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Balancing Water - Low Ta And Ch


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Ok, this is the first fill of a new (floor model) tub. Capacity of ~400 gallons. City water. Used pre-filter on end of brand new hose. No filter or softener in the house.

Before adding anything, the test strips (yes, I know the drop kits are better, but I had to order it, so I used what I had) showed TA of 20 ppm, pH of 7.2, and FC of 0.5.

Without being able to test better, I just added 2 TBS Dichlor to bring FC up to 10 ppm.

Second day, test kit showed up (Taylor K-2006):

CH 20 ppm

TA 20

I guess my city water is pretty clean and soft as it comes.


4 TBS calcium chloride to increase CH, ran pump 30 min.

8 TBS baking soda to increase TA, ran pump 10 min.

Tested again:

CH 30

TA 60

pH 7.6

FC 6

After soaking that evening, I added 1 tsp Dichlor to make sure FC didn't drop too much overnight.

On the bottle of calcium chloride, it mentioned waiting a long time before testing again to allow time for it to be absorbed; possibly that will come up further before I test again tonight.

My question: How far should I be trying to bring CH and TA up to? 110 each give a CSI of -0.04 on pool calculator; does that seem like a good target?

Edit: re-reading Nitro's approach, I see he recommends CH 130-150 and TA 80; I'll shoot for that.

Also, what should I be looking at to decide when/how much MPS to add?

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Well, the 1 tsp dichlor wasn't enough. When I got home from work, FC was at zero. Not too surprising with the bather load last night (4 person/hour). I guess I will need to check more often until it stabilizes.

This evening's test results:

CH 50 ppm

TA 70

pH 7.8

FC 0.0


2 TBS dichlor to bring FC to 10

3 fl oz calcium chloride to try and bring CH to 120

This should bring my CYA to ~20 so far

Will also try to keep an eye on pH

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Roughly speaking in a hot (104ºF) spa without an ozonator, every person-hour of soaking needs around 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of non-chlorine shock (43% MPS) to oxidize bather waste. 4 person-hours (did 4 people really soak for one hour in a very hot tub?) would need roughly 4-2/3 tablespoons of Dichlor.

This is just a rough rule and you can experiment around it to see how much you need to consistently register a residual FC reading by the next day.

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For heavily used spas, an ozonator will reduce oxidizer demand. For lightly used spas, an ozonator will tend to increase the amount of chlorine that needs to be added.

This is because ozone oxidizes bather wastes, but it can also react with chlorine and convert it into chloride or chlorate.

The silver won't have much of an effect either way on the amount of oxidizer demand.

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If the ozonator is working, then the amount of chlorine you needed to add would be around half what I indicated, so around 2-1/3 tablespoons of Dichlor for 4 person-hours so you probably added enough with your 2 tablespoons, but explains the zero chlorine level when you initially only added 1 teaspoon.

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