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An example is here: https://alpinespas.co.nz/content/uploads/2021/01/2020-Water-Care-Guide-Spa-Supplies-SSDEC08-WEB.pdf

It recommends ignoring test strips, using a "shock dose" of chlorine every two weeks, and putting a few teaspoons of chlorine after each use.

I am trying to understand the chemistry better to determine whether they are recommending too much chlorine.

Related question here asks what is the chemistry underlying the test strips failing: Why might an ozone UV spa pool cause test strips to fail for chlorine?

This question is asking about the argument for chlorination at the levels recommended in this and similar guides.

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  • $\begingroup$ This guide recommends both UV/ozone and chlorination. $\endgroup$
    – kennyB
    Commented Aug 28 at 9:01
  • $\begingroup$ Instead of posting another question, rather elaborate and extend your prior question about the same topic in prior query $\endgroup$
    – Poutnik
    Commented Aug 28 at 9:01
  • $\begingroup$ It is recommended to quote essential parts or summaries of provided links, as questions should be self-contained even in cases the links stop working. Also, users should be able to evaluate the question even without following the links. These should serve just for more details or reference. $\endgroup$
    – Poutnik
    Commented Aug 28 at 9:16
  • $\begingroup$ I've edited to include more relevant detail. $\endgroup$
    – kennyB
    Commented Aug 28 at 17:34

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You do not really need to understand the chlorination chemistry for proper dosing.

What you need to know is:

  • recommended concentrations of active chlorine (the link or search) in $c_0\ \pu{[mg/L]}$. IIRC it is:

    • the permanent/safe bathing concentration $1-3\ \pu{mg/L}$
    • shock treatment concentration $5-10\ \pu{mg/L}$,
      should be let to drop to safe one before next bathing, at least overnight.
    • apply-after-use concentration $2-5\ \pu{mg/L}$,
      should be let to drop to safe one before next bathing, at least few hours.

  • volume of pool water $V \pu{[m3]}$.

  • (estimated) mass of applied chlorination agent $m \pu{[g]}$.

  • specific content of active chlorine in the agent. $x \pu{[g/g]}$.

Then you can calculate the applied concentration $$c=\dfrac{m \cdot x }{ V} \pu{mg/L}$$

Imagine you have a pool $\pu{V = 6 \times 3 \times 1.5 m3}=\pu{27 m3}$, using commonly used trichlorocyanuric acid (TCCA) with 90% of active chlorine. You want concentration of active chlorine $\pu{c = 5 mg/L}$. Then required weight of TCCA is

$$m = \frac{c \cdot V }{ x} = \frac{\pu{5 mg/L} \cdot \pu{27 m3} }{ 0.9} = \frac{(\pu{5 g/L})( \frac{\pu{1 g}}{\pu{1000 mg}}) \cdot (\pu{27 L}) (\frac{\pu{1000 L}}{\pu{1 m3}})}{ 0.9} = \pu{150 g}$$

Therefore $\pu{30 g}$ for each $\pu{1 mg/L}$. You can check on scales how much it is by measuring cups count.

Or you can take the strip result, best with UV/ozone OFF for some time.

Be aware that the chlorine concentration decrease rate is bigger under Sun, at high temperature and at higher content of organic matter.

With intentional UV/ozone off, you may measure by stripes the rate of chlorine concentration decrease after a shock or after-use dose. Then you would know how soon to add another dose after a regular or shock dose. Experienced pool care-taker might eventually learn to estimate current concentration by chlorine smell intensity.

One reason for eventual overdosing can be if otherwise correct doses are applied too soon after prior ones.

Using UV/ozone additional sanitation, my opinion is that you could keep doses rather at their lower range ends.

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