I bought two bottles of Aquachek salt test strips this year because I thought the first bottle might be leading me astray. The first bottle I bought has an expiration date of 01/2010, and the second has an expiration date of 01/2011. I just tested the same sample of water leaving each strip in for 30 minutes, and here's the results:
exp date 01/2010 : between 3080 and 3330 ppm
exp date 01/2011 : 2670 ppm.
The reason I thought the first bottle was leading me astray was because the first test I did, the level was 2440. I added roughly 50 - 60 lbs of salt, and it didn't move. I bought the other bottle of strips, and my initial reading was 2460. I added another 40 lbs of salt, and now I get the readings I posted above.
My initial inclination is to believe the newer strips over the older ones, but when I run the numbers through the pool calc, it looks like the older strips match up more closely for the amount of salt that was added.
Methinks I might take a sample to the local pool store and see what they say, just for comparison.