Bluelab pH 7.0 Buffer 250ml Calibration Solution for pH Meters. To improve the accuracy and lifespan of your Bluelab pH device, you must clean and calibrate it regularly. Calibration is a simple process that is the same for all Bluelab pH meters and pens. After cleaning the pH probe, calibrate it using calibration solutions at pH 4.0 and 7.0. Then you can start measuring pH immediately. Repeat calibration after cleaning the probe or when the device requests calibration.