I've been using a combination of liquid chlorine (12.5%) and trichlor tablets for almost a year now. Starting a few days ago, I noticed that the chlorine was not rising to the expected levels when adding liquid chlorine. In other words, shortly after sunset, I'll add an amount of liquid chlorine that should bring up the total chlorine up by 1 ppm. When I check the levels the next morning (or later that night after the pump has been running a few hours), the chlorine levels only show a rise of about 0.4 ppm, less than half of what I expect to see based on my experience over the past year.
I'm using the same volume for my calculations that I've always used, and am measuring with a Taylor K-2006 kit using the .2 ppm accuracy test. I also checked the CYA, and it looks like it's down around 60 or a bit higher, so I don't see how this could be due to "chlorine lock." I'm also seeing zero combined chlorine in my tests, but is it possible to have high chlorine usage WITHOUT tests showing any CC? I would think that I would see at least SOME CC in my tests. It also seems like if the Cl was being used up at such a high rate, it would be zero by the next morning, but it is not. It seems like it's just lower to start with (after adding it), but that it's dropping at a relatively normal rate. Note that through all this, the water has never looked cloudy -- if it wasn't for the measurements there would be no indication of any problem.
The only thing that changed recently was that this is a new batch of chlorine. It's Hasa, which I would think would be as good as any, but is it possible to get a bad batch of chlorine? The measurements are behaving like the chlorine is only half as concentrated as it should be.