What Is My Chlorine Demand

[macgd016]

I have a 13,000 gallon pool and want to know what my Chlorine Demand is or should be. pH is 7.4 and CYA is 200, high I know but thats where it is and pool temp is 82F. The pool is crystal clear and has a very light load, just two people having a quick dip each day and the phosphate level is negligible. I have an automatic chlorine pump and have assumed that my chlorine demand would be around 1ppm per day so have added 23oz of 6.8% chlorine each day. This has been fine for the past 5 weeks but recently, as the temp as gone above 80F, I find that FC & TC are dropping daily so a)I can't see why this should be and other than by trial and error can't see how much chlorine I should be adding.

[waterbear]

Chlorine demand depends on many factors that interact so it is not something that can really be "tested" like you can test pH or CYA.

Part what you are seeing in your increased chlorine demand IS the increase in temperature and your high CYA is adding to it, most likely. You do not say where you are keeping your FC normally BUT with a CYA of 200 would require you to keep the FC at about 15 ppm to achieve normal levels of active chlorine in your pool since most of the chlorine would be bound up chemically with the CYA and held in reserve (and I question how you got that CYA number since chemical tests for CYA that are PRECISE, so I am not referring to test strips which are NOT, can only test up to 100 ppm without doing dilutions). At such a high CYA level the odds are very good that you are constantly fighting a nascent algae bloom, particularly as the temps increase, which will add to the chlorine demand.

I assume you are using liquid chlorine since you say you have an automatic pump (I assume a peristaltic dosing pump). However, I have never seen liquid chlorine come in a 6.8% strength. Most pool store chlorine is 10% to 12% while laundry bleach (same chemical but more dilute) ranges from 3% to 6% with 5.25% and 6% being most common.

To answer your question as to what 'normal' chlorine demand is...for most outdoor pools with NORMAL CYA levels (30-50 ppm) and normal to light bather load it is between 1-2 ppm per day.

Adding borates to you pool in a 30-50 ppm concentration can lower chlorine demand by preventing nascent algae.

If you would post a full set of test results not done with strips but done with a drop based kit such as the Taylor K-2005 or K-2006 for FC, CC, pH, TA, CH and CYA we could provide you with much more help.