How To Keep Cya At The Right Level

[macgd016]

Given that keeping CYA at the right level is such a problem, is there a standard calculation that shows how much water needs to be drained from the pool for each Trichlor puck that is used.

[chem geek]

Given that keeping CYA at the right level is such a problem, is there a standard calculation that shows how much water needs to be drained from the pool for each Trichlor puck that is used.

First figure out how much CYA you are adding every month. For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm. So if you have even a low 1 ppm FC per [EDIT] day [END-EDIT] chlorine usage, then this is 1*30*0.6 = 18 ppm CYA per month. Then figure how much you need to dilute based on your CYA level target range. If that is 50 ppm on the low end, then after a month it's 50+18=68 ppm CYA so the amount of water required for dilution is 18/68 = 26% which is a lot. That's why even with backwashing it's hard to keep the CYA level lower unless there is rain overflow as well. If your target CYA is higher, say 100 ppm, then that's half the dilution or 13%, but you may need to use a supplemental algaecide (PolyQuat 60) or phosphate remover to prevent algae growth.

[macgd016]

Given that keeping CYA at the right level is such a problem, is there a standard calculation that shows how much water needs to be drained from the pool for each Trichlor puck that is used.

First figure out how much CYA you are adding every month. For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm. So if you have even a low 1 ppm FC per month chlorine usage, then this is 1*30*0.6 = 18 ppm CYA per month. Then figure how much you need to dilute based on your CYA level target range. If that is 50 ppm on the low end, then after a month it's 50+18=68 ppm CYA so the amount of water required for dilution is 18/68 = 26% which is a lot. That's why even with backwashing it's hard to keep the CYA level lower unless there is rain overflow as well. If your target CYA is higher, say 100 ppm, then that's half the dilution or 13%, but you may need to use a supplemental algaecide (PolyQuat 60) or phosphate remover to prevent algae growth.

Thanks Richard, Now I see why it is such a problem. (I assume you meant 1ppm FC per day?) Is there any way of calculating the ppm one puck will add to 8000 gallons or 36000 liters

[chem geek]

Thanks Richard, Now I see why it is such a problem. (I assume you meant 1ppm FC per day?) Is there any way of calculating the ppm one puck will add to 8000 gallons or 36000 liters

Yes, I meant 1 ppm per day chlorine usage and I've edited my post to correct that.

You can use The Pool Calculator to see that one 3" puck weighing 8 ounces added to 8000 gallons adds 6.85 ppm FC and 4.2 ppm CYA. Some 3" pucks weigh 7 or 6 ounces. 1" usually weighs 3 ounces and 1/2" usually weighs 1 ounce so you can scale accordingly.

[macgd016]

Thanks Richard, Now I see why it is such a problem. (I assume you meant 1ppm FC per day?) Is there any way of calculating the ppm one puck will add to 8000 gallons or 36000 liters

Yes, I meant 1 ppm per day chlorine usage and I've edited my post to correct that.

You can use The Pool Calculator to see that one 3" puck weighing 8 ounces added to 8000 gallons adds 6.85 ppm FC and 4.2 ppm CYA. Some 3" pucks weigh 7 or 6 ounces. 1" usually weighs 3 ounces and 1/2" usually weighs 1 ounce so you can scale accordingly.

Thanks for the pool calculator, it is really useful but gives some worrying results. I have a 50,000 liter pool, the calculator shows that an 8oz puck will give me 4.2ppm FC and 3ppm CYA. Assuming for convenience a FC usage of just over 1ppm per day I will need 2 pucks a week which will increase the CYA by 6ppm. To bring the CYA back to a starting point of 50ppm will require 11% or 5500 liters of pool water to be drained and replaced weekly!! If the pool pump has a flow rate of 300l/min this is 18 mins of backwash. Is this right or have I got something badly wrong.

[macgd016]

I have a 50,000 liter pool, the pool calculator shows that an 8oz puck will give me 4.2ppm FC and 3ppm CYA. Assuming for convenience a FC usage of just over 1ppm per day I will need 2 pucks a week which will increase the CYA by 6ppm. To bring the CYA back to a starting point of 50ppm will require 11% or 5500 liters of pool water to be drained and replaced weekly!! If the pool pump has a flow rate of 300l/min this is 18 mins of backwash. Is this right or have I got something badly wrong.

Richard

Sorry to be a pest but do these calculations look about right, total accuracy is not important just the order of magnitude

Mac

[chem geek]

Mac,

1 ppm FC per day is 7 ppm FC per week and require 1.7 pucks for 50,000 liters if these are 8-ounce pucks (typical weight for 3" pucks). That increases the CYA by 4.3 ppm per week. So the CYA climbs from 50 to 54.3 and requires a dilution of 1-50/54.3 = 7.9%. And yes, that's a lot which is why no one does this. Actual backwashing of filters and splash-out do dilute the water somewhat and this does help the CYA not rise quite as fast, but it still rises fairly quickly though slows down as it gets higher since the percentage is applied to a larger CYA value already in the pool.

If you didn't want to worry about the CYA rise, then you could use a weekly PolyQuat 60 algaecide or a phosphate remover. Otherwise, you'll need to use unstabilized chlorine to prevent the rise in CYA.

Richard