Bluelab pH 7.0 Buffer 500ml Calibration Solution for pH Meters. To improve the accuracy and lifespan of your Bluelab pH device, regular cleaning and calibration is required. Calibration is simple and applies to all Bluelab pH meters and pens. After cleaning the pH probe, calibrate with calibration solutions at pH 4.0 and 7.0 to take immediate measurements. Repeat calibration after cleaning the probe or as required according to the meter display.