Does Chlorine Reduce Linearly Or Logarithmically?

[Dietitian]

Does chlorine reduce linearly or logarithmically? If it is linear, why do we measure chlorine demand in percentage terms instead of fixed, say "demand is 5ppm fc per 24 hours?"

[chem geek]

Like much in life, the answer is "it depends". The chlorine loss from outgassing or slow chemical reactions of chlorine with organics or other materials is proportional to the chlorine concentration so is logarithmic (i.e. an exponential decay -- losing a certain % per day, for example). The chlorine usage from bather load is linear to that bather load since the chlorine oxidizes such chemicals fairly quickly in hours (usually). So a more accurate description of chlorine demand would be a combination of no-bather-load chlorine demand as a % of chlorine level plus a fixed amount of chlorine demand proportional to the bather load.

For simplicity, we just measure the chlorine demand (CD) without bather load as a rough guide for how long chlorine will last day-to-day if the tub is not used and also as a rough guide for the amount of organics in the tub (i.e. if the water needs shocking or if the water needs to be changed). The person-hour bather load rule actually includes some CD in it given how it was measured, but it still seems to be a decent rule-of-thumb.

In addition to the above is a chlorine demand when one has an ozonator and that is usually a % per day since it is ozone reacting with chlorine so is dependent on chlorine concentration.

[Dietitian]

Like much in life, the answer is "it depends". The chlorine loss from outgassing or slow chemical reactions of chlorine with organics or other materials is proportional to the chlorine concentration so is logarithmic (i.e. an exponential decay -- losing a certain % per day, for example). The chlorine usage from bather load is linear to that bather load since the chlorine oxidizes such chemicals fairly quickly in hours (usually). So a more accurate description of chlorine demand would be a combination of no-bather-load chlorine demand as a % of chlorine level plus a fixed amount of chlorine demand proportional to the bather load.

For simplicity, we just measure the chlorine demand (CD) without bather load as a rough guide for how long chlorine will last day-to-day if the tub is not used and also as a rough guide for the amount of organics in the tub (i.e. if the water needs shocking or if the water needs to be changed). The person-hour bather load rule actually includes some CD in it given how it was measured, but it still seems to be a decent rule-of-thumb.

In addition to the above is a chlorine demand when one has an ozonator and that is usually a % per day since it is ozone reacting with chlorine so is dependent on chlorine concentration.

Excellent, thank you.