Quick question

Hello all, Today i went ahead and invested yet another hundred dollars or so on my set up, this is gettin pricy as iv now spent a little over a half grand on this "garden" of mine. Oh well, atleast im now prepaired to grow any time, almost any place! So i picked up a PH pen, and some calibration solution and of course ph up n down. My question is, i bought 2 PH calibration solutions, one is to set the pen to neutral 7.0, now i get wat purpose this serves , but i also got a solution the calibrates to 4.01. I was wondering wat the hell is the 4.01 for, and how would i go about using it. The instructions on the PH pen make no mention of a 4.01 solution, but they do say stick the fukr in the 7.0 solution to calibrate. PLZ any info would help, as id like to think i didnt just waste my precious dollars on some thing i dont need.THXS

pH meters are typically 2-point calibrated. You calibrate it to 7, using the 7.0 solution as a setting for that. And then calibrate it to 4, using that solution. There is usually a set screw for each calibration.

HTH