hydrometer is only as good as your calibration... make sure you calibrate it, and if its constantly not reading what the beads should be, recalibrate it to make sure its not the hydrometer... i use one of those heartfelt adjustables in my 150ct, but in my cooler, i use a $7 walmart job... i trust em both just the same though... i got a little 30ct humi to start with and it was a good size for a dorm room, just set it on a little end table i had set up... later, i picked up a 50qt cooler and stuck it in my closet with a pound of heartfelt beads as an expansion... worked fine, you'll be alright with that 50 even if you do need more room... just remember, Wal*Mart: 120qt cooler-- $43 ; digital hydrometer-- $7 ; add some beads and there ya go...