i bought one of these as a cheap way to test the ph of the water i'm feeding my veg.
It really doesn't appear to work. There is movement on the needle between 6.5 and 7 but this seems to be completely uneffected by what you put it in. I put it in a bottle of tap water and it read 6.5, an hour later it read 7, so I tried it in a bottle of filtered water with identical results. So i then I tried it in a bottle of vinegar - the needle should go off the scale but got the same results as the wate. Finally i tried it in a bottle of vodka and got the same results again. This thing is completely useless.
I consulted a top botanist friend of mine who laughed and told me that with these 'mickey-mouse' testers you get what you pay for, they're totally unreliable, which is why professionals use digital testers (which start at around £12
lesson learnt :)