Pool Chemistry Question

[shamus57]

Hello. I have an in-ground, plaster pool, approximately 21000 gallons (121 linear feet, average depth is 5 to 5.5 feet), salt water generator (Jandy). I saw a small algae bloom the other day and shocked with 3 gallons of 6% chlorine bleach, set my SWG to 100%, and let it run. I used a Taylor K-2005 test kit for the following readings:

Free Chlorine: 40ppm? (I diluted the sample with tap water, per the kit instructions, and it was still off the chart)

Total Chlorine: 40ppm? same as above

Combined Chlorine: unknown

Salt level: 3100ppm (per the Jandy sensor)

pH: 7.4 (although it tends to fluctuate pretty dramatically. I'll put about 1/2 gallon per week of muratic acid to keep it there, otherwise it gets 8.0+)

Total Alkalinity: 170

Calcium Hardness: soft endpoint - approx 750? (about 30 drops at 25 ppm a drop)

Cyanuric Acid: 58ppm

I'm trying to get the calcium hardness down (or whatever else it takes to get the water back into good shape). Any suggestions would be appreciated.

[chem geek]

You are being too aggressive with the chlorine. By the way, how do you get a CYA of 58 ppm from your test kit -- are you interpolating between the lines? At around 60 ppm CYA, your shock level should be around 24 ppm, not 40+ so your chlorine is too high. Also, you can get a FAS-DPD chlorine test to be able to measure high levels of chlorine more accurately. You can get the Taylor K-1515 here or here with the latter kit having more volume of reagents so is more comparably priced per test.

[shamus57]

1. Yes, I was interpolating my CYA.

2. I'll turn down my SWG for a day or two to get the chlorine down. Most of the time my chlorine isn't that high. I just wanted to be sure to kill all the algae =)

[PaulR]

The only practical way to lower calcium hardness is to do a partial drain and replace it with lower-hardness water. Or in the southwest (Arizona, SoCal) there are companies that can do a bulk reverse-osmosis treatment of the water to get most of it out. With your high CH and TA, you want to keep the pH no higher than 7.5 to reduce the risk of calcium scaling.

Reducing your TA will help keep the pH more stable, and also make pH control less critical.

--paulr

[shamus57]

duplicate post. sorry.

[shamus57]

I tested my tap water with the following results:

Free Chlorine: 1.5 or 2.0 (its tough for me to distinguish the colors)

Combined Chlorine: 1.5 or 2.0 (I didn't see any color change)

PH: 8.0 (acid demand was 3 drops to 7.4 or 7.6, which seems pretty good)

TA: 100

Cal. Hardness: 175

(Note: as in my pool water, the solution starts out a uniform red, then as I add drops, the red gathers into beads/globules in a clear solution. I called Taylor and they suggested I add 5 drops of the titrant (sp?) (in this case R-0012) into the untreated water, then test as normal. At the end, take the number of drops needed to affect change and add 5. It took 2 drops to affect change at the end; when added to 5 (the amt initially used), that gave a total of 7 * 25ppm = 175ppm)

I'm going to go try that with my pool water. I'll post the results.