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I not sure about the brand as mine is a cheapskate ph pen that cost around $12. It can cost from $12-$100. LFS are around $30 onwards. The better one are those that comes with temperature probe and able to auto calibrate so you don't have to use screw driver to adjust. It should cost around $60 onward if I didn't remember wrongly.
Like all measurement equipments, PH pen needs to be calibrated to precise accuracy before using. The calibration/buffer solution comes in ph 4, 7 & 9. If it's the dry form, you need to mix with the prescribed amount of water. Measure the temperature of the solution and adjust the PH according to the PH/Temperature chart. For more precise pen that measures 2 decimal place, you need to calibrate with 2 solutions else the normal cheapskate one, any of the 3 solutions will do. But I think if you are measuring lower ph, best to use lower ph cal. solution vise versa.
If you need accurate reading or didn't use for a long time, cal. before use. For me I just cal. once a month as I don't need very precise reading.
XD If you are cheapstake like me and don't mine that +-0.1 error, you can use freshly made baking powder solution. It has the ph 8.3 when freshly made but drop to 8 when it lose it carbonate over time. Before using baking powder, you need to at least cal. your pen with the cal. solution and then prescribe your own baking powder formula that has the ph of 8.3. As too diluted or due to your tap water factors you might not get a ph 8.3 solution. Or you can use potassium chloride as ph 4 solution is actually potassium chloride. Potassium chloride is used by AQ as ferts which is quite easily available.
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