Error when testing for nitrates

I'm going away for a few days so I am doing water changes before I leave. I used a new API test kit and found my readings were not too bad. For some reason I decided to test them again using my old, almost empty bottles. The results were about 10 ppm difference!

Before everyone tells me about shaking, hitting, banging, throwing, and gently placing bottle on top of a nuclear reactor 2 seconds before detonating... I assure you I did "most of that! Also I'm well aware of the expiration date (in this case the bottles lot numbers reflect 05-08 and 08-08), so I do not think that's the reason.

The only other thing I can think of is that the crystals in the older bottle may have slowly settled each time I used it and over time creates a higher reading. Now I'm not sure what to believe!

Here is a picture of the results of each test (side by side) of all three tanks:

 

no no no its not 2 seconds it 4 seconds on a nuclear reactor.. and i had trouble with new set... but then maybe its the old set- ask you LFS if they can test it for you, and yes shake the bottle alot new bottles are harder to mix so shake it for a good couple minutes straight.

 

:;laughing nice visual of the banging!
I dont keep my kits past a year old (its my own phobia) but the kits are suppose to be good for 3 years from those dates...maybe this is proof that hmm, im not so crazy after all LOL

 

Love the visual of the nuclear reactor and the detonating! That was a good one!

 

I only bought the test kits about 6 months ago but the date on the bottles were a year old. Since I have 6-8 tanks running (depending on what month it is:;laughing) I go through a lot of bottles. 10 drops per test and 7 tanks, once a week; that's 280 drops a month! That's why I buy two boxes at a time! (I really need to save up for the digital tester:;money:;money:;money)

I run twice as fast as you:;dogrun