Chasing pH problems lately, I decided to try out one of the cheap (13 bucks) pH meters that are on amazon. I'm not advocating for this one in particular, I thought just I might share my experience with it.
First off, here is the link to the one I got:https://www.amazon.com/Dr-Meter-0-1pH-Accuracy-Tester-Measurement/dp/B00PU0W35K/ref=sr_1_2?ie=UTF8&qid=1483931972&sr=8-2&keywords=dr+meter
When I got it, I followed the calibration instructions and noticed right off the bat that it was REALLY unstable. I stood 6ft away from the meter and it would change readings as I walked toward it or away, and forget touching it. Anyway, so I followed the calibration and it read correctly for the 6.89 and 4.01 buffer solutions if I gave it a many minutes to settle in and stood far away from it. So, I washed it with distilled water per the instructions and put it away (wet). The instructions are pretty minimal, and say nothing about storage solutions or anything.
Two days later, I am going to brew so I pull it out and do some sanity checks on it. First, I noticed that the instability of the thing is gone and it reads nice an stable now. I assume this is because the sensor in it got dried out and being in the buffer solutions and put away wet allowed it to rehydrate and so it plays nice again. So I put it back in the buffers and recalibrated it again to the 6.89 buffer (it was +0.5 off at this point). Now it was time to test some stuff and see if it gave the expected results.
First was the 4.01 buffer provided with the meter. I found that it took about one or two minutes to settle to a number, and read 4.00. Close enough for me. Next, I did a distilled water mash of pilsner malt which should be 5.9 according to the malt analysis sheet, and after a minute of settling the meter read 5.89. Again, close enough.
Next I brewed some beer (czech lager) and compared mash the results of the spread sheet to what I actually got (spread sheet provided by our LODO friends, thanks). The predicted mash pH for this batch was 5.3, and the measured value with this meter came in at 5.29. Again, there seems to be a systemic 0.01 error most likely from my calibration. In any case, close enough for my needs.
So I can't speak to the longevity of the thing, but at least at first whack it was well worth the 13 dollars. When I bought it I was pretty skeptical that it wouldn't be total junk but figured for the price I would try one it out just to find out (and let others know what I found as well). If I have problems with it in the future, I'll post it.