Why test CYA?

Well, a little provocative, but I'm just trying to figure this out.

Between many pool store testings and my own drop kit testings I'm fairly confident my CYA has been at 50-60 for the last six months or so in my salt-chlorinated pool. Now -- if I don't add stabilizer nor any chemical that has it in it (IOW, using BBB or just salt for the SC) and since you can't lower CYA with any method other than draining and refilling, just why would I have to test for it again at all?

To put it another way, there's nothing that can lower it (or you'd just say "do this" instead of draining/refilling) and AFAIK you can only raise it by adding stuff to the pool that would do that, so is there really any reason to test for it every few days? How about once a week (or even once a month)? What the heck would I be testing it for?

Splash-out and backwashing remove water that has to be replaced by either rain or the hose. CYA will drop. However, there's no reason to test more than a few times a season.

The answer is that you don't have to test it very often under the circumstances you describe. If you don't have a lot of splash-out and you use a cartridge filter which never requires backwashing (cleaning, yes, but washing using pool water, no), then you are correct that the CYA level will stay pretty constant throughout a swimming season and I have found that to be the case in my own pool.

However, over a winter, you may find that your CYA levels drop. If you keep your pool covered during the winter, then some forms of anaerobic bacteria will breakdown CYA. Of course, that implies that your chlorine level may drop at some point during the winter -- if you keep your chlorine level up then you will likely preserve your CYA as well, but some people just let their pool water sort of go "bad" over the winter and then deal with cleaning it up at the start of pool season.

Generally speaking, you should test your chlorine levels (FC, TC to get CC) most frequently (say, daily, unless you have a pool cover), then your pH level (once or twice a week is usually plenty unless you use Tri-Chlor -- bad for other reasons -- or your pH rises quickly and you add acid frequently), then your TA (monthly is usually OK unless you have a lot of pH rise and add acid frequently), and finally your CYA (twice a season may do it unless you have a sand or DE filter with backwashing and/or lots of splash-out).

[EDIT] JohnT wrote his post as I was writing mine so sorry for the duplicate information [END-EDIT]

Richard

Thanks! That's all good information.

Eventually I'm going to learn all this stuff <g>