The problem with strips in general are that they are sensitive to moisture and compounds on your hands. The individual pads on each strip are treated with chemicals to produce a certain color when exposed to moisture and various concentrations of the subject of the test (ammonia, trites, trates, pH, etc)
The strips become less accurate over time because each time you open the container and remove a strip, you are exposing the rest to small amounts of moisture and the compounds for which you are testing.
A brand new canister of strips should be inherently as accurate as liquid tests in the right circumstances. Each time you open the container to test, the reliability of your results drops dramatically.
To summarize, if the bottle in the picture is brand new, it should give you a ballpark answer. If the strips are older, I wouldn't trust them to change colors accurately at all.
It's the same basic theory as the dipstick urine tests that your doctor does at the office. It gives a clue, but for accurate results, you go for a direct urine sample measurement.
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