Opening Shocking: Dichlor vs Bleach & Granular CYA

[JesseWV]

After about a month of taking in all that TFP has to offer and maintaining my own pool I feel I have a pretty good handle on the Chlorine & CYA relationship.

When I opened the pool this year I foolishly went to Wal-mart and got a box of six 1lb Dichlor bags. I ended up adding 4 of them. It seemed like a lot at the time but now it seems I may have got lucky.

From 4lbs of dichlor I would have raised FC by 17ppm and CYA by 15.3ppm. My current level of CYA is 80. So it turns out my CYA level was probably around 65ppm before I added the dichlor. The initial reading for CYA before I added dichlor was zero. In hindsight what I should have done was use liquid bleach to raise FC to at least 2ppm and THEN tested the CYA. Would it have shown up then? Are the test strips the problem when reading CYA when there is no FC or does this happen to the turbidity test as well?

Luckily I'm still at a manageable level of around 75ppm. Hopefully this will slowly come down over time with rain water and refilling.

I've read a few threads where pool owners really do have zero CYA when opening their swamps for the first time. I know you guys always seem to recommend adding bleach and then the CYA separately through a sock in front of the return.

My question is wouldn't this be the perfect time to make use of the already stabilized chlorine and just use dichlor instead of two products? I understand that you would need to discontinue use of the dichlor during the shocking process if it took several days of shocking. One could always calculate the maximum amount of dichlor that could be used until having to switch over to un-stabilized bleach.

 

[JesseWV]

Thinking more about this it occurred to me that dichlor would result in less active sanitizer which would actually slow down the organic oxidation rate. I wonder if I answered my own question.

Oh well, I already typed it all out... perhaps it will be helpful to someone.

 

[Richard320]

Congratulations! You figured it out!

Personally, I would just use bleach to shock. You can buy 6% at Walmart or Target or any grocery store. Or 12.5% at a pool store, possibly Lowes or Home Depot (Watch out for old, weathered boxes from last year!)

 

[jblizzle]

Sounds like you get it. Remember the higher the CYA, the higher FC levels you will have to achieve to reach shock level. Thus, I think generally it is recommended to start the shocking process with low/no CYA and then when things start to clear you can used the stabilized chlorine to raise the CYA up to the appropriate level.

Are you using test strips or drop tests for all you levels?

 

[JesseWV]

Sounds like you get it. Remember the higher the CYA, the higher FC levels you will have to achieve to reach shock level. Thus, I think generally it is recommended to start the shocking process with low/no CYA and then when things start to clear you can used the stabilized chlorine to raise the CYA up to the appropriate level.

Are you using test strips or drop tests for all you levels?

I do the drop tests most of the time but the strips are useful for quick check ups now that I know what real levels the colors actually correspond to. I took a picture of a test strip when everything was perfect according to the drop tests. I suppose I'll have to do this again when and if I ever buy more test strips. I bet each brand of strips and probably each batch of strips from the same company are probably not quite the same.

 

[chem geek]

From 4lbs of dichlor I would have raised FC by 17ppm and CYA by 15.3ppm. My current level of CYA is 80. So it turns out my CYA level was probably around 65ppm before I added the dichlor. The initial reading for CYA before I added dichlor was zero. In hindsight what I should have done was use liquid bleach to raise FC to at least 2ppm and THEN tested the CYA. Would it have shown up then? Are the test strips the problem when reading CYA when there is no FC or does this happen to the turbidity test as well?

The FC level won't affect the CYA reading. It's more likely that your test strips readings are bogus and I wouldn't trust even your most recent readings. I don't understand why you don't have a drop-based test kit (by now). The turbidity test, while not the easiest to read, is generally much more consistent. The only issue is to warm up the water sample to room temperature before doing the test or else the reaction will be too slow (i.e. won't be complete after 30 seconds) so will under-estimate the CYA level.

 

[dmanb2b]

Ditto, is that CYA of 75 from strips or the TF100/K2006? IMO, strips are so unreliable and even if I went through 1 TF100 a season (I'm just now refilling mine in season 3 ) at $70 a season and knowing my results are reliable and consistent, is worth the added expense, vs messing w/strips.

Also, double check whether those bags of shock from walmart were actually dichlor...it could be, but by me, they only carry HTH shock in 1lb bags and it's actually cal-hypo, which would only add CH and not CYA.

Sounds like you get it though and that is what BBB is about...knowing your water chemistry and how certain chemicals impact that chemistry.

 

[JesseWV]

The FC level won't affect the CYA reading. It's more likely that your test strips readings are bogus and I wouldn't trust even your most recent readings. I don't understand why you don't have a drop-based test kit (by now). The turbidity test, while not the easiest to read, is generally much more consistent. The only issue is to warm up the water sample to room temperature before doing the test or else the reaction will be too slow (i.e. won't be complete after 30 seconds) so will under-estimate the CYA level.

I do have a TF-100 now but did not at the time of initial testing. I'm wondering if the strips only tested for stabilized chlorine and not CYA without chlorine bound to it. That would explain why it would indicate zero until at least some chlorine is present and bound up.

 

[JesseWV]

Ditto, is that CYA of 75 from strips or the TF100/K2006? IMO, strips are so unreliable

TF-100 results. But now I know what color corresponds to 75 on the crappy strips. It at least seems to be consistently exactly the same color every time.

Also, double check whether those bags of shock from walmart were actually dichlor...it could be, but by me, they only carry HTH shock in 1lb bags and it's actually cal-hypo, which would only add CH and not CYA.

They were AquaChem ShockPLUS+ which is for the most part dichlor. I wish I knew what to do with the 2lbs I have left. I can tell you they'll never go in the pool. Maybe I'll use it to sanitize the kiddie pool a teaspoon at a time.

 

[chem geek]

I do have a TF-100 now but did not at the time of initial testing. I'm wondering if the strips only tested for stabilized chlorine and not CYA without chlorine bound to it. That would explain why it would indicate zero until at least some chlorine is present and bound up.

I doubt it, but you could experiment with them over time at different FC and known CYA levels if you want to find out. I suspect they are just plain unreliable and inconsistent since that's what many people experience.

 

[JesseWV]

I do have a TF-100 now but did not at the time of initial testing. I'm wondering if the strips only tested for stabilized chlorine and not CYA without chlorine bound to it. That would explain why it would indicate zero until at least some chlorine is present and bound up.

I doubt it, but you could experiment with them over time at different FC and known CYA levels if you want to find out. I suspect they are just plain unreliable and inconsistent since that's what many people experience.

I would probably do that just to find out if I had some standalone CYA. Unfortunately all of the CYA in my pool was already there or added with dichlor.

I could add some dichlor to a bucket and allow the FC level to drop to 0 I suppose. I'm not sure that would duplicate what happens to the pool over winter. I believe I read somewhere that soil has some effect on it. I'm thinking a handful of dirt?