Pool Chlorine Demand and Chlorine Lock | What Are They? 

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


Your pool is hungry for chlorine and needs to be fed and tested on a weekly basis.  If you keep adding chlorine, and maybe get a reading after testing, but the chlorine doesn't hold, or keeps decreasing, you might just have what is known as "chlorine demand".  This demand means your pool needs more and more chlorine just to keep a proper reading. 

Your swimming pool residual chlorine (the chlorine that is left to kill organic matter) 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. 

Definition Of Chlorine Demand

Let's keep this simple.  Chlorine does three things: oxidize, sanitize, and kills organic matter in the pool.  That's it.  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 your salt cells will give you a good and proper swimming pool shock.

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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.

If you pool shock on a refill at 10-11 ppm, the next day you should have about 7 ppm. 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 - 11 ppm, 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.

Free Chlorine - Total Chlorine - Combined Chlorine

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

  • Perform a good pool shock (or Hot Tub) up to 10 ppm 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.  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 combined chlorine 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.

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What Is Chlorine Lock or Chlorine Block

This is actually one of my favorites sayings about chlorine lock/block.  The definition of chlorine lock is : "The pink and purple winged unicorn flying over the rainbow eating cupcakes."  To put it simply, it doesn't exist.  But when you ask certain people what's going on with your chlorine, they have to say something, don't they?  Or else they'll look foolish.  Your pool's chlorine is never "locked up". 

My friends, chlorine can only be one of two things.  It's either effective or not effective at doing its job.  What's its job?  At killing organic matter in the pool.  And chlorine's effectiveness is dependent upon your CYA and pH level.  That's chemistry and that's how this stuff works.  

Check your pool's chlorine demand so you'll have a proper foundation upon which to understand and build.

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