Inconsistent bleach quantity to FC increase in PoolMath

[jesper]

I’m using PoolMath for iOS on my iPhone.
I started a slam this morning as I have higher than usual chlorine demand, despite the water being clear.
I started with FC 1.5 and with a CYA of 40 the slam target is 16, for my pool it said that would be 3.3 litres of 15% chlorine like


I checked about an hour later and FC was only 11, so I added another 1.2 litres as suggested by PoolMath. However I was surprised by the high chlorine drop in just one hour, so I used “effects of adding” to check, and it says my 3.3 litres is an increase of FC by 12.0, not 14.5 as would be required for my original FC 1.5 to get to slam level FC 16

Is this a bug in the App?

 

[mgtfp]

It looks like in the "Effects of Adding" PoolMath calculates with 12.5% instead of 15% bleach strength.

That might be an intentional feature because 15% bleach loses its strength very quickly and 12.5% might be a more realistic chlorine content by the time you actually use the bleach. The question is, why this feature was only implemented in the "Effects of Adding"?

@Leebo , can you help out here?

 

[Msch99]

Is the pool volume correct?

 

[CrystalRiver]

I just checked, using your numbers in my Android version of the app. My results on Effects of Adding were the same as yours, but on the FC page it told me to add 2180 mL. Double check your pool volume in the app settings.

 

[mgtfp]

I also checked it, and got the same numbers as in the OP. Just did it again, checked the pool volume, and still got the same.

 

[jesper]

I also checked it, and got the same numbers as in the OP. Just did it again, checked the pool volume, and still got the same.

It is my best estimate - but even if it was wrong, it is the same volume used in both cases, so the result should be the same.

To be clear, I'm not asking why I didn't measure close to 16 FC when I re-measured. The difference can be for several reasons like
- Incorrect volume, if my pool is larger than what I added in PoolMath, the added chlorine will not result in as large an increase as PoolMath reported.
- The strength of the chlorine can have dropped, again resulting in a reduced increase in FC.
- Chlorine used up by organic stuff.

My question is why two different calculations from PoolMath results in different FC increases for the same addition of 15% chlorine, and the same pool volume.
- The FC page says in order to increase FC from 1.5 to 16, that is a 14.5 increase I should add 3341 ml of 15% chlorine
- The "effects of adding" page says the effect of adding 3341 ml of 15% chlorine is an FC increase of 12.0

I would have expected the two to agree.

 

[jesper]

I just checked, using your numbers in my Android version of the app. My results on Effects of Adding were the same as yours, but on the FC page it told me to add 2180 mL. Double check your pool volume in the app settings.

In the FC page, it will use the volume of your pool, not my pool. Doesn't that explain the difference ?

I also checked it, and got the same numbers as in the OP. Just did it again, checked the pool volume, and still got the same.

Not sure I follow. Above you said you saw different number, here you're saying you see the same as me. Did I misunderstand ?

 

[mgtfp]

Hey Jesper,
Sorry, that was phrased badly. When I wrote that I got the same, then I meant that I got the same results as you. I'm with you here, something is not consistent.

 

[Leebo]

Can you post a screenshot of the label showing the active ingredients of the chlorine?? 15% is very rare and I'd like to first verify that. I see you're overseas so it's plausible your manufacturing process may have different guidelines, but let's check there to start.

 

[mgtfp]

And I just confirmed it with a hand calculation.

The 3341ml are definitely correct to raise FC by 14.5 ppm with 15% bleach:
0.15 X 3.341l / 34600l = 14.48ppm

The 12ppm rise from the effects calculation would be correct for 12.5% bleach:
0.125 X 3.342l / 34600l = 12.07ppm

For some reason, PoolMath uses 12.5% instead of 15% in the effects calculation.

 

[jesper]

Can you post a screenshot of the label showing the active ingredients of the chlorine?? 15% is very rare and I'd like to first verify that. I see you're overseas so it's plausible your manufacturing process may have different guidelines, but let's check there to start.

Thanks for looking. Attached a photo below, it says 150 gram/litre, I assume that is 15%, however even if it is less it doesn’t explain why two different pages of PoolMath gives different results. However it can explain why I didn’t see as large an increase in FC as I had expected.

 

[jesper]

And I just confirmed it with a hand calculation.

The 3341ml are definitely correct to raise FC by 14.5 ppm with 15% bleach:
0.15 X 3.341l / 34600l = 14.48ppm

The 12ppm rise from the effects calculation would be correct for 12.5% bleach:
0.125 X 3.342l / 34600l = 12.07ppm

For some reason, PoolMath uses 12.5% instead of 15% in the effects calculation.

Thanks for checking the numbers!

 

[CrystalRiver]

In the FC page, it will use the volume of your pool, not my pool. Doesn't that explain the difference ?

No, because I temporarily changed the settings so that my pool has your pool's volume.

 

[mgtfp]

But apart from that app inconsistency, that I'm sure Leebo will sort out: I guess there are two reasons for your "missing" chlorine:

1) You are SLAMming for good reason, added Chlorine can get consumed very quickly when in the middle of an algae bloom. What's the exact reason why you are SLAMming? Algae, or is it unclear where your chlorine demand comes from?

2) 15% chlorine won't contain 15% for very long. Depending on the actual manufacturing date, it will have lost some (or a lot...) chlorine already. Is there a date somewhere on the container?

 

[mgtfp]

I just checked, using your numbers in my Android version of the app. My results on Effects of Adding were the same as yours, but on the FC page it told me to add 2180 mL. Double check your pool volume in the app settings.

But 2180mL would raise FC only by
0.15 X 2.180L / 34600L = 9.45ppm
That also doesn't make sense.

 

[jesper]

But apart from that app inconsistency, that I'm sure Leebo will sort out: I guess there are two reasons for your "missing" chlorine:

1) You are SLAMming for good reason, added Chlorine can get consumed very quickly when in the middle of an algae bloom. What's the exact reason why you are SLAMming? Algae, or is it unclear where your chlorine demand comes from?

It is not obvious to me why I have increased chlorine demand, I suspect I might have the beginning of an algae bloom, I've been losing 4-5 ppm of FC a day, when my normal is in 1-2 ppm range. I use an opaque safety cover, so the loss due to UV is minimal, especially as we usually use the pool only in the afternoon where it is in the shade of the house.
This all started because my SWG stopped producing chlorine, or at least significantly reduced the output, as it had some scaling. As a result FC dropped < 2 a couple of times, and I might have got a bit of algae.

I cleaned the SWG a week ago, but with the increased FC demand, it still could not keep up with my usual run time. So started a slam to make 100% sure that I don't have an algae outbreak, at the same time as trying to adjust the SWG run time.

2) 15% chlorine won't contain 15% for very long. Depending on the actual manufacturing date, it will have lost some (or a lot...) chlorine already. Is there a date somewhere on the container?

No date that I can decode, but I bought it directly from the manufacturer about a month ago, so my guess it that it is at most a few months old.

I'm not too worried if the strength is reduced, I can just add a bit more.

 

[mgtfp]

Looks like you are on the right track, just keep slamming.

Have a look at this thread regarding breakdown of bleach:

Breakdown of bleach over time by storage temperature

I found a nice web page that will predict the loss of strength of stored bleach over time based on the storage temperature. Take a look here.

www.troublefreepool.com

But as you said, doesn't matter too much as long as you get your FC to where you want it, but you might have to add more than PoolMath tells you.

 

[jesper]

Looks like you are on the right track, just keep slamming.

Have a look at this thread regarding breakdown of bleach:

Breakdown of bleach over time by storage temperature

I found a nice web page that will predict the loss of strength of stored bleach over time based on the storage temperature. Take a look here.

www.troublefreepool.com

But as you said, doesn't matter too much as long as you get your FC to where you want it, but you might have to add more than PoolMath tells you.

Thanks for that, it is pretty hot here, day time peak temperature between 35-42 degrees C (95-107F) the past few weeks, average across day and night for the past two weeks is just under 30 degrees (86F). So for sure it will have lost strength - probably I'll assume it is 10-12% instead going forward.

 

[Planenuts]

replying to an old post but the original issue/question does not seem to be answered and I have the same issue, in the App, setting the SLAM button on sets my SLAM target FC to 14 from the actual 4.5 I have now. This suggests to add 233 oz of 2.88% bleach by volume to reach target (increase of 9.5). If I go to the effects of adding section and put the amount to the suggested values of 233oz, FC rise would be of 18.0, in this section I would only need to add 124 oz to get a 9.5 rise in FC. which section of pool math is correct.

 

[CrystalRiver]

replying to an old post but the original issue/question does not seem to be answered and I have the same issue, in the App, setting the SLAM button on sets my SLAM target FC to 14 from the actual 4.5 I have now. This suggests to add 233 oz of 2.88% bleach by volume to reach target (increase of 9.5). If I go to the effects of adding section and put the amount to the suggested values of 233oz, FC rise would be of 18.0, in this section I would only need to add 124 oz to get a 9.5 rise in FC. which section of pool math is correct.

Post 10, @mgtfp confirmed the Effects of Adding section is wrong. The section where you selected SLAM is correct.