Bluelab CF PenBluelab CF Pen Don't overfeed or underfeed plants! Monitor your nutrient strength & temperature with a Bluelab CF pen. It doesn't cost much, but it's just what you need. You can hold readings and choose your units. To make life easy, calibration is optional. Since temperate affects CF, your CF pen has automatic temperature compensation. That way, you get an accurate reading every time. It also takes your nutrient temperature, so you can keep it inside