjust the CYA, then adjust TA to 70 and pH to 7.6, then adjust FC to 4-5 ppm (because of the higher CYA level) by adjusting cell output and/or pump run time. Once FC level is stable and on target monitor pH. When it rises above 7.8 drop it to 7.6 and no lower by using your acid demand test and MEASURING THE ACID. Just pour the acid slowly into a return stream. No need to walk it unless you really need the exercise.

3100 salt, 50 borates.

Good here.

Thanks again.

So to summarize:

1, adjust CYA to 80 ppm (Very Important for pH stability)

2. adjust TA to 70 ppm (Very Important for pH stability)

3. Bring up CH to keep water balanced if needed (important for plaster, probably important for fiberglass)

4. Adjust cell output and pump run time to maintain FC at 4-5 ppm (lower will not work with the CYA at 80 ppm)

5. Monitor pH and do not lower it below 7.6. wait for it to climb above 7.8 before adding acid and dropping it again. Your pool should stay between 7.6 and 7.8, particularly if you have also added the borate buffer system, which is highly recommeded.

[waterbear]

The merry go round seems to be 10lbs of bicarb one week then 1 gallon of MA the next. Broad casting the bicarb and walking the acid (17k pool-mine).

Well here is the problem. I just did the math. In 17k gallons 10 lb of bicarb will raise the TA by a bit over 40 ppm! (and the pH by about .1 but it will rise more as CO2 outgasses, depending on the initial level). One gallon of 20 baum muriatic acid will drop the Ta by about 30 ppm so you only have a net TA rise of 10 ppm when you are done. This is an expensive way to get a 10 ppm rise in TA. This much acid will also drop the pH by almost 2 (not .2) so you will need to raise the pH again or you will ruin your heat exchanger. (I assume this is why you kept adding bicarb again?) This is just not good pool maintenance!

Like I said initially, adjust your TA UP with bicarb to target level. If you need to compute how much to add may I direct you to the Onbalance adjustment tables. They are the best ones I have come acroess. There is also an extended table of formula numbers to use in the adjustment calculation from the first table. and a method to chemically determine the volume of a pool if it cannot be caclulated easily.

Why were yo adding so much acid at one time? How much were you trying to raise the TA each time you added the bicarb. I just don't understand this at all. I really makes no sense.

[AFiremanFirst]

Great explanation! Rookie mistake! It makes perfect sense that the SWCG aeriates, I hadn't accounted for that! And it makes sense that by raising my CYA, it will not run as often, which means less aeriation, less pH rise! I realize I had a net of 10ppm on my TA But with the SWCG aeriating and raising my pH, a gallon a week was not uncommon! I look forward to making the adjustments! You're knowledge is priceless.

[HouTex1962]

So to summarize:

1, adjust CYA to 80 ppm (Very Important for pH stability)

2. adjust TA to 70 ppm (Very Important for pH stability)

3. Bring up CH to keep water balanced if needed (important for plaster, probably important for fiberglass)

4. Adjust cell output and pump run time to maintain FC at 4-5 ppm (lower will not work with the CYA at 80 ppm)

5. Monitor pH and do not lower it below 7.6. wait for it to climb above 7.8 before adding acid and dropping it again. Your pool should stay between 7.6 and 7.8, particularly if you have also added the borate buffer system, which is highly recommeded.

I have very high CH levels (my test from my TF-100: 800, plaster contractor: 570, Leslie's: 400) with the following other results:

FC 4.5

CC 0.0

pH 7.2 (only after adding 64 oz of MA)

TA 90

CYA 70

Borates 80

Salt 3700

My plaster has developed some mottling that my pool contractor thinks should be fixed with a no-drain acid wash, then a full drain, a power wash, and then a refill. He thinks my water has been too "aggressive." I'm still adding about a gallon of MA per week due to the pool being new, spill over spa, SWCG, and a 2.0 hp pump. So my pH goes from say 8.2 down to 7.2 after I add acid. Lately, I've gone to adding less MA but more frequently to not let the pH rise above 7.8. So it's going from say 7.2 after I add about a quart of MA up to say 7.8 after 2-3 days.

After the acid wash and refill, assuming I get the CA to around 300 or so, what are my target numbers? I've used the pool caclulator and it seems to say I should target 7.8. But this is contrary to what the literature from Wet Edge says I should target (7.2-7.4). What do you suggest?