Chlorine Demand & Pool Chlorine

Swimming Pool Chemistry



Residual swimming pool chlorine demand is something every pool owner should know and understand. Your swimming pool chlorine needs to kill off the algae and bacteria so your pool needs the right amount of chlorine to do the job.

Your swimming pool chlorine demand (residual chlorine) might be the the single most important reading you do with your pool, yet residual chlorine is the most overlooked aspect of swimming pool care.




Definition

Residual chlorine is when any organic matter (animals, swimmer sweat, urine, saliva, green pool algae, brown algae, grass clippings, leaves, etc...) enters the pool and your pool chlorine oxidizes this matter and kills it.

The more organic matter you have in your pool the more chlorine is needed. This is not to be confused with combined chlorine or free available chlorine.

Chlorine demand is the amount of pool chlorine (or Bromine) your swimming pool, Spa, or Hot Tub is using and knowing this will also tell you how much organic matter is in your swimming pool.

Remember that neither a pool ionizer nor a salt chlorine generator will give you a good and proper pool shock.

Low or high cyanuric acid levels could contribute to a high residual chlorine demand.


How To Measure Chlorine Demand

To measure your residual chlorine (and this can be done for either your swimming pool or Hot Tub) you should:

  1. Measure your pool chlorine level with a good pool water testing kit. I recommend a Taylor Reagent FAS-DPD K-2006 pool water testing kit.

  2. Then perform a good pool shock (or Hot Tub) up to 10ppm or maybe even a little over. I prefer Calcium Hypochlorite but if you have high pH water or high calcium hardness, you may want to use Sodium Hypochlorite/ liquid chlorine or bleach.

  3. Allow the swimming pool to sit for 24 hours. If you're doing this with a Hot Tub, you can cover it.

    4. Don't use the swimming pool or Hot Tub during this process or you will get a wrong reading, along with very itchy skin.

  4. After the 24 hours, re-test the chlorine once again

  5. Take the 2nd measurement (after the pool shock) and divide it by the 1st measurement (your initial chlorine reading)

You now have the information you need. The above will tell you how much pool chlorine is left in the body of water after 24 hours.

You'll want to have the highest number possible, but 100% is reaching for it. This means your swimming pool is not using any chlorine, which is highly unlikely. A 0% chlorine demand is not good either because you're using up all the chlorine very quickly.

If you have asthmatic bronchitis you'll need to keep up on the chlorine and pool shock as the rising bacteria and chlormines in the swimming pool may put you at risk.

If you have any kind of chlorine allergy, you may either want to reduce the amount of swimming time or look into natural swimming pools.


Understanding Chlorine Demand

To get the best reading of your residual chlorine it's best to measure it after you have refilled your swimming pool or Hot Tub and before you use it.

Your goal is to get as close as possible to 25% residual chlorine, meaning you've only used up 25% of your pool chlorine which leaves the other 75% to do its job.

If you pool shock on a refill at 10-11ppm the next day you should have 7.5ppm. That's the goal you want to hit. Obviously after you start using the swimming pool or Hot Tub the residual chlorine will rise.

If after using the swimming pool and a good pool shock and your residual chlorine is a bit higher, say 50 - 75%, it means you didn't use enough pool chlorine to decontaminate the water. Simply do another shock up to 10 - 11ppm, then re-test after 24 hours.

Continue to shock and test until you get as close to the 25% range as you can. Make sure you keep up on your filter maintenance and backwash when needed.

By using this and other swimming pool maintenance tips, you will surely have a pool of which you can be proud.

Go swimming and have fun.

Go To:

Swimming Pool Chlorine From Chlorine Demand


footer for chlorine demand page