Why different alkalinity numbers?

I've tested with two test kits P234 & 5-Way HTH. Why am I getting two different alkalinity results with each test? Shouldn't the results be the same? HTH was 240 yesterday and 210 today. P234 was 190 yesterday and 160 today. Comparatively, the numbers went down the same amount from yesterday. However, I don't feel confident with the numbers and don't know how much more muriatic acid to add if I can't even start with "good" numbers! I'm inclined to go with Ben's numbers.....:confused: BTW...PH went up to 7.5...

Has your pH changed since each test. pH has an effect on your TA reading. Lower pH will cause an apparent shift downward of the TA. It really hasn't changed much unless you airate and raise the pH. If the water has been airating and the pH was low on the first test and was higher on the second that is why the numbers went down, you have actually lowered the TA. The chemistry behind it is a bit complex so I will spare you that for now. As far as the accuracy of the tests I would trust Ben's over the HTH!

The Ph has gone up from 7.2 to 7.5. I've added another 16oz. of acid to see if I can get the alk in the 100-120 range. I guess the concern is that people who don't have Ben's kit might be adding more chemicals to their pools than might be necessary. I purchased the 5-way HTH while waiting for the P234 and only broke the plastic cellaphane yesterday. The kit can't be that far off ...can it??:eek:

HTH ain't the company it used to be. Take that for whatever it's worth.:rolleyes: