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I personnaly use tap water (pH 7.4) and didn’t see any difference between distilled, osmosed and demineralized water. You can use any water with an average pH (6-8) because the powder will saturate the water at the correct pH level. You should read the product instructions but usually you have to put the content of the bag into 250ml of water and stir.
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I suggest the powder instead the liquid because it’s cheaper, expire after longer and the powder can’t be contamined (vs bottle). There are 3 common buffer solutions used for pH measurement: pH 4.01, pH 6.86 and pH 9.18 (7.0 and 10.0 exists). The op amp board just convert the millivolts of the probe into to something acceptable for Arduino (positive between 0 and 5v). This value is too small and other tech stuff like impedance make it unusable directly with an Arduino that’s why you need an “op amp”. Depending the pH it output a positive or negative voltage of a couple of millivolts. The probe is like a (tiny) battery when placed in a liquid. Less than 2 and more than 12 is almost impossible to determine something (too acid or too basic). It means that you can almost measure a linear value between 4 and 9 pH. The curve (log10) of the pH is a mix between linear and exponential as you can see in pH_curve.png. The pH is a value between 0 (acid) and 14 (basic) with a neutral value of 7. Some words are in italic to avoid “experts” saying it’s incorrect (because it is incorrect) but for our small projects and for understanding I intentionaly use those “simple” terms. And when the subject is speaking about the correct card, the information is incorrect or computation are made with random numbers coming from the sky. Most topics are related to other boards (Atlas, Sparkfun, Sparky’s Widgets, etc.) and nothing is working. I also bought this little board from China and it is difficult to make it working. The one with only 6 pins in front of the BNC plug (not on the side) as you can see in image Pinout1.jpg. First of all, I’m only speaking about the board labelled “ and “Logo_PHsensor v1.1”.