How Do I Know I've Finished Shocking?

[Jaywalker]

This is undoubtedly printed somewhere for easy reference, but I don't see it in Pool School, and the search engine won't look for 'shock,' as it's too common a word.

I understand pool shock is a process rather than one rise of chlorine, but how do I know when the process is completed? The only thing I can think of is to check for an absence of Combined Chlorine. (Note - I have a DPD chlorine tester, but not a FAS.)

Thanks.

 

[Bama Rambler]

Here are the criteria for knowing you're done shocking.

• An overnight chlorine drop test loses less than 1ppm.[/*:m:1bapp4se]
• And you have 0.5ppm CC or less [/*:m:1bapp4se]
• And your water is clear[/*:m:1bapp4se]

You really need a FAS-DPD chlorine test. It gives you much more flexibility.

 

[gordiec]

You can find it in the Pool School -- scroll down to the "How To" section and click on the "Shock Your Pool" link and it is listed under step 3.

 

[Jaywalker]

Thank you. I read that and missed the completion criteria.

 

[Jaywalker]

I'm still having trouble understanding this:

Shocking:

1. Measure the FC level
2. Add enough chlorine to bring FC up to shock level
3. Repeat steps 1 and 2 as frequently as practical, as often as once per hour, and not less than twice a day, until:
1. CC is 0.5 or lower;
2. An overnight FC loss test shows a loss of 1.0 ppm or less;
3. And, if you don't have a FAS-DPD test kit, the water is clear.

Step 1 - fine
Step 2 - fine, using the table that specifies shock level from CYA
Step 3 - here's where I have the problem. If I add chlorine every hour, the FC is greater than 5 (DPD kit). I can't know CC until I can calculate it from the FC and TC readings. I therefore can't know the CC level until FC falls below 5.

So, back to my first question, how do I know when to stop adding chlorine? Shall I shock, then allow the chlorine to drop down to 4, check it again and if inadequate, run the chlorine level back up again?

Thanks.

 

[bk406]

This is why you really need a FAS based test. You need to be able to read FC much higher than 5 when you are going throught the shock process. The idea is to add enough chlorine to reach shock level (based on your CYA) and hold it there. When the FC starts to drop, that means its working on the algea. But, if the FC keeps dropping and no more is added, the algea can start growing again and take hold. Again, the key is to maintain shock level and hold it there until you kill all the algea. I know I'm finished when I do an overnight chlorine loss test and lose no more than 0.5-1 ppm. If you dont have a FAS test, you can dilute the water with distilled and back calculate the FC. However, this isnt very accrurate. I would really encourage you to get a Fas based test. It's really the only way to accurately test the FC and shock the pool properly.

 

[JasonLion]

Just to elaborate on one aspect of what bk406 was saying, if you don't have a FAS-DPD chlorine test (which can measure FC and CC accurately even at very high levels), you will need to overdo the time spent at shock level a bit to be sure you have gotten it all.

 

[Jaywalker]

Thank you. I appreciate the context so that I can understand what's going on at the higher chlorine levels; I'll order a FAS tester for after I'm finished with this shock. May I suggest, however, that the current wording of "Shock Your Pool" is misleading in that it suggests that (1) the DPD will work, but (2) that the suggested methodology isn't practicable, i.e., "every hour" and "check CC" are mutually exclusive.

Thanks. I'm not adding any more chlorine now and will check the CC tomorrow or the next day when it's below 5. I appreciate the help.