Please help with pool chemicals

I have a new pool, just over a year old, and having some problems with PH. Here's the Stats
850,000 Gal, 55 meters long, 25 yd across, and from 7 1/2 feet to 13 feet deep. It Pumps 2700 gpm, with 6 sand filters inline, UV filter, Acid Feeder, co2, and a Pulsar4 chlorine tablet feeder.

The problem I am having is that our automated chemical system is set to keep PH at 1 7.3. It does its job, but I keep getting different readings from the water in the pool. I get a 6.8 reading on the shallow end, and a 7.0 on the deep end. I am wondering If I need to change what our chemtroller is set to to make up for the difference.
Other numbers:
Alk: 60
TC: 3.0
Ca: 520

Also, the company that built the pool recommends a 7.3-7.4 PH for our pool, with the diamond-brite bottom, and bulkheads

***** OTHER POSTERS PLEASE NOTE => THIS IS A *LARGE* *****
***** COMMERCIAL COMPETITION TYPE POOL *****
Here's a picture:
http://www.gjsentinel.com/images/pho...ar_600x500.jpg
www. gjsentinel. com/images/photos/ imgkit_sized/pomar_600x500.jpg
www. mesastate. edu/purchasing/documents/AppendixAEquipmentSpecifications.pdf )


Note for Chem_Geek: they're using UV for chloramine reduction, and have combined a cal-hypo tablet feeder with CO2 based pH control! They did install floor inlets, so they have better than average circulation.

Hi MAV;

Thanks for subscribing. Let me caution you, as I did others here (above) your pool is quite different from what all but a few here have worked with. The underlying chemical principles of pool care remain the same, but there are some very large practical differences.

OK. Where to start?

FIRST:
Unless you have an exceptionally well designed circulation system (possible), your water volume is so great, and the distances from flow input so long that the pool will not have uniform mixing rates. This will tend to lead to variations in chemistry.

SECOND:
Your pool will have a greater tendency toward 'wobbly' pH than most large indoor pools. The reason is that your "acid" is a partially soluble gas that will tend to exit the pool water during aeration (ie, during warm-ups at a big swim meet!), causing the pH to rise. (Chem_Geek, will the TA also tend to rise?)

THIRD:
Because you are using CO2 (via carbonic acid) to lower pH, AND calcium hypochlorite to chlorinate . . . you are going to have problems with hitting your contract targets:
pH 7.4 - 7.6
Calcium Hardness 200 - 400 PPM
Total Alkalinity (Calcium Hypochlorite) 60 - 80 PPM
Langelier saturation index -0.3 - +0.3
( http://www.mesastate.edu/purchasing/...ifications.pdf )
Frankly, I'm not sure that it's possible, without changing to a different source of chlorine or a different source of acid. Trying to operate a pool by feeding carbonic acid (HCO3) and calcium hypochlorite which reverts to calcium chloride (Ca(OCl)2 => CaCl2) will be an interesting challenge.

THIRD:
If you have a Chemtrol PC3000 or equivalent per the specs . . . you do NOT have a system that can "measure" chlorine levels; you have a system that infers (guesses at) the chlorine level based on the ORP. This is a half-way reasonable thing to do, with an indoor pool (it's altogether stupid on an outdoor pool!), but you still have to remember that your system measures ORP, not chlorine. And ORP does NOT equal chlorine, though higher chlorine levels do tend to cause ORP to increase.

FOURTH:
pH / ORP controllers like the PC3000 have to be calibrated when they are new, and frequently, thereafter. When I was selling a competitor product, I ended up locking the boxes, and putting tape over the windows, because the operators were reading the gauges rather than doing actual water tests . . . and a result, failing to notice when the unit had drifted out of calibration.

FIFTH:
If you are testing pH with a meter, you MUST calibrate your unit with fresh buffer solution REGULARLY. Meters *can* be more accurate than 'eyeball' colormetric tests, but unless you calibrate them regularly, they tend to be much LESS accurate. On the other hand, with an 'eyeball' test kit using phenol red drops, you can't reliably distinguish 6.8 from 7.0.

SIXTH:
Your Facebook indicates you are a student there. If so, I need to be careful how I answer you. Someone is very proud of that pool and its design (and there are some very good features), but such a person doesn't really want to here things like, "You never should have spec'd and installed an XYZ system". To put it another way, there's advice I could offer that would be appropriate for someone with total authority over the facility . . . but giving that advice to a student employee would only get him in trouble if he tried to follow it. :(

So here's what I suggest:
1. Make sure your pH tests are valid -- either use a wet test OR make sure your meter is (and stays) properly calibrated.
2. Find out if your pool finish contractor (Diamond Brite) is still on the hook for the finish warranty. If he is, make SURE you stay within (and log!) his requirements. Otherwise, if you stray outside much, and if there is ever a problem with the finish, he'll likely get off the hook by blaming it on the 'pool maintenance' errors.
3. Find out who's responsible for maintaining calibration the Chemtrol unit.
4. Do NOT allow your pool to rise in pH quickly -- it could turn milky instantly.
5. Let us know what you find out, and how I can help you.

Ben
PoolDoc

Thank you for your help. A few other notes, we are using a becsys5, for chemical control, and the it does only give an orp. For chlorine and PH tests we are using a La Motte kit, and using taylor for the alk, and CA, The numbers I gave you were from the Palintest kit, and it was calibrated.
Our orp is roughly 780, +/- 12, as programmed. I was at 840 for a while as we had an algae problem before we got our alkalinity down to within reason.

As far as the Diamond Brite warranty, It is being refinished this summer, when we can drain the pool, as the school only has had control of the pool since last May (it was run by the company who built the pool before then) And then when I got control in September, it took until November to get our Alk balanced, as we got a acid feeder installed.
I am a student, but I am also a CPO, and have been the pool manager since September.
The other problem we have is trying to keep our Ca low, but the water we add is 400ppm, but below 500 is what the pool contractor wants, so I have been dumping and repalcing about 100,000 Gal, every week.

As Ben noted, ORP is not a good absolute measure. It's best used for process control to maintain an FC level, but that means creating a "setpoint" and then retesting periodically where any long-term change may require a resetting of a new "setpoint". That is, you get the FC level to where you want it and then lock in that ORP reading as your target.

Quote:

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Originally Posted by PoolDoc

Your pool will have a greater tendency toward 'wobbly' pH than most large indoor pools. The reason is that your "acid" is a partially soluble gas that will tend to exit the pool water during aeration (ie, during warm-ups at a big swim meet!), causing the pH to rise. (Chem_Geek, will the TA also tend to rise?)

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Injection of carbon dioxide will lower the pH with no change in TA. It is the exact opposite of outgassing of carbon dioxide. If not all of the gas dissolves and some outgasses, that's fine and won't have any affect on the water chemistry (i.e. the gas just passes through the water with no effect -- only that which dissolves into the water has an effect). I don't think the pH will be any less stable due to using carbon dioxide, per se, especially since the TA level is fairly low to begin with. If the TA were higher, then one could be fighting rising pH from outgassing while trying to lower it by injecting. Given the large pool size and implied poor circulation, that is probably the source of inconsistency since the feedback loop (measurement of pH to make further adjustments) is very time-delayed (hysteresis).

When using carbon dioxide for pH control one normally does see a rise in TA over time, but this is due to adding hypochlorite sources of chlorine that have some excess lye in them. Cal-Hypo has less than most, but it still contains some calcium hydroxide and calcium carbonate. For typical Cal-Hypo, for every 10 ppm FC it will increase TA by 0.5 ppm (and CH by 7.1 ppm) so the rise in TA is usually slow.

Quote:

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Originally Posted by mavswimmer

The other problem we have is trying to keep our Ca low, but the water we add is 400ppm, but below 500 is what the pool contractor wants, so I have been dumping and repalcing about 100,000 Gal, every week.

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That's a real challenge, but one can run a pool with a higher CH level if one keeps other water parameters such as TA and pH somewhat lower. In fact, a near-zero saturation index would be at a pH of 7.4 with TA of 60 and CH of around 600. Does the pool contractor understand that a higher CH will be OK so long as the pH and/or TA are lower such that the saturation index is still near-zero? Also, if you are using a lot of carbon dioxide to try and keep the pH lower, then your pH target may be too low as well.

Thanks, ChemGeek.

PoolDoc

PS. Mav., since I know who you are, I went ahead and bumped you to "Subscriber" so you can post w/o moderation.