CYA lag time?

[TreeFiter]

I've read over and over again that there is often a lag time when adding CYA to a pool. It is usually attributed to the slow rate at which it dissolves, especially when it is poured into the skimmer. I've never been a big fan of this idea. It just doesnt' seem to make much sense to me. In order for the granules to fit through the skimmer basket, and end up in the filter, they must first get smaller. They get smaller as they are dissolved. They then have to fit through the pump basket, where they are further dissolved. Yes it is entirely possible that a portion is captured in the filter, but with the rate of flow through that filter, its hard for me to believe that the CYA remains in the filter more than a few hours at most.

Today I had an experience with a pool that makes me suspect that the lag has nothing to do with CYA being trapped in the filter until it dissolves. I added 4lb of CYA to a pool yesterday afternoon. This morning I returned to the pool, and tested the CYA to find that it hadn't increased yet. Without thinking, I backwashed the filter. After doing so, I remembered that CYA is supposed to take time to dissolve, and you are supposed to wait a few days before backwashing. So if there was CYA in the filter, it should be gone now.

Since this pool is pretty much on my way home, I stopped by again to check on it. I was going to add more CYA since I had backwashed away everything I added the day before. I decided to test the water one more time. This time the CYA had come up by about 10 ppm. So I decided not to add more, and see just how far it comes up. I'm going to try to check on it tomorrow again.

It seems to me that even if some of the CYA is trapped in the filter, a significant portion of it has to dissolve before it will even pass through the skimmer basket. Why doesn't that portion show up right away?

 

[Crackerjack4u]

It can take up to a week for the CYA level to rise when using the powder form, and the sock method, not pouring it into the skimmer, is the only recommended way to add CYA to your pool.

 

[TreeFiter]

It can take up to a week for the CYA level to rise when using the powder form, and the sock method, not pouring it into the skimmer, is the only recommended way to add CYA to your pool.

But what causes the lag?

 

[singingpond]

I think this is an interesting question too, and will keep an eye on this thread.

And why does the CYA in dichlor show up pretty much right away (judging by my once annual use of dichlor, anyway - can measure the CYA rise an hour after addition of dichlor). I've assumed that the dichlor molecule is more soluble than the plain CYA, and that the plain CYA is perhaps in suspension (tiny particles) rather than fully dissolved for some period of time after we add it to the water, and that it can't combine with the melamine reagent until it is fully dissolved...? It would be nice to know what the truth of the matter is.

 

[Crackerjack4u]

LOL Your guess is as good as mine on exactly why the lag time. Perhaps someone a lot smarter then me will answer it for us. I do have my best guess on it but I'm not sure it's close to being correct though.

Here is my thoughts: I think the lag time IS consistent with the time it takes for it to dissolve.
Like in your case, if some had dissolved in the filter from yesterday, then it was dispersed into the pool through the return, then it would in fact be registering. Backwashing may not have removed it all, and left some behind to register later in the day? and Perhaps with you adding it through the skimmer instead of using the sock method, the undissolved CYA particles however small they may be after going through the skimmer are lighter than the sand, or whatever material is in there, and they may sit on top of the medium, until they completely dissolve, and only then be able to filter through?

That's why liquid CYA has an immediate effect, and dry form can take up to a week to raise the levels. I actually added some CYA via sock method to my pool today, and it is already showing up in the levels. So I think the faster it dissolves, the faster it dissipates, and the faster the levels register, then it can do what it was put in there to do. Anyway, lol that's my theory, and I'm sticking too it, At least until someone tells me I'm wrong. LOL sorry I couldn't be of more help. I will be watching this thread for all the other answers though. Have a nice day .

 

[chem geek]

You aren't buying CYA pellets, but rather granular CYA where the granules will easily slip through the skimmer and the pump basket to mostly get caught in the filter. Yes, if you dump a bunch of once it doesn't flow through immediately, but it's not stuck because it's too big but rather because you put so much in at once that you create a "bridge" across the size of the opening. That's why in a sifter you can pile a bunch of sand and not have it drop through but if you move back and forth you can get all the sand through if it's all individually smaller than the sifter openings.

As for the flow rate in filters, it's MUCH slower than through the skimmer or over a return flow. The flow rate through a skimmer might be around 15 GPM or so depending on your pump speed. The water velocity over a 6" diameter would be 0.17 feet per second or 2 inches per second. My oversized 4-cartridge filter has 340 square feet of area which only has an average water velocity through it of 0.0038 inches per second (0.1 millimeter per second). Compare that to the effective filtration area for a sand filter of 3-4 square feet, so 100 times faster water velocity (2 feet per minute or 0.4" per second) or a DE filter of 30-50 square feet so 7-11 times faster water velocity (about 2.5" per minute; only 0.04" per second). Nevertheless, the water velocity in the sand filter is 5 times slower than the skimmer, the DE filter about 50 times slower, and the cartridge filter over 500 times slower.

Again, there is no mystery here. If you add CYA directly to your skimmer then much of it will get caught in your filter. It might not all get backwashed and some might have slipped through (depending on type of filter; sand filters let more slip through) to continue to slowly dissolve in the pool. When I add CYA to my pool I put it into a T-shirt (so double-layered) in the skimmer and it dissolves usually overnight with the pump running and I measure nearly all the expected CYA level the next day.

Cyanuric Acid is a slow chemical to dissolve. Dichlor dissolves much, much faster as does sodium cyanurate in a slurry in liquid CYA products. It is absolutely, positively the case that the delay in CYA measurement is related to how slow it takes for it to dissolve and that to dissolve it faster you need more water flow (velocity) through the crystals.

 

[TreeFiter]

But how would this explain the increase in CYA after backwashing the filter, but not before?

 

[Griswald]

The test is +/- 10, so if you see a 10 point rise, it could be the test method and not really a rise.

 

[chem geek]

Also some CYA may have been in the pool and not in the filter and dissolved so slowly that it only got to 10 ppm coincidentally with after you backwashed. However, 10 ppm isn't enough to be definitive about anything regarding CYA. The fact is that MOST of the CYA you added got into the filter and removed in the backwash. Again, you haven't seen anything different from what we already know. If you add CYA to the filter, say via the skimmer, then it will take longer to dissolve -- how long depends on filter type. If you instead use a sock over a return or a T-shirt or skimmer sock (perhaps two if they are too thin/porous) in the skimmer then it will dissolve more quickly and register on tests faster.