Question about FC loss and SWG settings

[run53]

Hi,

I was wondering if there is any "general guidelines" as to how much FC loss to expect on a daily basis.
Hypothetical: if water temp is in the mid 80's and weather is in the 90's with full sun from 8:30am till 5:00pm.

Is there any rough formula for expected FC loss? I think I read somewhere that it could be as much as 50-65% of FC. For example, if FC is 4 then FC loss on a sunny day as described above would be about 65% or 2.6ppm.

I want to know because I just completed a SLAM and the rate of FC loss seems more at the beginning (when FC=20ppm) and gradually gets smaller. My SWG has been set to 65% throughout the process. I want to make sure that once FC reaches about 4ppm (CYA is about 70), that I can keep it there with the SWG.

So, if I can assume a 65% FC loss on a sunny day, then my SWG (dialed to 65%) needs to produce 8.3oz of chlorine gas to generate the lost 2.6ppm (of course, this assume no OCLT loss). But since I usually get an OCLT of 0.5 to 1.0 then I really need to generate 3.1ppm - 3.6ppm (or at least 10oz - 11oz of chlorine gas)

So, if I run my SWG for 12 hours at 65%, i can generate 11 oz of chlorine gas and i should be all set.

Looking for any holes in my thinking, especially around my assumptions for FC loss.

 

[woodyp]

Consider 2 to 3 ppm loss daily a good all around "average". Your mileage may vary. Mine does!........up to 3.5 or 4.

 

[JasonLion]

Daily FC usage depends on both your CYA level and your starting FC level. Loss is a percentage of the starting FC level, with the percentage determined by your CYA level and how much direct sunlight the pool gets.

For a typical pool following our guidelines you can expect daily usage of roughly 2 ppm, but that can vary quite a bit (from less than 1 ppm to over 4 ppm) depending on how much sun your pool gets and what your CYA level is. And if you start at shock level your chlorine usage will be much higher.

 

[run53]

Loss is a percentage of the starting FC level, with the percentage determined by your CYA level and how much direct sunlight the pool gets.
.

Thanks. That's what I thought. Being an engineer and mathematician, I was wondering if anyone knew the theoretical formula for these relationships. Or is it just way to complex?

For example if I assume the # hours of direct sunlight and CYA, can I calculate FC loss?

 

[jcowart]

Hi,


I want to know because I just completed a SLAM and the rate of FC loss seems more at the beginning (when FC=20ppm) and gradually gets smaller. My SWG has been set to 65% throughout the process. I want to make sure that once FC reaches about 4ppm (CYA is about 70), that I can keep it there with the SWG.

Might want to check your OCLT again. It is advised to turn off you SWG when you are clearing a problem. That way you only test what YOU added and not what the SWG produced. Yes it's not really a fixed ppm but a percentage loss. So the higher the FC the higher the loss. When it comes back down to maintenance levels your loss should be lower and your SWG should keep up.

 

[run53]

Hi,


I want to know because I just completed a SLAM and the rate of FC loss seems more at the beginning (when FC=20ppm) and gradually gets smaller. My SWG has been set to 65% throughout the process. I want to make sure that once FC reaches about 4ppm (CYA is about 70), that I can keep it there with the SWG.

Might want to check your OCLT again. It is advised to turn off you SWG when you are clearing a problem. That way you only test what YOU added and not what the SWG produced. Yes it's not really a fixed ppm but a percentage loss. So the higher the FC the higher the loss. When it comes back down to maintenance levels your loss should be lower and your SWG should keep up.

OCLT is consistently (last 4 nights) at 0.5 or 1.0. I don't run pump/SWG overnight so OCLT readings should be accurate.

 

[chem geek]

Being an engineer and mathematician, I was wondering if anyone knew the theoretical formula for these relationships. Or is it just way to complex?

For example if I assume the # hours of direct sunlight and CYA, can I calculate FC loss?

Unfortunately, it's just way too complex because there are many different factors involved. The chart in this post can give you a rough idea of chlorine loss rate due mostly to sunlight at various CYA levels with the FC/CYA constant (except for the entries with 0 ppm CYA). For these SWG examples, the daytime loss rate is 1 to 1.5 ppm FC over 8 hours of equivalent noontime sun and there might be an additional 0.5 ppm FC or so loss the rest of the day. When we quote 2-3 ppm FC is being a typical loss, that is for non-SWG pools so assuming once a day dosing where you start out higher in FC/CYA ratio and end up with an FC that is 7.5% of the CYA level (compared to the SWG pool that has the FC mostly constant at 5% of the CYA level). Non-SWG pools use more chlorine because they have a higher active chlorine target and a swinging FC level that has to start out higher. So at 50 ppm CYA and a minimum FC of 4 and a 40% daily loss rate (which includes some non-daylight loss), you'd have to start with 6.5 ppm FC to end up at around 4 ppm FC after 24 hours and that's 2.5 ppm FC loss.

Though most of the chlorine loss is due to sunlight, there is still a lot of loss from chlorine oxidizing CYA in the pool (around 0.25 to 0.35 ppm FC per day) and oxidizing organics blown in, bather waste (though that's usually a low amount since 1 bather-hour of a non-urinating bather in 10,000 gallons is 0.1 ppm FC), pool covers, etc. Unlike the loss from sunlight, these chlorine oxidizing losses are dependent on temperature where the rate doubles roughly every 10ºF increase (not 10ºC -- most chlorine reactions are more temperature sensitive). This is why chlorine loss rates drop so dramatically in the winter when the water is cold and the pool is either covered or the days are no longer sunny.

 

[run53]

Being an engineer and mathematician, I was wondering if anyone knew the theoretical formula for these relationships. Or is it just way to complex?

For example if I assume the # hours of direct sunlight and CYA, can I calculate FC loss?

Unfortunately, it's just way too complex because there are many different factors involved. The chart in this post can give you a rough idea of chlorine loss rate due mostly to sunlight at various CYA levels with the FC/CYA constant (except for the entries with 0 ppm CYA). For these SWG examples, the daytime loss rate is 1 to 1.5 ppm FC over 8 hours of equivalent noontime sun and there might be an additional 0.5 ppm FC or so loss the rest of the day. When we quote 2-3 ppm FC is being a typical loss, that is for non-SWG pools so assuming once a day dosing where you start out higher in FC/CYA ratio and end up with an FC that is 7.5% of the CYA level (compared to the SWG pool that has the FC mostly constant at 5% of the CYA level). Non-SWG pools use more chlorine because they have a higher active chlorine target and a swinging FC level that has to start out higher. So at 50 ppm CYA and a minimum FC of 4 and a 40% daily loss rate (which includes some non-daylight loss), you'd have to start with 6.5 ppm FC to end up at around 4 ppm FC after 24 hours and that's 2.5 ppm FC loss.

Though most of the chlorine loss is due to sunlight, there is still a lot of loss from chlorine oxidizing CYA in the pool (around 0.25 to 0.35 ppm FC per day) and oxidizing organics blown in, bather waste (though that's usually a low amount since 1 bather-hour of a non-urinating bather in 10,000 gallons is 0.1 ppm FC), pool covers, etc. Unlike the loss from sunlight, these chlorine oxidizing losses are dependent on temperature where the rate doubles roughly every 10ºF increase (not 10ºC -- most chlorine reactions are more temperature sensitive). This is why chlorine loss rates drop so dramatically in the winter when the water is cold and the pool is either covered or the days are no longer sunny.

Great info! thanks