Lowering chlorine after shocking

Discussions that do not fit into any of the
swimming pool categories and general topics.
bobobi-wan
Pool Enthusiast
Pool Enthusiast
Posts: 25
Joined: Tue 14 Apr, 2009 11:37
My Pool: university pool(150,000 gal)and 2 spas.pentair pumps and filters
Location: phx az

Lowering chlorine after shocking

Postby bobobi-wan » Fri 29 May, 2009 11:38

I have 150,000 pool and would like to know the procedure for lowering chlorine after shocking.I have the Poolcomm monitors and heard that using thiosulfate could possibly harm the probes.I usually broadcast the chlor-out over a wide area.any suggestions?I usually shock because of algae altho I mainly brush the pool and let the chlorine in the pool kill the algae without shocking.


chem geek
Pool Industry Leader
Pool Industry Leader
Posts: 2382
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Lowering chlorine after shocking

Postby chem geek » Fri 29 May, 2009 14:04

Does the chlorine not drop quickly enough during the day with exposure to sunlight? Or is this a pool not exposed to the sun (e.g. indoor pool)?

Thiosulfate is the most common chlorine reducer. I don't know about which kind of chlorine reducer would work without harming your probes (haven't heard of that before).

Technically, if you've got Cyanuric Acid (CYA) in the pool, then the active chlorine level even when shocking is very low. If you shock so that the Free Chlorine (FC) is 40% of the CYA level, then that's technically equivalent in active chlorine (hypochlorous acid) concentration to a pool with 0.6 ppm FC and no CYA -- less than most indoor pools. Of course, the EPA says not to have more than 4 ppm FC, but that's based on a drinking water standard where one drinks many quarts of water every day. Personally, I'd much rather be in a pool with 12 ppm FC and 30 ppm CYA than in a pool with 1-2 ppm FC and no CYA. My wife experiences this difference every year when she uses an indoor pool with around 1-2 ppm FC and no CYA and her swimsuits degrade (elasticity gets shot) after just one winter season of use and her skin is flakier and hair frizzier. In our own outdoor pool with 3-4 ppm FC and 30 ppm CYA (technically equivalent to around 0.1 ppm FC with no CYA) the swimsuits have lasted for years and there is far less effect on skin and hair. The difference is most likely due to the factor of 10-20 time difference in active chlorine (hypochlorous acid) concentration.

Is there some reason you can't maintain a higher Free Chlorine (FC) level relative to the Cyanuric Acid (CYA) level in order to continually oxidize bather waste and avoid having to shock at all unless there is some sort of "incident" (or the CC level climbs too high)? In a residential manually-dosed pool, an FC that is at least 7.5% of the CYA level will prevent algae growth (even at high phosphate levels). In a commercial/public pool, an FC that is 20% of the CYA level would oxidize bather waste more quickly yet still be a fairly low active chlorine level, equivalent to around 0.2 ppm FC with no CYA.
bobobi-wan
Pool Enthusiast
Pool Enthusiast
Posts: 25
Joined: Tue 14 Apr, 2009 11:37
My Pool: university pool(150,000 gal)and 2 spas.pentair pumps and filters
Location: phx az

Lowering chlorine after shocking

Postby bobobi-wan » Fri 29 May, 2009 15:28

county regs are that spas to be 3-5 and pool 1-3 altho all last summer and fall pool stayed at 4-7 and spas 4-8.it was a continual fight to keep the fc in range.it's an outdoor facility.I like to keep cya 25-40.my problem was the orp vs fc.poolcomm insists that orp be 650 but that kept the fc high.ever since the spa heaters went down(chemical corrosion?)the fc has stayed in range but I had to lower orp to 615.also I discovered my Lamotte test for ch was defective and giving me a 300 read but was actually 600+.I found out when I received a new ch reagent bottle and got my higher tests.I had tried to verify with my Taylor kit but would stop after 40 drops thinking I was reading the color change wrong,actually I stopped too soon.live and learn
chem geek
Pool Industry Leader
Pool Industry Leader
Posts: 2382
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Lowering chlorine after shocking

Postby chem geek » Fri 29 May, 2009 15:55

The county regs, and indeed virtually all pool/spa regs across the country, have no concept or understanding of the chlorine/CYA relationship even though it's been known definitively since at least 1974 as described in the paper in this link . The active chlorine level with 1-3 ppm FC and no CYA such as in an indoor pool is WAY too high while the active chlorine level with 1-3 ppm FC and 25-40 ppm CYA is marginal with the low end not able to prevent algae growth (without supplements).

The ORP is roughly measuring (logarithmically) the active chlorine (hypochlorous acid) level, though it's affected by pH and temperature in ways that don't perfectly correlate with the active chlorine level. Also, different ORP sensors will get different readings on the same water and even if you "calibrate" them with a setpoint at a certain FC and CYA level, doubling the FC will result in different mV increases with different sensors. The whole idea of ORP being an "absolute" standard is a stretch -- a rough guide, OK, but not much more than that.

Your ORP numbers sound somewhat close to an Oakton sensor I modeled (and not close to Chemtrol or Sensorex). The following are FC, CYA and predicted ORP for that sensor (all at pH 7.5 and 84F temp):

FC . CYA .... ORP
1 .... 40 ..... 580
1 .... 25 ..... 600
2 .... 35 ..... 616
3 .... 40 ..... 628
3 .... 25 ..... 650

To prevent algae growth if your pool is being automatically dosed and assuming good circulation, the FC would need to be at least 5% of the CYA level and preferably 7.5% or more. The 1 ppm FC with 40 ppm CYA wouldn't cut it and the 2 ppm FC with 35 ppm CYA is marginal. You could live within the county regs if you had 3 ppm FC with 30 ppm CYA, though you still wouldn't be at 650 mV. As I said in my previous post, for a commercial/public pool, it's probably better to have an FC that is around 20% of the CYA level so at 30 ppm CYA that's 6 ppm (and will get you around 677 mV ORP), but I don't know how you will reconcile that your county regs. This is technically equivalent in disinfection and oxidation power to having around 0.2 ppm FC with no CYA.
bobobi-wan
Pool Enthusiast
Pool Enthusiast
Posts: 25
Joined: Tue 14 Apr, 2009 11:37
My Pool: university pool(150,000 gal)and 2 spas.pentair pumps and filters
Location: phx az

Lowering chlorine after shocking

Postby bobobi-wan » Mon 01 Jun, 2009 12:49

if you run for president,I will buy votes for you(altho I'm not from Chicago).this is the best guideline I've come across in all my searches for solutions to keep my pool chems in line.I am passing this info along to others.thanks for all your help.
bobobi-wan
Pool Enthusiast
Pool Enthusiast
Posts: 25
Joined: Tue 14 Apr, 2009 11:37
My Pool: university pool(150,000 gal)and 2 spas.pentair pumps and filters
Location: phx az

Lowering chlorine after shocking

Postby bobobi-wan » Mon 01 Jun, 2009 13:12

p.s. does temperature affect your chart?before I lost my 2 spa heaters,I kept the 3300gal at 103,the 6300gal at 98.this summer i expect the main pool to reach 90-95.(I use the areator method to cool it down.)I hope to soon have my spa heaters replaced.
chem geek
Pool Industry Leader
Pool Industry Leader
Posts: 2382
Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Lowering chlorine after shocking

Postby chem geek » Mon 01 Jun, 2009 17:00

Unfortunately, it's hard to predict what will happen with temperature since there are two competing effects and one of these hasn't been verified with ORP controllers and the other hasn't been verified against hypochlorous acid concentration.

My best guess is that in pools with CYA, a 10F temperature rise will increase the ORP by around 10 mV due to the increased hypochlorous acid concentration since more is in equilibrium with CYA at higher temperatures. Were it not for this effect, then at the same hypochlorous acid concentration a 10F temperature rise would decrease ORP by around 14 mV.

Also don't forget that I said that different sensors behave differently so this is something you just need to check for yourself using a good FAS-DPD chlorine test kit (and a decent CYA test) and then see how things vary with temperature but at the same FC, CYA, pH readings. If you do this experiment, let me know what you find out.

If you are a masochist, you can try using my spreadsheet here but note that I have line 225 "Use Temp. Dependent Cl-CYA" turned off by default. Line 380 has the "Approximate Oakton ORP (mV)" info I used for the table.

Richard

Return to “Poolside - General Discussions”

Who is online at the Pool Help Forum

Users browsing this forum: No registered users and 0 guests