It's a cheapo ... probably only good for six months. I only trust it to about 0.5 accuracy. @Drew51 is right though, you have to keep the contacts clean at all times.
That can be true, but the product I bought is part water-soluble and part "standard" prilled sulfur. In my experience the water-soluble portion should be permeating the mix in about a week and as you said: the rest will take months. In the meantime I'm going to plant those blueberries and then put down a few inches of small fir bark on top. The latter will enable the bacteria to go to work. Also, my fertigation system will be reconnected this summer when the construction work is finished and that will sustain a desirable pH.