Ok so my pen got here just in time for me to mix up my first batch of nutes for my tent and I calibrated it with GH 1500ppm solution checked my tap water which is sitting at 141ppm which isn't too bad. So with my veg mix I'm using of 7ml A&B 1.1ml Roots Excelurator and 2.5ml of MultiZen comes out to 764ppm which is where I wanted to be for veg anyway 700-750ppm so I'm happy. Water temps were 73 it's cool to have a nicer pen I must say lol and this one feels nice and solid,well built and I'm kicking myself in the ass for not seeing this one last time cuz I paid $24 for the TDS EZ from HM Digital cuz it came with a pack of 1342 calibration solution but if only I had flipped through a few more pages I would've found the AP-1 and AP-2 (EC instead of ppm) but yea I suggest getting one if you don't have one already as they are needed to get the most out of your grow..