I opened my 38,000 IG vinyl pool this year to find that the 30-40ppm CYA that was present at the close in the fall is now at 0?? This is the second season that this has happened. I tested with the black dot test and then had two different pool stores check it. All came up 0. I do not drain any water off to close and had to only add about 1" of water this year when I opened it.

Any ideas??

Losing all of your CYA over the winter is not uncommon. We think that it is caused by the growth over the winter of certain kinds of bacteria that break down the CYA into ammonia.

When you lose CYA over the winter, there will be a substantial chlorine demand when you open the pool in the spring. It will probably take a substantial amount of shocking before you will be able to hold a FC level.

As usual, Jason is right on the spot.

chem geek, one of our gurus on this forum, documented his case involving this very process here: http://www.troublefreepool.com/it-can-happen-to-anyone-zero-chlorine-cya-ammonia-t10974.html

The problem that he ran into and that Jason touched on is the potential for ammonia to rapidly consume your FC. During shocking, if you can't seem to get the FC up and keep it there overnight very well, it may be due to the ammonia. A simple and cheap way to check the ammonia level in your pool is with the use of an ammonia test kit from an aquarium supply place.

I had the same when I opened ours this year 0 CYA. It never occurred to me to get a test kit for for detecting ammonia in fish . Something I will add to the list to get. Thanks for the idea 257WbyMag !

One thing to keep in mind is that the ammonia test kits are interfered by monochloramine, so you want to measure both Free Chlorine (FC) and Combined Chlorine (CC) with your FAS-DPD chlorine test in addition to measuring the ammonia. You will likely measure 0 FC, but may also measure some amount of CC. To figure out the true amount of ammonia, you want to subtract 1/5th of the CC measurement from the ammonia measurement.

The minimum amount of FC that it will take before you start to hold the FC will be 0.6 * CC + 8 * Ammonia (adjusted).
The formula using the raw (unadjusted) measurements is 8 * Ammonia - CC.

It may take a lot more chlorine than this, so to figure it out more precisely you can do a bucket test -- or you can just keep adding chlorine till the cows come home and you eventually measure FC that reasonably holds.