Hey everybody.... I recently purchased a PH meter through an online store. It's a milwaukee PH 600. It was pretty inexpensive. I'm going to buy a nice all in one meter eventually. This is my first grow so I'm fairly new to all this. My question is how often do you need to calibrate them? I've had it for about 2 weeks now and checked it 3 times. Every time it's off by about .3-.5? I keep calibrating, and it keeps changing lol. I'm using 4.01 and 7.0 buffers.