Bluelab Replacement pH Probe The Bluelab pH Probe is used to measure the pH of a solution. The Bluelab pH Probe is easy to clean because of its bullet shaped glass bulb, fully waterproof and less fragile than all-glass electrodes because of its tough polypropylene barrel. The pH probe is gel filled (non-refillable) making it maintenance free and has a double junction reference which provides longer life! NOTE: Gel filled non-refillable pH electrodes are general purpose probes for use in moderate to high ionic strength solutions that are free of oils, suspended solids and proteins.