OK, please clarify the discrepancy between Dichlor + Bleach #'s and other common references

[kiva822]

Hi Folks,

I have been using the Dichlor + Bleach method for 3.5 months now and I love it. It's so easy. I hardly ever have to adjust my PH and everything is so stable. It's great. I did add a mineral stick for "insurance" in case my FC drops too low. Here are my current numbers:

TA: 60 (drop 7 doesn't impact color)
PH: 7.5 and hardly ever moves.
CYA: 30
Calcium Hardness: 130
Borate: 50

So, assuming that's all OK, here is my question: why does every other reference I can find say that the TA goal should be between 80-120 and that anything below 80 will cause corrosion? This came to a head recently on a Bullfrog FB group where somebody posted offering to help people with chemical numbers and I asked the above question. His answer was that anything below 80 will cause corrosion. I talked about how, if TA and PH are stable, what is the problem? He wasn't able to give me an answer about what chemically would be going on to cause corrosion in my situation, other than "TA below 70 will cause corrosion". I said my water seems stable but he insisted that PH should move some from time to time and the fact mine is stable at 7.5 is a bad thing and shows my TA is too low.

I said, "so we're destined to throw baking soda or acid at our spas forever"? His answer; yes - it prevents the corrosive condition.

Can somebody explain to me the discrepancy between TFP's recommendations via the "how to use chlorine" thread and the numbers seen elsewhere? To top if off, I ordered a test kit from TF and the card in the kit even says TA of 80-120.

I just want to make sure i'm not "destroying my spa from the inside out" per that BF forum. I even helped a friend set up his spa using this method and want to make double sure...

Thank you.

 

[ajw22]

This came to a head recently on a Bullfrog FB group where somebody posted offering to help people with chemical numbers and I asked the above question. His answer was that anything below 80 will cause corrosion. I talked about how, if TA and PH are stable, what is the problem? He wasn't able to give me an answer about what chemically would be going on to cause corrosion in my situation, other than "TA below 70 will cause corrosion". I said my water seems stable but he insisted that PH should move some from time to time and the fact mine is stable at 7.5 is a bad thing and shows my TA is too low.

As you saw that person lacks any scientific explanation to support his assertions.

TA does not directly cause corrosion. The only reason we care about TA is the effect it has on pH.

I suggest you read TA - Further Reading which I wrote which describes how TA affects pH.

And then you can read PH TA Relationship - Further Reading for a deeper dive.

Now low pH will cause corrosion. Low pH is pH into the 6's and below. It is your TA value that keeps your pH from getting that low. But it takes a TA below 50 to risk allowing the pH to get below 7. And even a TA of 30 or 40 does not guarantee that your pH will drop below 7.

So saying TA below 70 will cause corrosion is flat out wrong. Saying a very low TA can cause your pH to crash and the low pH can cause corrosion is science. But how low can TA go before pH drops below 7 is not black and white. And a TA of 60 is fine. A TA of 50 is skirting the danger zone. And a TA of 40 is playing with fire.

 

[kiva822]

As you saw that person lacks any scientific explanation to support his assertions.

TA does not directly cause corrosion. The only reason we care about TA is the effect it has on pH.

I suggest you read TA - Further Reading which I wrote which describes how TA affects pH.

And then you can read PH TA Relationship - Further Reading for a deeper dive.

Now low pH will cause corrosion. Low pH is pH into the 6's and below. It is your TA value that keeps your pH from getting that low. But it takes a TA below 50 to risk allowing the pH to get below 7. And even a TA of 30 or 40 does not guarantee that your pH will drop below 7.

So saying TA below 70 will cause corrosion is flat out wrong. Saying a very low TA can cause your pH to crash and the low pH can cause corrosion is science. But how low can TA go before pH drops below 7 is not black and white. And a TA of 60 is fine. A TA of 50 is skirting the danger zone. And a TA of 40 is playing with fire.

Thank you. I got the feeling he didn't understand why he believed what he was saying - at least scientifically.

Thank for the articles. I'll check those out.

Needing to adjust my PH like every 4-5 weeks sounds like it's standard, so I'll stay the course.