There are quick read and slow read strips. The quick read strips give you results within about 30 seconds. These strips give you a short window of opportunity to compare the test pad to the label... maybe 3 seconds? Even shorter in full sunlight. You dip the strip into the pool, hold it horizontally, and compare the test pad after about 25 seconds. After 30 seconds, the test pad will change rapidly so it's useless after that.
The slow read strip takes a bit longer (3 t0 10 minutes). You place the strip in a sample container with about an inch of water and the strip standing upright, with only the bottom of the strip in the water. There is a yellow strip along the top. When at least half of the strip turns black, your test is done. The strip will turn white along a numbered scale. This number must be compared to the scale on the test bottle that the strip comes in. I make special note of this because the strip is marked specifically to the scale on the bottle. If they are the same batch number, then you can, but they can be very different from bottle to bottle.
Above all, you must keep your hands dry when removing a test strip. Keep the bottle closed tightly. And keep the bottle out of the sun, or heat. Any of these conditions can compromise the rest of the strips and you might as well throw it away.
Electronic meters are, as Evan said, accurate as long as they are regulary calibrated.
Salt water aquarium Hygrometers are not within the range for pools, so are not applicable.
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