Today’s inquiry came to us from ‘Mandy’ who asked, “Our pool test kit show no chlorine at all even though the feeder box stays full of tablets all the time and we never let it get empty. The pool stinks badly like chlorine, too, so what is wrong? Do we need a new test kit? Are we using the wrong chlorine?”

Thanks for the question, Mandy!

When a person tells us they have no chlorine reading on their test kit usually one or more of the following factors has played a part in causing that to happen:

  • Chlorine demand greater than chemical feeder system can supply
  • Old or expired test kit reagents, test strips, etc. not properly reading chlorine levels
  • Lack of pool stabilizer (cyanuric acid)

The fact that you can smell chlorine, however, tells us that you may have another situation on your hands. Quite possibly your pool has combined chlorine but not much free chlorine. If you do not know the difference between combined chlorine and free chlorine then please take a look at an earlier posting of ours called What is the Difference Between Total Residual Chlorine and Free Chlorine?

For those not interested in reading the long version, the following will explain free chlorine, combined chlorine and total chlorine:

  • Free Chlorine does the active sanitizing of the pool water by neutralizing organic contaminants (i.e. bacteria, parasites, etc.). Traditional chlorine sanitized pools should have a healthy free chlorine level at all times.
  • Combined Chlorine started out as free chlorine, locked horns w/ an organic contaminant, and now has become attached to the contaminant it neutralized. While still partially effective as a sanitizer, it has nowhere near the sanitizing power of free chlorine… and it plays a big part in producing the strong ‘chlorine smell’ most people associate with pool water.
  • Total Chlorine represents the total concentration of free chlorine plus combined chlorine in a water sample.

Poolmaster 5- Way Test Kit
Poolmaster 5- Way Test Kit

So, Mandy, at this point we believe you should begin by making sure your test kit can test for both free and total chlorine. If it cannot then we suggest picking up a test kit such as the Poolmaster 5-Way Test Kit or, if you prefer a simpler chlorine testing method, the Pool Check 5-Way Test Strips should work quite well for you.

If your existing test kit can test for both free and total chlorine then you may need to replace your reagents (i.e. DPD-1 Test Tablets, DPD-3 Test Tablets, and/or DPD ReagentStrips). If stored near other pool chemicals, left out in the sun and extreme heat for long periods, or beyond their expiration date(s), your reagents may have gone bad.

DPD 1 & DPD 3 Test Tablets
DPD 1 & DPD 3 Test Tablets

Now that you have ruled out the test apparatus as the root of your chlorine troubles, test the pool for both free and total chlorine and then subtract the free chlorine concentration from the total chlorine concentration. Ideally you should get a very low number (zero would be perfect) but if you get more than a 0.5ppm difference you most certainly have a buildup of combined chlorine — and that would explain the strong chlorine odor you mentioned.

To get rid of the combined chlorine experts suggest ‘shocking’ (superchlorinating) the pool with a non-stabilized chlorine (contains no cyanuric acid) such as calcium hypochlorite powder or a non-chlorine ‘shock’ treatment such as monopersulfate (MPS).

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