I have been using the Taylor FAS-DPD kit for my chlorine testing. I use bleach to add chlorine. I have been using the 0.75 oz bottle of reagent R-0871 that came with my Taylor test kit. I'm trying to keep my FC in the range 4 to 6, since my CYA is 50. Recently my CYA was 4.0 in the morning. I added the amount of bleach that should have raised FC to 8.0. Later in the day I ran out of R-0871 reagent, so I went to Leslies to get some. They only had the 2 oz bottles (also made by Taylor), and the guy had to search for it until he finally found a bottle behind and under the counter.
When I tested the FC later that day, using the new 2 oz bottle from Leslies, it showed FC as 10.5. I was surprised it went up so high, given the amount of bleach I had added.
Several days later, I received a new 0.75 oz bottle of Taylor R-0871 that I had ordered online. I started testing the pool water samples using both bottles, and found that the new 0.75 oz bottle produced FC readings that were consistently 1 to 2 ppm lower than what the 2 oz bottle from Leslies indicated. I trust the lower readings from the 0.75 oz bottle more than the readings from the 2 oz bottle I got from Leslies, because my initial use of the 2 oz bottle indicated a much bigger FC increase than I had expected. My question is why the two bottles produced such different readings? I visually observed that that the droplet sizes from the two bottles appear to be the same, and the liquid is clear from both bottles. The 0.75 oz bottle I just received in the mail has an expiration date of 9/2017. The 2 oz bottle from Leslies has no expiration date on the label but just says "Lot #1122A". So I'm wondering if the bottle from Leslies was so old it has gotten weak? The color is still clear. I suspect Leslies does not sell that reagent very often since it was behind the counter, and it took the salesman a while to find it.
To add to the mystery, I did an overnight free chlorine loss test. The larger bottle from Leslies showed FC as 6.5 both the night before and the following morning, and it showed CC=0. However the smaller bottle showed FC=5.5 the night before, and FC=4.5 in the morning, and also showed CC a little under 0.5 in the morning. Has anyone had similar experiences with variability in the FAS-DPD test results? I will continue to use both bottles for a while and record the differences, but I'm thinking of just getting rid of the bottle from Leslies.