Chlorine Demand

Chlorine Demand and swimming pool water chemistry. Good weekly pool maintenance to give you the pool perfect for you.

Your swimming pool residual chlorine might be the the single most important reading you do with your pool.

Residual chlorine is the most overlooked aspect of swimming pool care. Neglect can cause cloudy pool water.

Low or high cyanuric acid (pool stabilizer) levels could contribute to a high demand for chlorine. 

How To Test Your Chlorine

FC - TC - CC?  Wait. What? 

Definition Of Chlorine Demand

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.

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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 your salt cells will give you a good and proper swimming pool shock.

Eliminate your chlorine demand and put more money in your pocket. Click on the eBooks below and get started on having a perfectly clear pool all season.

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:

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

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  • Perform a good pool shock (or Hot Tub) up to 10ppm or maybe even a little over. I prefer Sodium Hypochlorite/liquid chlorine or bleach. Calcium Hypochlorite will raise your pool pH too high because it has a pH of 12.
  • Allow the swimming pool to sit for 24 hours. If you're doing this with a Hot Tub, you can cover it.
  • Don't use the swimming pool or Hot Tub during this process or use neutralizing chemicals. You will get a wrong reading along with very itchy skin.
  • After the 24 hours, re-test the chlorine once again
  • 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 (hypochlorous acid) is left in the body of water after 24 hours.

Chlorine Demand

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% reading 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 chloramination in the swimming pool may put you at risk. You may also experience a nasty chlorine smell.

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

Understanding Chlorine Demand

To get the most accurate 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.

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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 swimming pool you can be proud and will reduce your pool maintenance cost.

Go swimming and have fun. 

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