Chlorine consumption and water temp

[Youfah Mizzum]

When I test my chlorine level, it's typically around 3 to 5. I have a SWG and don't really need to adjust it. And the water temp in summer without heating is around 25C. Maybe plus or minus a degree or two but 25 is the ballpark.
That is just a bit cool for my wife who likes the water temp 29C or 30C.
Very easy to get the temp up there as I have a pool blanket & a heat pump heater.

But making no change to the SWG chlorine generation, when I test the chlorine level when the temp is 30C the level is around 1 to 2. Water always remains crystal clear but is that a normal phenomenon, that chlorine tests show a lower level when the temp is higher?

 

[mknauss]

Temperature has no effect on the FC test. I assume you are using a FAS-DPD test.
More FC loss is possible with higher water temperatures. More people in the water, etc.

 

[mgtfp]

The temperature itself shouldn't have an influence on FC. But maybe the way you increase the temperature? How often do you use the heater or the blanket? Maybe there is some algae on the blanket or in the heater or the pipes to and from the heater?

Maybe it is just that the pool is being used more while the water is warmer? Not just you, but also your wife using the pool, and you stay longer in the water? Maybe the kids enjoy the warmer water that much that they get a bit sloppy in going to the toilet?

If you really want to get to the bottom of it, you should probably run a few more FC tests over the course of a day. With and without blanket or heater, during the day and over night as a real OCLT. Seeing when the chlorine disappears should help to understand the root cause.

 

[Dirk]

Humans sweat. And they sweat under water. Increased water temperature will likely lead to more people in the water, people staying longer in the water, perhaps being more active in the water, and all of them, for all that extra time, will be sweating more underwater. Sweat is one of the many organic materials that humans leave behind (there are others) on which chlorine goes to work. Suntan lotion. Hair. Skin. And, well, lets just say "other stuff," too. That is at least part of the reason you're losing more FC.

You'd have to eliminate all people from the pool, for two days, one day at one temp, and the next day at another temp, to prove or disprove this theory...

 

[Youfah Mizzum]

You'd have to eliminate all people from the pool, for two days, one day at one temp, and the next day at another temp, to prove or disprove this theory...

I don't think it's because there's more people in the pool. Like Dirk says, I test without pool action. When the temp is cooler I get readings around 3 to 5, and when I crank up the heat (brand new pool, brand new heat pump, brand new pipes etc) to around 29 to 30 (that's celcius people!) without anyone in the pool, the FC test shows around 1.5 to 2. Blanket is on all the time whether heated or not. We only uncover the pool to swim then cover it again when not swimming.

I know in the OP I said the clarity of the water is the same but actually the clarity of the water is ever so slightly hazier when warm, I guess this would possibly be a result of higher chlorine consumption when warmer.

So based on the feedback (i.e. I can rule out the extra pool activity) just a peculiar phenomenon.

 

[mgtfp]

Blanket is on all the time whether heated or not. We only uncover the pool to swim then cover it again when not swimming.

Sources of chlorine loss without bather load or UV (i.e. what you would see in an OCLT) apart from organics like algae, are basically "stuff" that chlorine oxidizes, and these oxidization reactions will to a certain degree be temperature dependent.

As long as there is not much to oxidize, you usually don't see much of a temperature effect, as it just vanishes in your testing accuracy.

In a perfectly clean pool, there would still be the CYA that gets oxidized by chlorine - which is one of the reasons why, apart from splash-out or backwashing, CYA disappears over time, especially in hot climates.

Another typical source can be a pool cover. Even when clean, the material itself gets oxidized, and faster so at higher temperatures.

You could run an OCLT with and without cover to work out if that is a factor in your case.

 

[mgtfp]

know in the OP I said the clarity of the water is the same but actually the clarity of the water is ever so slightly hazier when warm

This could indicate that you have some algae in the water that is just being controlled by sticking to the FC/CYA table, preventing an actual bloom.

When warmer, the algae will become more active, faster metabolism which will make it easier for chlorine to get into cells and kill them, so you see more dead algae in the water, turning it dull.

With CYA, the HOCl concentration actually increases with higher temperatures, which makes the chlorine more efficient, increasing the above effect further.

I would definitely suggest an OCLT, with and without cover, and proceed with a SLAM if required.

 

[Dirk]

You may have eliminated people as the possible cause of the "peculiar phenomenon," but there is something that is reacting to the extra heat that is using up chlorine. It's not unexplainable, it's just that we've yet to explain it! @MyAZPool introduces some reasonable alternative causes...

On a side note: I won't argue the benefits of a cover, but this thread certainly supports my preference not to use one... it puts another tick or two in that column, anyway...