Water Balance Suggestions

[freeradical]

Hello,

I have had my spa about 5 weeks. Just received my Taylor K-2006 test kit. My spa is 500 gallons with a Nature 2 silver cartridge and an ozonator. I have used a combination of MPS/Dichlor for oxidation/sanitation. We have been using the tub pretty regularly, almost daily. I add 1 oz (2 Tbls) MPS about 5-10 min prior to entering the spa. After exiting, I add 3 to 3-1/2 tsp dichlor per person-hour. My readings these evening (approximately 21 hours later) follow:

T ~99 degrees F

pH 7.0

Total Alk 30 ppm

CH 190

Free Chlorine 17.5? ppm

Combined Chlorine 1.5 ppm

CYA 75 ppm

MPS in "OK" range by test strip

Water clarity- colorless

Per total alkalinity increaser label, added 6 oz to increase TA by approx. 50 ppm. Circulated water x 30 min. New result TA = 100 and pH 8.0. Acid demand is approx. 1.23 oz sodium bisulfate 93.2% (Table F in Taylor test manual--required 2 drops).

Where do I go from here? Could the elevated CC and FC be due to the MPS? I don't know why the FC would be so high after almost 24 hours. Also, I am not sure why the TA keeps dropping; I added additional alkalinity 1 week prior (pre-treatment TA was 41 by pool store measurement, added 4 oz alkalinity increaser). Thank you for any help.

[chem geek]

Both MPS and Dichlor are acidic (Dichlor is net acidic when accounting for chlorine usage/consumption) so that will have the TA drop over time. 3-1/2 teaspoons of Dichlor in 500 gallons would drop the TA by around 1.7 ppm so over a week that's nearly 12 ppm. The 1 ounce volume of MPS may drop the TA by 3.8 ppm so nearly 27 ppm in a week, though this depends on whether the MPS is somewhat pH balanced (this varies by brand of MPS) and I've assumed 43% MPS (some MPS is less concentrated).

MPS will also register on the chlorine test, usually as CC, but at higher levels sometimes bleeding through as FC as well.

Your pH is too low, though the high FC levels will tend to make the pH test invalid, though usually has it read too high so your low reading is probably valid.

You don't need to add as much oxidizer (either MPS or Dichlor) when you have an ozonator and use the tub daily. The ozonator will handle some of the bather waste so less oxidizer is needed. This is probably why your FC level is so high. Why haven't you been testing daily? You need to do that until you get a better understanding of your water and the chlorine demand.

Your CYA will continue to climb if you continue to use Dichlor. For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm. You might consider using the Dichlor-then-bleach method.

[freeradical]

Thank you for your help, ChemGeek.

After I filled spa and initially added chemicals to balance, an MPS-only method was recommended by the dealer (and was told to ignore manufacturer recommendations which they said were "overkill." The dealer recommended to either add 6-8 Tbls MPS once a week or 2 oz MPS after each use. Tried the once-a-week method first, and my MPS was always low using the MPS test strips. Then tried the MPS addition only after each use. Water then got cloudy about 1 week ago. "Shocked" with MPS, but this did not help. Dealer no help as the gentleman testing the water did not know why the water was cloudy. So purchased dichlor after reading posts on this site. I shocked the spa with dichlor, and water cleared about a day later. At this point I ordered the Taylor test kit, which did not arrive until a couple of days ago. Anyway, will be testing daily to get a handle on the FC and other levels. I may consider just using MPS until I make a water change in a couple of months. Do you think this would be OK, or, do you think a water change may be in order sooner? I know you have said that the MPS level for sanitation is a bit harder to calculate. I could follow the Nature 2 instructions to test for MPS before entering spa, and adding 2 Tbls until level ok by test strips and then adding 2 Tbls after exiting spa.

Thank you again.

[freeradical]

Updated values, approximately 18 hours later (11 am today). No additional chemicals added:

pH 7.6, TA 100, FC 4.0, CC 1.5, MPS low to borderline OK by test strip

BTW MPS is 44.7% by weight with an equivalence to 4.7% active oxygen.

I wonder if the previous FC measurement was an error. It was my first time using the test kit. But more likely, it was correct.

Is the elevated CC value of 1.5 within expected error range due to the confounding presence of MPS? Maybe I should get the Taylor MPS neutralizing reagent.

[chem geek]

The CC may very well be from the MPS. If you are going to continue to use MPS regularly (i.e. the N2 + MPS method), then you either need to ignore your CC and focus on the MPS test strips or you need to get the interference remover. With the N2+MPS method described in the Nature2 manual, it says to shock with Dichlor "as needed", so clearly getting cloudy water means it's needed and as you found out it clears up the water rather quickly.

You should decide which way you want to go with sanitation. N2+MPS with weekly or as needed Dichlor vs. (N2 +) Dichlor then bleach. You can switch to the latter at any time though you'd just go directly to using bleach since your CYA is high and you would likely need to lower your TA through acid addition as your pH would likely rise (and may need to use 50 ppm Borates as well). Remember that a higher TA will tend to have the pH rise unless you are adding acid (which includes Dichlor and MPS which are acidic). So it's up to you what to do -- perhaps it would be easiest to stick with the N2+MPS+weekly Dichlor for now and consider switching to Dichlor-then-bleach after a fresh refill.

[freeradical]

I think I will stick N2+MPS+weekly Dichlor on an as needed basis. Will likely switch to dichlor-then-bleach at next refill. Thank you again.